Insect (insecta)

Table of Contents

Insects (or Insecta in scientific Latinfrom the word insectum “cut, divided”) belong to the Class of the Arthropoda Phylum, which, after the systematic revisions from the last decades of the 20th century, was included in the Hexapoda Superclass, they are considered as the largest group in the animal kingdom on Earth: the species described are over a million and their number increases with the ongoing discovery of new species. 

Their scientific importance is exceptional since, among other things, many of the greatest discoveries in general biology resulted from studies on insects; their practical importance for men is enormous since the damage that phytophagous species cause to agriculture is huge and they can carry very serious and numerous diseases. 

Insects appeared in the Paleozoic era and the first traces of their presence belong to the middle Devonian period with fragments of the, probably winged, species Rhyniognatha hirsti, initially classified as springtail. With their remarkable ability to adapt, they invaded all the inhabitable parts of the earth’s surface, including some areas (decomposing substances, cavities, and tissues of several organs, thermal waters, petroleum waters, etc.) in which there are few to no other animals. 

The Insecta Class represents the animal group that has had the most evolutionary success on planet Earth, colonizing every kind of habitat in all the continents. The number of living species of insects is still unknown, but it could be around 40 million. Also regarding the number of individuals, no terrestrial animal has such numerous populations. Along with plants, they represent the fundamental organisms on which terrestrial ecosystems are based. The reasons for their evolutionary success are their anatomy, which is simple and efficient, their high reproductive rate and their ability to adapt to environmental changes. 

At the end of the Carboniferous orders such as Blattodea, Ephemeroptera, Odonata, Orthoptera already existed; in Permian, many of today’s orders appeared such as Neoptera: Coleoptera, Neuroptera, Mecoptera, Psocoptera, while many of the most ancient groups disappeared during the great extinction of the Permian-Triassic. In the Mesozoic Era, Diptera, Lepidoptera, Trichoptera spread after they had appeared with primitive shapes at the end of the Paleozoic era. From the Early Cretaceous many successful modern groups developed alongside (coevolution) Angiosperms, i.e. flower plants, that, in turn, have increased their number exponentially. In the Cenozoic era, the first social groups of Hymenoptera spread out (they had first appeared in the Cretaceous).

Insects, which have a shell (exoskeleton) that allowed them to evolve in infinite varieties of shapes, and which are equipped with honed instincts, responsive sense organs, the ability to adapt to every kind of environment, occupy a “dominant” position on Earth compared to other groups of animals and they greatly affect the general balance of nature. 

The life span of insects, except for some social taxa and a few others, is generally short: most of them live for a few months, a few weeks or for a few days. However, the duration of the preimaginal phase (the group of stages of the insects’ metamorphosis that precedes the adult stage) can last even for years. They spread (actively or passively) in an extraordinary way, sometimes migrating even in groups of a million individuals, through continents and oceans. They live alone, in groups or in societies (which can be homogenous or heterogeneous; seasonal or persistent; etc…) of remarkable complexity and often in symbiosis (with plants or animals) made of 2 or 3 symbionts. 

Morphology and anatomy of insects 

The nervous system has a central complex, a visceral complex, and a peripheric complex. 

The central complex is made of a cephalic supraesophageal ganglionic mass (cerebrum), united with a ventral ganglionic chain through a girdle. 

The visceral complex consists of a supraintestinal part and a subintestinal part.

The circulatory system is made of a dorsal vase where the so-called “blood” proceeds from the rear to the front, after being aspirated through the ostioles. It circulates in the rest of the body through the organs’ gaps, regulated by a system of diaphragms and often forced to follow certain routes from pulsing accessory centers.

The respiratory system is tracheal; it is, in fact, made of tracheae that open externally through stigma and they branch in secondary tubules and in tracheoles. Many aquatic insects breathe oxygen dissolved in water by using tracheal gills. In other cases, breathing can happen without specific organs and through the integument.

The digestive system consists of a more or less developed intestine, divided into three main parts: foregut, midgut, and hindgut. Digestion and absorption take place in the midgut. The excretory system is made of malpighian tubules, by nephrocites, etc. The adipose tissue and the integument also take part in the excretory function. 

The secretory system is made of various glands, that can be uni- and pluricellular, spread and confined, some of which emit secretions of great value in the market of insects and are used sometimes from man (silk, wax, hairspray, etc…) 

The reproductive system includes, in general, an equal number of sexual glands (testicles and ovaries), which communicate through equal gonoducts (deferents and oviducts) with an unequal gonoduct (ejaculator or vagina) which opens ventrally after the 8th abdominal segment, in front or ventrally to the intestine’s outlet. 

Most times it has receptacles and accessory glands. Sexes are normally separated. The post-embryonic development happens through a series of more or less remarkable transformations that conduct the young organism to reach the imago stage and, with it, somatic and germinal maturity.

These transformations constitute the metamorphosis which is almost absent in Apterygota and in wingless Pterygota, is partial in many Pterygota and more profound in others. Reproduction happens in various ways; the more common are sexed reproduction and parthenogenesis; the rarest ones are pedogenesis and polyembryonic reproduction. 

External morphology

The features that distinguish insects from other arthropods are the following: body covered by a chitinous dermal skeleton and divided into three distinct morphological sections: head, thorax and abdomen. The head is not divided into segments; it is generally provided with two eyes, with 1-3 ocelli, a pair of antennas and 3 pairs of mouth appendages (maxillae, mandibles, and labium).

The head is a rigid capsule (skull) with two openings: a front one (mouth opening), and a ventral posterior one (foramen magnum); it includes a pair of important organs, the antennae, inserted on the forehead, made of few or many segments, of variable shape and with many sense organs; two eyes (which are rarely missing); 1-3 ocelli (which can be totally absent) and appendages that form the so-called buccal apparatus. This can be different depending on the group but it is mainly always made of the same parts (in dorso-ventrally direction: labrum, mandible, maxilla, and labium) though it can go through more or less visible transformations, specializing to certain diets. 

In the chewing buccal apparatus (which belongs to shorter insects: Blattodea, Orthoptera, Termites, Coleoptera, etc.) the labrum is a small scleritis, unequal and medial, that contributes to the gripping of food and it is needed as a front roof for the mouth opening. Mandibles are equal pieces, intact, generally shaped like pinchers. Maxillae are made of several parts (hinge, jamb, external lobe, internal lobe, palpigerous, etc.) and they carry an appendix with 1-7 segments, the maxillary palps. 

The labium, which is probably formed by the fusion of a second pair of maxillae is also divided into various pieces and it has two appendages with 1-6 segments, the labial palps. In its internal portion (dorsal) you can see in many cases a sort of prominence that goes all the way to the front of the mouth, through the mandibles (prepharynx).

The lambent buccal apparatus, which is common of almost all Apoids Hymenoptera is a chewing buccal apparatus in which, among other things, the maxillary lobes, the first two segments of the labial palps and the ligula (resulting from the fusion of the internal labial lobes) get longer and spatulated. Among the sucking buccal apparatuses there are some which are unable to pierce, like in Lepidoptera, in which the maxillary lobes stretch enormously and link together to form the coiled proboscis, mandibles reduce or disappear, maxillary palps regress; and there are others with which instead the insect stings plants or animals, in which (like in Hemiptera) the mandibles and maxillae are transformed in 4 bristlelike stylets and the labium in an elongated rostrum that holds them, but doesn’t penetrate the wound; or (like in many Diptera) the labrum and the prepharynx are also elongated and styliform; or (like in the Diptera from the Stomoxys genus) the labium is attenuated at the peak, is chitinized and capable of cutting the skin.

