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Insects also are the most highly developed class of invertebrate animals, with the exception of some mollusks. Insects such as the bees, ants, and termites have elaborate social structures in which the various forms of activity necessary for the feeding, shelter, and reproduction of the colony are divided among individuals especially adapted for the various activities. Also, most insects achieve maturity by metamorphosis rather than by direct growth. In most species, the individual passes through at least two distinct and dissimilar stages before reaching its adult form.
In their living and feeding habits, the insects exhibit extreme variations. Nowhere is this more apparent than in the life cycle of various species. Thus the so-called 17-year locust matures over a period of 13 to 17 years. The ordinary house fly can reach maturity in about ten days, and certain parasitic wasps reach their mature form seven days after the eggs have been laid. In general the insects are very precisely adapted to the environments in which they live, and many species depend on a single variety of plant, usually feeding on one specific portion of the plant such as the leaves, stem, flowers, or roots. The relationship between insect and plant is frequently a necessary one for the growth and reproduction of the plant, as with plants that depend on insects for pollination. A number of insect species do not feed on living plants but act as scavengers. Some of these species live on decaying vegetable matter and others on dung or the carcasses of animals. The activities of the scavenger insects hasten the decomposition of all kinds of dead organic material.
Certain insects also exhibit predation or parasitism, either feeding on other insects or existing on or within the bodies of insect or other animal hosts. Parasitic insects are sometimes parasitic upon parasitic insects, a phenomenon known as hyperparasitism. In a few instances an insect may be parasitic upon a secondary parasite. A few species of insects, although not strictly parasitic, live at the expense of other insects, with whom they associate closely. An example of this form of relationship is that of the wax moth, which lives in the hives of bees and feeds on the comb that the bees produce. Sometimes the relation between two species is symbiotic. Thus ant colonies provide food for certain beetles that live with them, and in return the ants consume fluids that have been secreted by the beetles.
One of the most interesting forms of insect behavior is exhibited by the social insects, which, unlike the majority of insect species, live in organized groups. The social insects include about 800 species of wasps, 500 species of bees, and the ants and termites. Characteristically an insect society is formed of a parent or parents and a large number of offspring. The individual members of the society are divided into groups, each having a specialized function and often exhibiting markedly different bodily structures. For discussion of the organization of typical insect societies, see articles on the insects mentioned above.
Although the superficial appearance of insects is extremely varied, certain characteristics of their anatomy are common to the entire class. All mature insects have bodies composed of three parts: head, thorax, and abdomen (the abdomen and thorax are not always differentiated in larvae). Each of these parts is composed of a number of segments. The head is made up of several segments, usually so fused they are scarcely differentiated. On the head are two antennae; a pair of jaws, or mandibles; a pair of auxiliary jaws, or maxillae, that in turn bears a pair of palps; and a fused second pair of accessory jaws, the labium, that also bears a pair of palps. The antennae, usually attached to the anterior part of the head, are segmented. In some insects, the antennae carry organs of smell as well as organs of touch. The mandibles are large, heavy jaws on either side of the mouth. They close horizontally and are used for grasping food and crushing it. The maxillae, or inner jaws, are lighter in structure. The mouths of many insects are adapted for piercing and sucking rather than for biting. The eyes of the insect are also situated on the head.
All insects have three pairs of legs, each pair growing from a different part of the thorax, called, from front to back, the prothorax, the mesothorax, and the metathorax. Many larvae have, in addition, several pairs of leglike appendages called struts, or prolegs. The forms of the legs vary, depending on their uses, but all insect legs are made up of five parts. In winged insects, the wings, usually four in number, grow from the thorax between the mesothorax and the metathorax. The upper and lower membranes of the wings cover a network of sclerotized tubes, called veins, that stiffen the wing. The pattern of veins of the wings is characteristic of most insect species and is extensively used by entomologists as a basis for classification.
Insect abdomens usually have 10 or 11 clearly defined segments. In all cases the anal opening is located on the last segment; in some species, such as the mayflies, a pair of feelers, called cerci, is also present on this segment. The abdomen is devoid of legs. In female insects, it contains the egg-laying organ, or ovipositor, which may be modified into a sting, saw, or drill for depositing the eggs in the bodies of plants or animals. Insect sexual organs arise from the eighth and ninth segments of the abdomen.
Insects have an external rather than an internal skeleton; this exoskeleton is a rough integument formed by the hardening of the outer layer of the body through impregnation with pigments and polymerization of proteins, a process known as sclerotization. The exoskeleton at the joints does not become sclerotized and therefore remains flexible.
Most insects possess wings during at least part of their life cycles. Insect wings are large folds in the exoskeleton composed of two sheets of cuticle permeated with stiff supportive veins. The wings are powered by two sets of muscles that independently drive the upstroke and downstroke of the wing movement. The frequency of wingbeats ranges from 4 beats per second in butterflies to nearly 1000 beats per second in some gnats.
Insect wings not only move up and down but they also move forward and backward in an ellipse or figure eight pattern that provides both lift and thrust. Given their shape, speed, and stroke pattern, it has never been clearly understood how insect wings can generate enough lift to sustain flight. Recently scientists discovered that insects generate a vortex, or spiral air motion, along the leading edge of their wings. This vortex flows out toward the wing tip in widening spirals. The whirling cylinder of air above the insect provides the extra lift that makes flight possible.
Certain species of insects breathe through the body wall, by diffusion, but in general the respiratory system of members of this class consists of a network of tubes, or tracheae, that carry air throughout the body to smaller tubelets or tracheoles with which all the organs of the body are supplied. In the tracheoles the oxygen from the air diffuses into the bloodstream, and carbon dioxide from the blood diffuses into the air. The exterior openings of the tracheae are called spiracles. The spiracles are situated on the sides of the insect and are usually 20 in number (10 pairs), 4 on the thorax and 16 on the abdomen. Some water-breathing insects have gill-like structures.
The circulatory system of insects is simple. The entire body cavity is filled with blood that is kept in circulation by means of a simple heart. This heart is a tube, open at both ends, that runs the entire length of the body under the exoskeleton along the back of the insect. The walls of the heart can contract to force the blood forward through the heart and out into the body cavity.
The digestive tract of most insects is divided into the foregut, the midgut (or stomach), and the hindgut. In the foregut, a food passage, or gullet, from the mouth is followed by a crop and a proventriculus. The crop serves as a storage space for food. Salivary glands open into the gullet, and their secretions are mixed with the food during mastication. Digestion takes place primarily in the midgut, and the products are absorbed in the midgut and the hindgut. The food waste passes to the hindgut, or intestine, for elimination. Connected to the forepart of the hindgut are a large number of small tubes, called the Malpighian tubules, that float in the blood of the body cavity. Waste matter in the blood passes through the walls of these tubes and into the hindgut, from which it is eliminated from the body of the insect.