The thorax is divided into 3 segments (prothorax, mesothorax, and metathorax); it carries 3 pairs of segmented legs and dorsally 2 or, more commonly, 4 wings. The abdomen is made of a various number of segments, typically 11. The thorax has 3 segments (prothorax, mesothorax, and metathorax), which can be simple or sub-annular but, usually, they have a surface divided in secondary sclerites of various extensions and various shapes. Their dorsal region is generally called tergum, the ventral one stern, the lateral ones are called pleurons. Thoracic segments often have very different developments. In Apocrita Hymenoptera the thorax is linked with the first abdominal segment, called propodeum or propodium.

Along with the special, more or less visible, tegumental processes which can decorate it, the abdomen is equipped with the main locomotion organs, namely wings and legs. Wings are dorsal appendages that can be, in living forms, up to a number of 4 (2 at the mesothorax and 2 at the metathorax), sometimes they are 2 (usually at the mesothorax), but they can also be absent or appear more or less reduced (aptera, subaptera, microptera forms) or transformed in tegmen, hemy-elytra and elytra.

They articulate to the segment which they belong to in the meeting point between the tergum and the pleuron, through little axillar sclerites (pterals) and they are lateral expansion (para-tergum) made by two foils stuck together, which are crossed in various directions by linear chitinous formations that can be empty or full (There are main and secondary venations, longitudinal and transverse, and there are spaces in the cuticle that are limited by venations that are named cells.)

The legs are ventral appendages in the number of 6 (two for every segment) articulated in the meeting point between the stern and the pleuron. They consist of the subcoxa (trochanter) and of the free limb, made of 6 pieces articulated together: coxa (or hip), trochanter, femur, tibia, tarsus (that in turn includes 1-5 segments) and pretarsus that has several organs (nails, pulvilli, empodium, etc.). Legs can go through considerable changes, they can adapt to special functions (swimming legs, fossorial legs, raptor legs, jumping legs, etc.) they can reduce or even disappear. 

The abdomen is typically made of 11 somites or urites (12 in Protura), but it is rare that all of these segments are clearly distinct; some, in fact, specifically the firsts and the lasts, change more or less profoundly and their number is thus apparently variable. It contains the most important insides, or part of the insides, and can present various appendages (ancestral, larval transitional; sensory, genitals, etc.). The abdomen usually has a simple structure, but its shape is not at all constant; sometimes it articulates to the thorax with a large base (sessile abdomen), sometimes with a peduncle of 1-2 segments (pedunculated abdomen). In can be equipped with several appendages, that are persistent or transitional and have different functions. We will remind the ancestral appendages (styles and saccules), the larval transitional ones (pseudo-legs, pseudo-gills, etc.), the cerci and the gonapophyses situated around the genital orifice that function as organs in reproduction and oviposition.

Muscular and integumentary systems – Locomotion and flight

The body of insects is covered by a kind of shield (exoskeleton or dermal skeleton), which is more or less thick depending on the order or on the post-embryonic state, but it’s always thin and bendable at the level of joints and the limitations of the various segments. In the integument, proceeding from the outside to the inside, we can see the following parts: cuticle (made of a specific substance called chitin), chitinogenous epithelium (of which cells secrete the cuticle itself) and a basal membrane. The external surface of the cuticle is rarely perfectly smooth and even; more often it has various modifications (tubercles, points, hulls, striae, etc.) that form the so-called sculpture; it can have processes of different dimensions and often very evident (prominences, thorns, horns, etc.); lastly it can have integument appendages, i.e. articulated formations (hair, bristles, scales). The, sometimes very lively, colors that adorn insects are due either to pigments situated inside the tissues, which in most cases look like intrinsic pigments, (chemical colors); or to structures’ peculiarities that originate reflection, interference, and diffraction phenomena (physical colors or the combination of the two previous ways.)

The complex of chitinous apophysis which the exoskeleton sends inside the body and that is needed for the attachment of muscles and to support certain organs, is called endoskeleton. It is well developed in the head (where it mainly consists of the tentorium) and in the thorax (where it mainly consists of phragmata, apodemes, and gallows), it is less developed in the abdomen. The insects’ muscles are all made of striated fibers. The vibrating muscles of wings have a specific structure (yellow or atypical muscles). The muscular strength of these Arthropoda has been often considered as enormous because only the relative muscular strength has been taken into consideration, which is higher as the animal gets smaller; however, absolute muscular strength, i.e. the contractile strength of a section’s area, is much more inferior to the vertebrates’ one.

Hexapoda locomotion happens with the minimal effort possible, by shifting the center of gravity forward and outside and letting the body rest alternatively on a supportive triangle made by the resting points of the two external legs on one side and the middle one on the other side. On smooth surfaces, on vertical plans, and on vaults, this can happen with the help of empodiums and pulvilli, which adhere to the support under the action of atmospheric pressure. Apodal forms move by using the peristaltic movements of the muscular shell of the body, with the help of processes and appendages of their integument and, sometimes, of various organs.

Jumping happens with the action of the hind legs, on which the muscles on the femur are remarkably developed and the tibia is elongated, but it also happens through special mechanisms or specific movements. Insects that walk on the surface of the water are supported both by the surface tension of the liquid and by the fact that their body and legs are equipped with hair that has hydrophobic substances. Those that swim underwater have, instead, special behaviors, specially modified organs, and other features.

During the insects’ flight, the whole wing can be leaning to constitute a single lifting table, but mostly just a part of the wing’s foil acts, with movements that can happen in three axes that cross in a right angle (horizontal movement, vertical movement, and rotational movement), which obviously have to be composed in pairs to obtain the lifting and propulsion effect. In general, the weight supported by their wings is much more inferior to the one supported by birds. In the smaller forms, the minimum wing surface has a biologically useful excess of the airfoil which is, sometimes, used (like in predatory Hymenoptera) for the transportation of the victims by flight.

Nervous system and sense organs

The nervous system consists of a central nervous system, a visceral system (or sympathetic) and a peripheral sense system. The central nervous system is made of a supraesophageal ganglionic mass (cerebrum), of a subesophageal ganglionic mass, united to the first mass through a peri-esophageal girdle, and of a ventral ganglionic chain, that runs through the thorax and the abdomen, and that should present 3 ganglia in the thorax and 11 ganglia in the abdomen, but that, actually, always has less (the brain is normally situated in the cephalic capsule and lies dorsally to the esophagus; it is divided into three parts that are deeply connected: the protocerebrum, that innervates eyes and ocelli and it’s the center of the vital activity, the deutocerebrum, that innervates the antennae and it is the center of olfactory perceptions, the tritocerebrum, that innervates the labrum and part of the front region of the intestine and it’s the center of taste perceptions.

The subesophageal ganglia innervate the mandibles, the maxillae, and the labium. The thoracic ganglia are the motor centers of wings and legs; the abdominal ganglia assure the regularity and the coordination of breathing movements; the last one of them controls the movements of the reproductive system. The visceral nervous system consists of the stomatogastric system (or supraintestinal sympathetic) which starts with a frontal ganglion, which is anterior to the brain and dorsal to the intestine, and that it’s made of nerves and ganglia situated dorsally and laterally to the intestine itself (it innervates part of the intestine, the ventricle, and other organs), and of the sub-intestinal sympathetic that originates from the subesophageal ganglionic mass and it is connected to the ventral ganglionic chain (it innervates stigma and tracheae). The peripheral sensorial system consists of a group of fibers and of multi-polar nervous cells, it is placed mainly under the basal membrane, of which elements are connected to the external world on one side and to the central nervous system on the other. 

Sense organs, sometimes scattered in the integument, sometimes grouped in specific areas, consist of very different structures. 
There are sensilla, trichois, basiconica, styloconica, placoids, celoconica, ampullacei, etc. Tactile sensilla (aphids) are often scattered on the whole surface of the integument but can be seen especially on the antennae, on the palps, on the legs, on the cerci, etc. Olfactory and taste sensilla are the same. The taste sense seems specifically confined in the palatial vault and on the mouthparts, but it can be also found elsewhere; the olfactory one (or at least the sense that we compare to the olfactory) seems to be concentrated mainly on the antennae and it is a sense of enormous importance for insects, because it constitutes the base of orientation of every one of their vital activity (the search for food, prey, reproduction, oviposition, etc.).

In some groups, it has reached an extraordinary finesse. The organs of sight are the ocelli and the eyes. Ocelli can be lateral in the Holometabola larvae and dorsal in the Heterometabola nymphs and in adults. Eyes consist of several (from one to tens of thousands) elements (ommatidia) and they allow to perceive a single and straight image of objects. The perception of colors is not the same in all of the insects, and, while certain species perceive some colors well, others are blind regarding them, some others seem to be able to even perceive the ultraviolet. Hearing sense organs can be simple, like the so-called chordotonal organs, while others are more complicated, like tympanic organs. They occupy various areas of the body, but they are not to be considered as real hearing organs, they are sensilla that are able to perceive the vibrations of solid bodies. There are other sense organs in insects but their functions, like the ones of many other sensilla, already named, are still unknown or less known. 

Organs that produce sounds and light

Many species of insects can produce high-intensity sounds of various nature, by banging their head or prothorax against external objects (Anobiidae, Termites, etc.) or by rubbing a part or an appendix of the body against another (Orthoptera, Coleoptera, Heteroptera, Hymenoptera, etc.) or by vibrating their wings and letting air through stigma, or, finally, through proper organs. Among these there’s the very well know device of Cicada, which consists of two big ventral cavities of the abdomen’s base, each divided by a vertical partition into two parts, on the smaller one (tympanic cavity) there’s a membrane (tympanum) that is stretched and vibrates under the influence of the contraction of specific muscles. In the European species, this organ is only present in the male.

Light in insects is produced by phosphorescent bacteria or by specific organs. True luminous species all belong to the Coleoptera and specifically to the Malacodermata group (local genii Lampyris, Luciola, Phosphaenus, Lamprohiza, Phausis, Photinus, etc.) and to the family of Elateridae (Purophorus, Photophorus). In these species luminescence is typical of all the development phases; it is widespread in the egg, but in the post-embryonic stages is much more localized. The luminous organs are fundamentally made of an external layer of cells grouped in lobules, under the cuticle and crossed variously by numerous tracheae (photogenic layer), and by an internal layer of adipose cells, full of urinal concretions (reflecting layer). The luminous phenomenon is determined by the action of two substances that react together: luciferin and luciferase, the second one works as an enzyme of the oxidase group. Insects’ luminescence has remarkable superior profitability compared to other known forms of light, this results from the minimum loss of energy for heating irradiation and from the small existing proportion of chemical reactions, caused by the presence of a specific fluorescent substance in the blood (pirophorin). Of all the luminous insects, south-American Pirophora are the most powerful ones; with 30-40 individuals you could light up a room with the same intensity of a candle. 

Digestive system

The insects’ intestine is typically represented by a tube of various calibers and of various lengths that run under the pulsing vase and over the ventral ganglionic chain; it can morphologically be divided into three parts: foregut, midgut, and hindgut; the first and the last, which are provided with a chitinous inside, are considered as ectodermal invaginations, respectively cephalic and anal; the midgut, which doesn’t have an inside, probably (the issue is still debated) derives from the endoderm. In the foregut, there are two expansions, which can be co-existing or not, of the hind portion of the esophagus: the ingluvius and the gizzard. The first, that in some orders is very voluminous and in others is shaped like a bag connected to the esophagus through a duct, works as a temporary tank for food and, in ants, as a “social stomach”, since the liquid that is stored in it is later regurgitated in favor of all the members of the anthill. The midgut (or stomach, or chyliferous ventricle) usually has a superior caliber compared to the front and hind portion; it’s often provided of diverticula that are placed on one or more levels (gastric cecum); it doesn’t have, as said, a chitinous inside, but it’s instead provided with a peritrophic membrane, derived from the activity of special cells or from the midgut epithelium itself and it works sometimes as a protective membrane of the epithelium and as a dialyzed diaphragm.

In most Apocrita Hymenoptera larvae, in Neuroptea Planipennia, in various Diptera, etc., the midgut is closed at the back and doesn’t communicate with the hindgut; Hemiptera Homoptera and especially the shorter Sternorrhyncha have very particular dispositions of the digestive tube and can equally have the midgut closed at the back. The hindgut sometimes has a very developed colon, that can also be provided with a blind pocket (colon’s cecum). The rect, which is generally short, usually has special folds and papillae (rectal papillae), which we don’t really know the function of. In the midgut there are the important phenomena of digestion and absorption; this last one can apparently happen in the hindgut, at least for some substances. Not a few insects have special diets that correspond to special digestive processes. Those that eat wood, wool, fur, wax, have enzymes that transform these substances into other digestible ones. Some host symbiotic organisms (Protozoa, Fungi, Bacteria) which contribute to use the substances ate by the guests; others (like Dysticus and Neuroptera Planipennia larvae, etc.) inject an intestinal liquid in their victim’s body, through the mandibles and thanks to anti-peristaltic contractions, which digests in situ the victim’s tissues. 

Respiratory system

In most insects breathing happens thanks to ectodermic tubules, tracheae, which have an inside (endotrachea), an epithelium, and proper membranes, which branch profoundly through the various organs until they divide into very small branches (tracheoles) and they can, rarely, constitute a system by themselves, i.e. they can anastomize each other and constitute complicated systems. Tracheae open outside through holes (stigma or tracheal spiracles) situated generally in the pleurons of thoracic and abdominal segments in various numbers. Japyda, among Apterigota, have 11 pairs, but no species has more than 10 (2 at the thorax and 8 at the abdomen). Other reductions are very frequent. Depending on the number of existing spiracles we have holopneustic forms (with 9-10 pairs of open spiracles), hemipneustic (with a lower number of stigma, from 1 to 8 pairs), apneustic (with all the spiracles closed or missing). Stigma have a chitinous cercin (peritrema) and sometimes special processes that protect the opening or real closing systems.

In many winged species, tracheae have stretchable expansions shaped as vesicles and often very evident, that are named aerial bags; they store air that can be used during flight and decrease the specific weight of the body. When, as it has been seen, there are no tracheal spiracles or tracheae are actually missing, the exchange of air happens through the thin integument (cutaneous respiration). Finally, in the pre-imaginal stages of many aquatic Insects, there are special protrusions containing tracheae (pseudo-gills or tracheal gills), with very thin walls and variously shaped, confined in various regions of the body, externally or inside the intestine walls (Odonata, Anisoptera), that can be coexisting or not with functional spiracles, that allow these insects to use the oxygen dissolved in water. True gills (Bloody gills) are rare and are found also in terrestrial forms. 

The physiology of breathing in insects it not really known well yet. The, almost general, lack of hemoglobin found in their blood has led to thinking that the circulating fluid has, in these Arthropoda, a secondary importance in respiration and that air runs directly through the tracheal system to bring oxygen to the tissues; but the existence of forms lacking in tracheal systems, the discovery of copper in the blood of many species (which leads to thinking that there is hemocyanin in some Arthropoda) and other facts lead to sustain that the blood of insects certainly has a function, and remarkably in specific cases, in breathing. Breathing movements, which are entirely reflexes, are thoracic in certain groups, abdominal in others, and they increase with the rising of environmental temperature. According to some authors, there is a more or less defined, circulation of air through the tracheal system, and there are functional stigma that work as inhalant openings and functional stigma that work as exhaling openings; according to others, this is not true.

Carbon dioxide is mainly eliminated through the integument. Among insects that live in water many (water-dwelling insects) have tracheal spiracles and breathe at the surface, others (aquatics) breathe through pseudo-gills, but with often different and complicated processes. Some species physically breathe on the surface and chemically breathe underwater, bringing air reserves under the helytre or elsewhere; in others (such as Helmida among Coleoptera) the thin layer of air that is spread along the body is continuously oxygenated for the Exchange of CO2 with oxygen dissolved in water; some others take advantage of the oxygen coming out of the under-water vegetables or of the one contained in plants, etc. Hymenoptera and Diptera larvae which are endophagous parasites of other insects use the oxygen that is in the blood of their host if they are apneustic; they find out how to put their tracheal spiracles, if they are provided with them, in contact with the outside or with the guests’ tracheae.

Circulatory system

Blood in insects flows only for a relatively short section, closed into a dorsal vase and in accessory vases, but in most of the body, it flows through the organs’ gaps, regulated at best, in its flow, by a system of diaphragms. So circulation results partially vascular and mainly lacunar; but while in vertebrates blood is pushed in vases by the contractions of an organ, the heart, that works as a pump and is generally placed under a positive pressure, in insects, once it’s been spilled from the aorta in the free spaces of the head, it flows slowly towards the abdomen under the aspiring action of the dorsal vase that is felt up to the head; tt regularly is under negative pressure.

The dorsal vase is a kind of tube that runs dorsally to the abdomen and the thorax and ends opening in the head; it can be divided into two parts: a posterior one (ventriculus), divided into ventriculitis and communicating with the pericardial sine, where it lies through ostioles, little side openings that open in couples in every ventriculitis; a front one (aorta) which is not divided into compartments and it doesn’t have ostioles. The accessory pulsing organs, that force the blood to follow certain routes, are apparently shaped like tubules or bags and they lie in various parts of the body; they act through suction like the ventriculus.

When diaphragms are completely developed, the cavity of the body remains divided into three parts from two septa: the dorsal diaphragm, which is stretched over the intestine and under the dorsal vase and which limits (ventrally) the dorsal or pericardial sine; the ventral diaphragm, which is stretched under the intestine and over the ganglionic ventral chain and which limits (dorsally) the ventral or perineural sine. Between these two diaphragms, there’s the visceral sine. Sometimes a diaphragm shows pulsations that contribute to making the blood circulate; sometimes that happens in the ventral diaphragm (Leydig chord); some other times a sine is provided with transverse septa, which have an opening provided with a sphincter, thanks to which it’s possible to stop circulation in a specific region of the body and to make the blood go elsewhere. The circulating fluid (hemolymph or blood) is a rather dense liquid, colorless or brightly colored with special pigments that combine with the proteins that are contained in it. In contact with dusky air for the oxidation of the albuminoids and for the precipitations of black-greenish granules of uranidine. This is composed of a fluid part, serum or plasma, and of various species of cellular elements (amebocytes). Overall, circulation appears to be very imperfect, and the temperature of the various parts of the body reveal differences which are often remarkable (even of 10°). In pre-chrysalises, in chrysalises and in adults Lepidoptera there is a periodic inversion of ventricular peristalsis; i.e. an alternation of the blood flow from the back to the front and from the front to the back.

Regarding animal heat, insects seem to be part of a special group, in between homeothermic and heterothermic. In most adults form temperature can increase very much, especially under the influence of solar light, on top of the external one; whereas in larvae, it is approximately the same, or slightly superior, to the environmental one. Everybody knows that, for example, in hives, it oscillates in every season between 32° and 33° and that, in winter, Hymenoptera inevitably have to produce a heat of around thirty degrees. Since during this season breathing exchanges in bees are more active, it is evident that they fight against the cold by increasing combustions. Some Insects (Coleoptera, Orthoptera, Hemiptera, Lepidoptera, Hymenoptera larvae, Symphyta, etc.) can produce, after breaking exsertile vesicles, or the integument in a less resistant point, or after a detachment of an articulation membrane or in other ways, a certain quantity of blood, that later gets mainly reabsorbed. This Phenomenon, that appears to be connected to glandular organs and with secretion, is known as auto-emhorrea. 

Excretory and secretory systems

The most important excretory organs of insects are nephrocytes and Malpighian vases. The main groups of nephrocytes are dorsal nephrocytes (or pericardial cells) and ventral nephrocytes. The pericardial cells are cellular elements of variable dimensions, with 1 or 2 nuclei, which can be isolated or stuck together to form a real pericardial tissue. They lie in the pericardial sine, at the sides of the dorsal vase, and can be considered as closed bloody glands, with a merocrine secretion, that are able to neutralize alkaline substances excessively contained in blood, to absorb some colloidal products from the blood, to transform in simpler elements and to throw them again in the blood at the state of crystalloid compounds, that are later eliminated through Malpighians vases. Ventral nephrocytes are not really widespread and usually (like in Diptera Larvae) they form chords of cells situated under the foregut.

Malpighian vases are considered as the real kidneys of insects, they only rarely miss (in Collembola, in some Thysanura, in Aphids), they are shaped as tubules closed at the free extremity, they can be simple or branched, they can have different lengths, and they open in a various number at the beginning of the hindgut (or at the end of the midgut), while remaining freely immersed in the blood of the visceral sine. Their walls are made of an external peritoneal tunic, of a proper membrane and of an epithelium of big cells. The catabolic substances stored in these are discharged in the central cavity and from here they go into the intestine, from where they are later eliminated together with feces. Calcium Carbonate, that is often situated in Malpighian vases, is used by some Insects (Cerambicida, Lariida) to pack or close the pupal chamber; in others (some Coleoptera, Neuroptera, Planipennia, etc.) the vases secrete silk, that is used to spin the cocoon in which pupation happens.

To the excretory organs mentioned before we must add oenocytes, urinal cells, labial glands, muting glands, and also the integument and the adipose tissue. In the Insects’ body, there are numerous glands that secrete substances that are destined to be used in several ways and that often free the blood of dissimilation products that are bad for the organism. There are unicellular ones and pluricellular ones (simple or composed), widespread ones and confined ones. Among the widespread integumental ones we will remind you wax-glands that are particularly developed in Hemiptera, Homoptera, and Apoidea Hymenoptera; lac-glands that are specific of some Coccidia; cutaneous silk-glands of Diaspina Coccidae; urticant glands are frequent in various larvae and especially in Lepidoptera ones, etc. Among the confined ones we have mandibular and maxillary glands; labial glands (properly named salivary), sometimes venomous ones and frequently transformed in silk-glands, thoracic glands; pygidial glands; venomous glands which are specific to the female sex of Hymenoptera, where they are connected to the gonpophysis (sting, terebra). The venomous system generally consists of an alkaline gland and an acid gland that are variably developed; the venom results from the union of two secretes; it is needed as a defense, as an offense and, in Terebrants, is injected with the eggs to determine various favorable effects for the larvae’s life.

Reproductive system

Insects are animals that typically have separated sexes, but there are cases of gynandromorphism and intersexuality. The reproductive system is fundamentally made of properly said sexual glands (ovaries in the female and testicles in the male), from equal gonoducts (respectively oviducts and deferents) and from an unequal gonoduct (respectively vagina and ejaculator). In some insects, Tisanura, Dermaptera, Hefemeropetra, the gonoducts can be more or less completely equal. The most primitive conditions regarding this are seen in Ephemeroptera, where the deferents of the male and the oviducts of the female each have a distinct opening to the outside. To these parts, we must add other accessory ones: spermatheca, copulative bag, seminal vesicles, ejaculating ampoule, annexed glands and, finally, the genital armor.

The reproductive system is generally situated in the abdomen, placed on an oblique plane, over the ventral ganglionic chain and at the sides of the digestive tube. Its opening is always ventral or anterior to the intestine’s one and it usually is the 8th or the 9th urosternite, or in the intersegmental membranes between the 8th and the 9th, i.e. between the 9th and the 10th urosternite. In the male reproductive organs the testicles are generally 2, the can have various shapes, they can be uni- or pluricellular, sometimes (like in Coleoptera Lamellicornia) they are divided into testicles, some other times they are covered by a communal membrane (scrotum). Deferents have different caliber and length and they have one or two seminal vesicles. The ejaculating canal expands sometimes near ita proximal extremity and forms the ejaculating ampoule. Accessory glands, that can be different in number, shape, and length, have a function that is still unknown. In some species, their secret is needed for the formation of spermatophores. The copulation organ (or aedeagus) is an evagination of the ventral face of the abdomen, reinforced by chitinous pieces and complicated in various ways, in which the distal portion of the ejaculating channel is included.

In female organs, the ovaries don’t always have a constant shape and they generally are made of a various numbers (from 2 to a few thousand) of ovarian tubes (ovarioles) of different types. Those in which the vitellogenous cells are missing are called panoistic and those that have vitellogenous cells are called meroistic, they are connected to the oocytes (polytrophic), or confined to the extremity of the ovariole (acrotrophic). Oviducts often widen in a chalice. The spermatheca (or spermathecae) that may or may not have a proper gland and communicates with the vagina through a cuniculus, is rarely missing and works as a deposit for the sperm, that can be stored even for years (Insects’ females living in persistent societies) in perfect conditions. The copulative bag is less common, it can reach big dimensions in some species and in superior Lepidoptera, it opens at the 8th urosternite with a proper opening (ostium bursae), that works as a copulative ostium. Accessory glands, which are also called collateral, are very common and they reverse their secrete in the vagina; they provide the necessary material to build the ootecha, and they wrap and fix the eggs, etc. Venomous ones belonging to Hymenoptera, of which we have talked about, belong to this type. The genital armor, made by gonapophysis, constitutes systems of various shape, and with different and often complicated developments (ovipositor, terebrum, etc.), that work in relation to the deposition of eggs.

Reproduction and prolification 

Usually insects reproduce in an anphygonic way; but parthenogenesis alternated with anphygony is very common among them. Other forms, like pedogenesis and polyembryony, are rarer. In anphygonic reproduction the male, which is more agile and active, generally moves to look for the puber female, which it usually senses at a great distance. Often males are less numerous than females and have to, sometimes, fertilize more than one; and, in various circumstances, they are bound to have one coupling. Preliminaries before the wedding and the pre-copule vary a lot in the different groups; and that’s the same for the properly named copule, that can happen in a state of quiet for the two individuals, i.e. during the flight (nuptial flight). The position of sexes during coupling is very different and there are strange and paradoxical cases; the length of the copule can be short or very long and it can last even for days; in some species, it can happen when the female has carried the germs to full maturity, in others, it can happen before. There are usually cases of neoteny, while progeny is less frequent.

Parthenogenic reproduction can happen in many forms like a sporadic and accidental phenomenon (accidental parthenogenesis), in other cases is constant (facultative or haploid parthenogenesis and thelytoky parthenogenesis), but mostly alternates regularly or irregularly, with anphigony in heterogenic cycles (regular cyclic parthenogenesis and irregular). Geographical Parthenogenesis (or polyploid) is not rare.

Pedogenic reproduction was discovered in insects (in Diptera Nematocera of the Miastor genus, and precisely, in M. metrologa Mein) and was recorded then in other species of the genus and in other genii of the same Cecidomyiidae family. This form of reproduction strictly connects to the cyclic parthenogenesis and it’s not different from it except for the fact that it happens in individuals at their larval stage; so it can be considered as a parthenogenic progeny. In Aphids too, there’s a kind of pedogenesis when the development of eggs starts in the embryons; this was named endo-pedogenesis. Pupal pedogenesis which is found in Diptera Chironomida, instead, represents just an early parthenogenesis, that generally happens in the adult but it can happen in the pupa. Polyembriony is known until today as peculiar to Microhymenoptera (Chalcididae, Proctotrupoidea, Braconidae, and Bethylidae) and of Strepsiptera. From one single egg, there can be more than a thousand individuals that, however, have all the same sex. In some species from a single germ, there are two types of larvae, some of which have reproductive organs and generally evolve into adults, others (asexual larvae) don’t have said organs and they die without developing any further.

Most insects are oviparous, so they lay eggs from which, after different periods of incubation, larvae or nymph shell; ovoviviparous and viviparous forms are not rare. Fertility varies in prodigious ways: from species which are only able to lay a few germs, we move to the ones which emit thousands, hundreds of thousands, and even millions. Eggs look very different regarding dimensions and shape and they usually belong to the centrolecithal type; they have two casings: a vitellous membrane and a corion (this last one is provided with one or more micropyles), they have a bilateral symmetry and, in the ovary, they show same orientation of the one of the maternal organism (Hallez law). At the moment of birth of the larva or the nymph the corion breaks with no constant modality, i.e. it cracks along the lines that are less resistant. Eggs are usually laid in isolation, but when the environment allows the nutrition of a lot of larvae, they are emitted in any group, or with a specific and characteristic look (boat-shaped, ribbon-type, ring-type, etc.). Some females not only reunite the eggs, but they enclose them in special cases, that are named ootecha and that can be made with external material stuck together and cemented through the secretion of the accessory glands of the reproductive system (Locustoidea’s ootecha, etc.), or just made by the secretion of named glands (Blapta Mantis’ ootecha, etc.). Many insects protect their germs by simply saddling them to the support; but there are others (Lepidoptera from the Perthetria and Nygmia genii, for example) that cover them with hair fallen from their abdomen, or with silk wires (Corrodenta, Embioptera) or wax wires (Coccidea, Pseudococcidae), or with special ovibags (Coccidea Monophlebidea), or even with their own body (Coccidea Lecaniina), etc. There are many places where insects can lay eggs: water, decomposed organic substances, alive or dead plants, victims from every species, homeothermic vertebrates’ hair or skin, soil, waste, manure, many foods, heterogenous artifacts, processed wood, etc., the female of a north-American Hemyptera from the Serphus genus and the one from the European genus of the same order (Phyllomorpha), even entrust their germs to the male, that keeps them on its back until the hatching of the nymphs. To reach a suitable environment, that often requires remarkable and prolonged efforts, the mother uses various organs (gonapophysis, mouthparts, legs, etc.). In general, we can say that, except for some, the eggs are laid in a place that is near to the area where the young organisms can find sufficient living conditions. 

Nidification, progeny supply, hunt and capture of the prey

Insects’ nests are shelters excavated or built for the protection of species during their development; they can be built by the mother, or sterile females (Social Insects) that build them for the unborns, i.e. the larvae that put them together for their advantage. Flying over the numerous types of more simple nests, we can mention pedotrophic ones, in which Insects’ larvae, to which mothers provided necessary food, develop and live until the eclosion. One of the less complicated behaviors is the one of Coleoptera Silphidae from the Necrophorus genus (commonly known as Beetle), that bury the carcasses of the small Vertebrates in situ and they entrust them with their germs. The modalities used by Coprofagous Coleoptera Lamellicornia are much more specialized, they dig one or more galleries or chambres and in these they deposit, along with the egg, parts of manure previously chosen, manipulated and sometimes modeled in special forms. Solitary Mellifera Hymenoptera take advantage of holes, of cavities, and of pre-existing galleries, or they expressively make them themselves in branches, stems, logs of dead or emaciated plants, in wood, in soil, where they often build cells of various shapes, using various materials: resin, vegetable hair, soil, plant foliage, flower petals, etc.

Some, like Chalicodoma, build very strong nests with mixed soil, and it is difficult to corrode them or break them, even with a strong knife. The cells are provided with honey and pollen and the egg is deposited on the mass. Predator Hymenoptera (Sphegida, Psammocaridae, Vespoidea) dig galleries and dens in the soil (or more rarely in wood) and they drag or transport the prey that they have captured and paralyzed there by flight for the breeding of larvae. Some (like Sphegida from the Sceliphron genus, Psammocarida from the Pseudagenia genus, various Eumenida, etc.) The mother, generally, sticks an egg to the victim’s body or to one of them and closes the nest, abandoning the larva and the supplies to themselves, but there are Sphegida (Bembex, Monedula, some Stizus, etc.) that supply the progeny for the day, bringing them the necessary food from time to time.

In one way or another, the Hymenoptera larvae eat the alive, but still, body of the sacrificed. Finally, in social insects (Termitidae, Apidae, Vespidae, Formacida) they reach their highest architectonic ability and reveal the most extraordinary (instinctive) phenomena in the factory of their houses and in the breeding of larvae. Just remember the nests of some Termites, like the ones of Eutermes pyriformis from Australia that are very solid and can be up to 6 meters high and 1 meter large, or the ones of Apicotermes Kisautuensis from Congo, that are engineering masterpieces, crossed by a series of canaliculi on their outskirts; the combs of domestic bees which have cells, that are hexagonal prisms, that are regular and contiguous, closed proximally by a pyramidal surface with three rhomboidal faces; the nests of oriental Ants from the Oecophylla and Polurhachis genii made only by silk or by leaves and silk, that is emitted from the larvae, that the ants themselves handle as work instruments.

Post-embryonic development

Post-embryonic developments starts with the outlet of the larva or the nymph from the egg and, with more or less visible metamorphosis, it ends when the insect reaches the adult stage, i.e. the somatic and germinal maturity. The young organisms shells from the egg by breaking its corion with their mouthparts, so by moving their body, but there are some which have special processes, that determine or help to determine this break. The mute phenomenon was considered to be dependent only by the growing needs of the insect; modern researches demonstrated that is not fatally connected to growth but is related to the necessity of facilitating the internal transformations and the formation of new organs and to free the organism from catabolic products stored in a very active phase of nutrition. 

The more or less remarkable transformations that insects have to go through in their post-embryonic life are called metamorphosis. Some insects shell out of the egg in apparently the same way (except for the height) as adults, which are primitively wingless. So they don’t have metamorphosis, in the strict sense of the word, and they are called ametabola (Protura, Collembola, Thysanura). There are others which, although they are part of the Pterigota subclass, have lost their wings too, together with other features, in their adult stage, for various reasons. These, in which newborns are similar to the imago, also miss metamorphosis, even though it’s acquired and not integral; they are called pseduo-metabola (Mallophoga, Anoplura and many other wingless species from other orders).

The third group of Insects includes those which are born in a shape that is similar but not equal to the adult, from which they are different mainly for the lack of wings. Their metamorphoses are modest and gradual. They are called heterometabola and they include 14 orders (Blaptodea, Orthoptera, Phasmatodea, Mantodea, Isoptera, Embioptera, Dermaptera, Psocoptera, Zoraptera, Plecoptera, Ephemeroptera, Odonata, Thysanoptera, and Hemiptera, except for the species included in the previous category). Among these there are two subgroups: paurometabola, in which young organisms and adults live in the same environment and hemimetabola, in which the young organisms (aquatic or hypogeic) and in adults (terrestrial or epigeic) attend different environments. The fourth and last group is represented by insects of which the young organisms (real larvae) are very different from the adults and in which the metamorphosis is radical. They are called holometabola and include 9 orders: Neuroptera, Mecoptera, Trichoptera, Lepidoptera, Coleoptera, Strepsiptera, Hymenoptera, Diptera and Aphaniptera. But in n holometabola there are some species that have a superior number of post-embryonic stages compared to the normal one (some endophagous Hymenoptera, various Coleoptera, some Neuroptera, Diptera, etc.) and that are named hypermetabola. They owe this behavior either to an early hatching from the egg (progenesis) or to secondary modifications of the locomotory organs and sometimes of the buccal ones, connected to special modalities of their biological cycle. 

In ametabola, pseudo-metabola and heterometabola insects the newborn is not a properly named larva, but an insect that is more or less similar to the adult, that can be called nymph and that will gradually collect, during its growth, the features of the perfect insect and the germinal maturity. On the other hand, in holometabola the larva eats intensively and grows until it reaches a condition of (larval) maturity and then is transformed in pupa. This, usually, doesn’t move, doesn’t take food, and it represents a transitional stage, during which the organs and the tissues of the larva, that are profoundly transformed, reorganize and form the ones of the imago, that will fly off in due course from the pupa itself. The complex of these hystolitical and hystogenetical phenomena is called nymphosis. On its determinism and on the one of metamorphosis, in general, there are many theories that have been proposed (fagocitary, asfictic, genital crisis, physiological arrest, autophagia, undifferentiated cells’ activity, etc.) but the problem has not been solved. For studying purposes we can distinguish three main types of holometabolous larvae: the cyclopiform type, the eruciform type, and campodeiform type and lots of secondary types. The larval period of holometabolous insects it’s the full period of nutrition. The quantity of food that the larvae, mainly phytophagous ones, can eat is enormous; some, at the end of the growth, increase their weight of thousands of times; others can increase their weight of 50-100 times after just one meal. The variety of the substances that constitute suitable nutrition for the different species is extraordinary.

In addition to the immense phalanx of the phytophagous forms that can eat any part of any plant, to the carnivorous, not rarely vigorous predators (Dytiscus larvae attack and devour even small aquatic Vertebrates), there are some that eat the weirdest foods (tobacco, leather, paper, wool, wax, humus, manure, etc.) and that affect the most resistant substances. Insects’ larvae are generally naked, but there are some (Chrysopa) that carry the remains of the Aphids that they have eaten; others (Cassida, Mycetophila, etc.) put together a sort of dorsal pad formed by their droppings; others (Cionus) secrete a glutinous liquid from the anus that forms a protective shell around the body; others build themselves pockets of objects or various substances (little shells, pebbles, twigs, leaves, etc.), that they connect with silk secreted from them (Lepidoptera, Trichoptera, Coleoptera, etc.) other larvae build themselves, more or less solid shelters, i.e. true nests, isolated or in company. The life span of the larva is much shorter the more substantial and abundant and easily reachable is the food. Some Diptera larvae eat decomposed animal substances, they go across the whole development phase in less than a week; others, Coleoptera larvae, live in wood, which is sometimes used for many years before reaching maturity. This rule can have several exceptions too. 

Three main types of pupae are known: pupa exarata or free, pupa obtecta or covered and pupa coarctata; this last one belongs to Diptera Cyclorrhapha. Pupae can be naked or anoic, suspended, succinct, partly protected (emioic), protected in a coherent and defined way by casings, pockets, silk cocoons, made of several substances cemented together, by intestinal secrete (evoic). In many insects, the metamorphosis has a regressive character because it leads to constitute adults with an involute organization. 

Life and behavior of adult insects

The complex of actions with which the imago (the perfect insect) comes out of the nymphal or pupal casing is named eclosion; it fundamentally consists in the break of said casing and, if it exists, of the cocoon in holometabola; in a much more simple procedure, so to say, generally, in a kind of last mute in ametabola and in heterometabola. The way through the cocoon is opened by the mandibles or other organs, i.e. with the help of special formations; it’s sometimes facilitated by the emission of special secretions that tender and macerate the tissue. In Odonata and in Orthoptera there’s the phenomenon of air swallowing that precedes the eclosion; after that, the intestine and the tracheal system occupy more space in the body cavity and compress the blood, that acts on the integument as a hydraulic press, tending to make it jump and ending up breaking it on the thorax’s and the head’s back or somewhere else. The insect that had just eclosed is, often, skimpy, with curled up wings, “turbid and stunned”, but in more or less time light, heat, circulation of blood, air ventilation, etc., lead it to its normal shape.

The two adults’ sexes of insects rarely look the same on the outside (sexual homeomorphism), more often they are different (heteromorphism) and they differ for a series of features that are usually visible, that can concern height (dimegetism), color (dichroism), shape (dimorphism) or even diet (dysphagia). Individual polymorphism cases are common (seasonal polimorphism, unisexual polymorphism), rarer are the cases of collective polymorphism (non-social polyphilia of Aphids and other Insects and social polymorphism of the forms living in societies).

Various species apparently imitate the behaviors of others or they look like them in their shape and color (phaneritic mimetism), and others behave similarly regarding any body or substance from the surrounding environment (cryptic mimetism); finally, in specific circumstances, some assume specific behaviors that, to us, are incredible (terrific behaviors), or, if they are teased or exited in any way, they eventually let themselves fall from the above, for example., from a plant, after retracting the legs and the antennae (thanatosis or death simulation). All of these behaviors, and many others, are generally interpreted as means of protection and defense of the individual, but their meaning is probably different; in fact, insects provided with mimetic clothes can hide from men or from big animals that certainly, in most cases, don’t represent forms that are able to affect the existence of the species in a significant way for them, but they don’t hide away from their true enemies, parasites, and predators, that can certainly find them and always with the help of smell.

Insects’ vitality, if compared with the ones of superior animals, seems very great. Injuries, wounds, mutilations and other facts, that lead them more or less quickly to death, find them resisting even for a long time or even completely indifferent. However, some substances that are fairly dangerous for us or even harmless (pyrethrum, hydrocarbures, etc.) prove to be fatal for them. The amputation of limbs, the perforation from side to side of the body, the greatest injuries, even decapitation are either bared with great resistance or, at least, don’t lead the insect to death in a short period of time. There are insects who were seen living for months and months after the complete amputation of all appendages and for several weeks with their body crossed by a pin; there are others that have kept active movements of the thorax after 10-90 days from their decapitation, and others which underwent reproduction one or two hours after the decapitation. 
Regarding chemical agents, the remarkable resistance of insects is due to the more or less water-resistance of the integument and to the ready and precise closeness of the tracheal spiracles. Naturally, the liquids that are not able to affect the cuticle and the eventual special secretions that cover the body and that are not equipped with high capillary penetration are, usually, less deadly compared to gas. Insects have a great resistance even towards physical agents. Some species bare -40°, others live in thermal waters at +46°.

Insects’ longevity is various. But we need to distinguish the pre-imaginal life span to the adult one. Sometimes the first phase is longer than the second; sometimes the second is longer than the first one; in some cases with enormous differences in time. Everybody knows the ephemeris that lives for a few hours as imago and that spends some years as a nymph in the waters of ponds and streams. There are some Buprestidae, some cicada and other species that spend tenths of years as larvae or nymphs and a few years as adults. Vice versa various flies, bees, ants, etc., have an imaginal life that is longer than the pre-imaginal phase. Regarding adults, considered by themselves, it seems that Coleoptera and particularly those who belong to the Tenebrionidae, Carabidae, Coprophaga, and Cerambycidae families are among the most long-lived; some of them are kept alive for more than 9 years. 

The conquest of new environments to be exploited is made generally by insects with active means. Exceptions are those (parasite) forms that are naturally spread from the host that dwells them and those which are spread passively by the wind, by water or in other ways. 
In fact, many of the species that are more dangerous and sadly known for their misdeeds were transported from region to region and from continent to continent from men, with their traffics and their trades. Migration doesn’t seem rare in insects and is determined sometimes by exhaustion, regarding the food of the environment inhabited until then; sometimes it is determined by other factors, and most of them are unknown. In general, migrant individuals undertake their journeys in big groups, but it’s not only, as it would seem, good flyers and winged ones that engage in this activity.

The hordes of locusts can reach an unheard power; they have been seen in clouds extended for many miles moving through vast regions at a height of several kilometers and at a speed of 5 miles an hour. Regardless of the migrations, there are species that reunite in hundreds and thousands of specimens in specific places and epochs. Some of these agglomerations are presumably due to the necessity to defend themselves from adverse environment conditions; other reunions still don’t’ have a sufficient explanation. 

The gregarious and social instinct

More or less defined gregarious aptitudes, i.e. the tendencies to form, individualistic colonies, in a more or less temporary way, are found in many species of Insects; some reunite, in fact, in numbers to spend the night, i.e. to build nests, but each of the components lives a separate life without worrying or being interested in their companions. Properly said societies are, instead, families, i.e. associations among relatives, generally made by the mother and its descendency. In some forms there already are, in various grades, some familiar behaviors, also called maternal or marital individualistic societies depending on the cases.

Among the firsts we can remember the ones of bedbugs (Elamostethus grisou L., Cantas ocellatus Thunb., Schirus sexmaculatus Rbr.) of Grillotalpidae, of Forficula, of Embia, in which the mother cares for the eggs and refrains sometimes with the young organism; among the seconds, the ones of Passalida, Silvanida Coccidotrophus socialis, and Eunausibius Wheeleri, of Scolitidi lignicoli of Xyleborus, Platypus, Pterocylon, Xyloterus, etc., that have social features that are stronger. Today, we don’t know less of thirty families belonging to eight orders (Blaptodea, Orthoptera, Dermaptera, Embioptera, Psocoptera, Isoptera, Coleoptera and Hymenoptera) that include subsocial and social species. Of these, 2 orders (various families of Hymenoptera and all of the Isoptera) present definitely social forms. Societies can be annual like in local Vespas from the Polistes and Vespa genii and in Apida from the Bombus genus, and persistent like in bees, in ants and in termites.

Social life has brought modifications in the components of societies and it also determined a series of other facts, of which the main ones are: development of castes, and polymorphism; increased fertility (usually much higher) of the fertile female; extension, often remarkable, of the life of adults (particularly in females); development of mutual stimuli transmission means, of distribution and nourishments and special secretions (ecotrophobiosis or trophallaxis); collection and provision of food reserves made in common; enlargement and perfectionism of nests, complication of costumes and of behaviors that become cumulative. The development of castes consists of the differentiation of the females (or females and males) in two forms: one is fertile: queen (or queen and king); one is sterile: neutral. These can further differentiate in workers and soldiers, and in workers from two or three categories.

Neutrals from Aculeata Hymenoptera are infertile females, smaller than the proliferating mother, winged in Vespas, in bumblebees and in bees, wingless in ants. They owe their state to the influence of specific nutritional conditions sustained in the course of their post-embryonic development (food or physiological castration), but there are others that, in specific periods, can lay eggs in a parthenogenetic way, and from them, only males are born (in the Cape honey bee, Apis mellifica caffra or mellifica unicolor, intermissa or tormentosa, the fertile female workers are not only really common, but they are able to generate female workers and even queens). It seems that female workers are born, generally, with ovaries and eggs able to develop and regress according to the living conditions that will be possible for them depending on the collectivity.

Generally, the eggs degenerate and are reabsorbed, but when the number of female workers that work in societies is higher than the one of the nutritional ones that are necessary for the ongoing breedings, the collection of food is more simple and the quality is better, at that point, the laying workers appear. Even in experiments, if a queen is eliminated during the full flowering, many workers become fertile because along with the queen the breeding of the progeny is gradually suppressed and the young attendants that only take care of this would keep the food destined to the larvae for themselves. From this comes the term nutricial castration (from the word nutrix) which is given to the supposed determinism of this definitive state of sterility of the workers. Termites’ communities are different from the ones of the Aculeata for their bisexual composition because the different morphological and biological specializations are distributed in both sexes (queen and king, female and male workers, female and male soldiers). For this reason, they are called conjugal societies and not maternal like the ones of Aculeata.

Social insects, since they have created specific environments around them, have found themselves exposed to the invasion of a crowd of intruders, associated, and parasites, that almost all belong to Arthropoda and mostly to insects. Comprehensively those which attend the nests of ants are called myrmecophila, the residents and the associates of termites are called termitophila, the inhabitants of the nests of Vespoidea are called sphecophila, and the ones of the Apoidea nests are called melittophila. Then, according to the different grades of relations among them and the host, they are divided into various categories: sinectri (predators), sineceti (commensals), trophobiont (special liquid excretors), Symphyla (true associates), parasites, etc.

Some Symphylas are able to determine extraordinary facts among ants societies. In fact, they not only nourish them, but they take charge of the caring of their progeny and sometimes push the aberration so much that they neglect the nutrition and the breeding of their larvae. This leads to incomplete development of the individuals that should become queens that, instead, seem abnormal, sterile, beings, like the female workers, and they are not able to work as the queen: they are called pseudogyna. This, for the anthill, is a ruin; the ants sacrifice their race for the ethereal fluid emitted from those sinophile and sinophilism, at this point, becomes a social disease like human alcoholism that leads to degeneration (ants).

Useful, indifferent and dangerous insects

Regarding men, animals, and cultivated plants there are useful, indifferent and dangerous insects. The useful ones can be that way in a direct or indirect way. The directly useful species, which are few in numbers, include those who provide us with refined substances (silk, wax, honey, lacquer, etc.) and those which, killed and mounted, are needed as ornaments or as jewels. Of the indirect useful species, some are like that because they free the soil from decomposing substances or from excrements that usually facilitate the multiplication of other pernicious insects, like flies; others are useful because they determine the crossed fertilization of several plants; others, which are very numerous, because they are predators or parasites of dangerous insects. The indifferent species are legions.

The dangerous ones are thousands and they cause many diseases for men and domestic animals or they compromise plants, furnishings, artifacts, our reserves in various ways. Most of the first one live temporarily or permanently on man’s and other Vertebrates’ bodies; they just nibble the necrotic portions of the epidermis and horns productions (Mallofaga); or they lay their own eggs on the host, from which there’s the shelling of larvae, that, in different ways, will invade the skin or penetrate the cavity, causing superficial, dermic or cavitary myiasis (various Diptera); or, finally, they penetrate the skin with their buccal apparatus and they suck the blood (Anoplura, Aphaniptera, Diptera and hematophagous Emiptera). The last ones are the most dangerous because they can transmit the germs of very serious diseases, like carbuncle, typhus, bubonic plague, yellow fever, Malta fever, sleep disease, and other trypanosomiasis, malaria, babesiosis, spirochetosis, heartworm, etc.

Then, other species, like domestic flies, don’t bite but attend infected places and substances and they are equally able to cause incalculable damage by transporting germs of diseases (dysentery, cholera, typhus, ophthalmia, etc.) and by spreading them everywhere. The second category of dangerous Insects includes the ones that damage plants, furnishings, artifacts, food, etc, that only in Italy destroy products for the value of one or two billion lire every year. There are species that eat leaves, flowers, gems, fruits, roots, stems, etc., species that dig galleries inside vegetables affecting this or that tissue, this or that organ; species that bite plants with the bristles of their mouth, that inject saliva that contains toxic substances; enzymes that are able to determine hydrolysis processes of polysaccharides, hemicellulose, pentosans, etc.; species that determine neoformations, abnormalities, alterations. Omnivorous insects and the ones with special diets are responsible for misdeeds on many products: tissues, fur, leather, paper, tobacco, spices, food, processed wood; even on the lead of telephonic cables and on the coatings of the chambers of fabrication of sulphuric acid.

Insects’ enemies

Among the enemies of insects, besides environmental physical factors (especially temperature and humidity that have a great influence on their life) there are Microba, Fungi, carnivorous Plants (Droseracea, Lentibulariacea, Nepentaceam Orchidaceae), Protozoa, Nemathelminthes, other Insects, various Arthropoda and a remarkable number of Vertebrates. 

Microbial flora is certainly not inferior for richness and variety to the one of the Vertebrates, but the number of entities well known is very limited. Insects are equipped with a natural immunity that is remarkable compared to most part of pathogenic microbes for Vertebrates, however, they succumb to the actions of others, that are harmless for Vertebrates; often a microorganism that causes a mortal disease in a specific species doesn’t find suitable soil in related and near species. However, it seems that insects, in general, are much more resistant to soluble bacterial toxins studied until now, compared to homeothermic animals. The ordinary form of infection in them is the septicemic one and, contrary to what happens to homeothermic Vertebrates, the influence of temperature on its progression is often very remarkable. There are many Fungi that develop in the body of insects and that produce sometimes real massacres, which become immense benefits for men when the species exterminated species is dangerous. Just remember the genii Empusa, Botrytis, Verticillium, Sporotrichum, etc. 

The insects that kill and thus broom out other Insects, either to feed themselves or to breed the progeny, are used to reunite in the two categories of predators and parasites. Predators hunt their victims and, once they reach them, they devour them. There are some that present those habits at the larval stage and at the imago stage (Cicindelida, Carabida, Ditiscida, Stafilinida, Hysterida, Coccinellidae among Coleoptera, etc.); others that behave like that only during the larval stage (Sirfida among Diptera, Crisopida among Neuroptera, etc.). Parasites (or parasitoids) look for their victim, they paralyze it and they entrust their germs of their descendency. The larvae that shell devour the host. This is the behavior of Hymenoptera of the families of Calcidida, Proctotrupida, Braconida, Icneumonida, Evaniida, Stefanida, Trigonalida, some Cinapida, some Siricida, etc.; Tachnida among Diptera and some other form. Similar behaviors to the ones described above are common to Hymenoptera Aculeata named Vespiforma (fossors or predators) of the Sfegida, Psammocarida, Scoliida, Sapigida, Mutillidae, Betilida, Crisidida families, most of which transport the paralyzed prey in previously prepared dens. Lastly, a particular class is the class of intraspecific parasites, constituted by solitary parasites Apida and Vespidae that are thought to be derived directly from the host species. Among parasites we know many biological behaviors that characterize specific groups, genii or species.

Ectophagous parasites, for example, are the ones of which larvae devour or suck the victim while lying on its body, endophagous are the ones that consume the tissues and the organs of the host from the inside; monophagous parasites constantly look for only one species of prey, oliphagous few species, poliphagous attack species of different genii, families, or even different orders indifferently; solitary parasites devour on or more victims while isolated, gregarious assault one victim while reunited in various number; in synchronous parasite, the biological cycle and the generations coincide with the ones of the host, in asynchronous (or temporary) that doesn’t happen; they multiply often at the expense of various prey (co-victims); primary parasites are the ones that destroy phytophagous host, hyperparasites (also called epiparasites by some) the ones that live at the expense of other parasites of different grade, co-parasites are the ones who look for the same victim together.

Parasites and predators, by eliminating endless quantities of dangerous insects, are very useful for us; hyperparasites, on the other hand, are unfavorable for us. Among non-Insects Arthropoda, many contribute remarkably to reduce the number of those. We will limit to remind Arachnida, Miryapoda of Scolopendra and Scutigera, and various Acari genii, among which it is important to remember the Eterostigmato Pediculoides ventricosus (Newp.) Finally, not few Vertebrates: some Fishes, insectivorous Mammals, Chiroptera (more precisely Microchiroptera), some Gnawings, Toothless (where they exist), Amphibians (especially Anura), Reptiles (particularly Saura) and, in a special way, birds devour many Insects and can have sensible importance for men.

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