INSECT

any member of the class Insecta, the largest class of the phylum Arthropoda, which is itself the largest of the animal phyla. Like all arthropods, the insects have segmented bodies, jointed legs, and, when present, external skeletons (exoskeletons). Insects are distinguished from other arthropods by their body, which is divided into three major regions: (1) the head, which bears the mouthparts, eyes, and a pair of antennae, (2) the thorax, which usually has three pairs of legs (hence Hexapoda, the name that was formerly given to this class) and usually one or two pairs of wings, and (3) the many-segmented abdomen, which contains the digestive, excretory, and reproductive organs. In a popular sense, insect usually means the familiar pests or disease carriers, from bedbugs, houseflies, and clothes moths to Japanese beetles and aphids; the annoyers, such as mosquitoes, fleas, horseflies, and hornets; and the conspicuous butterflies and moths. Many insects, however, are beneficial from a human viewpoint; they pollinate plants, produce useful substances, control pest insects, act as scavengers, and serve as food for other animals (see below Importance). Furthermore, insects are valuable objects of study in elucidating many aspects of biology. Much of our knowledge of genetics has been gained from fruit fly experiments and of population biology from flour beetle studies. Insects are often used in investigations of hormonal action, nerve and sense organ function, and many other physiological processes as well. member of the class Insecta, the largest class of the phylum Arthropoda, which is itself the largest of the animal phyla. Of all the animal species so far described by science, insects account for about five-sixths; in addition to the nearly 1,000,000 insect species known, entomologists estimate there are 1,000,000 to 4,000,000 yet to be described. Many of the features of insects are those of Arthropoda generallynotably jointed legs and a segmented body covered by a hard exoskeleton, composed, in part, of the protein chitin. The exoskeleton provides protection, much like a suit of armour, and it also serves as the point of attachment for the muscles. The exoskeleton must be molted (shed) periodically during growth. Insects are unique among the arthropods in that they have only three pairs of legs (all other arthropods have four or more pairs) and in that most insects bear wings (all other arthropods are wingless). Also, unlike most other arthropods, the insects show specific division of the body into three regions: the head, the thorax, and the abdomen. Insects exhibit a bewildering variety of forms; except in the most primitive insects, even individuals have slightly to strikingly different forms as larvae and as adults. Insects range in size from the barely visible to several inches in length or wingspread; their lifespans range from hours to many years; they may be solitary or social. Insects feed on plants, animals, and organic debris; their interactions with their food source range from mutualism to parasitism to predation. Despite their diversity, all adult insects share some basic external and internal anatomical features. In all species, for example, the head contains sensory structures and mouthparts. In most insects the mouthparts consist of several structures, including an upper lip (labrum), a pair of jaws (mandibles), a pair of sensory structures (maxillae), and a lower lip (labium). These basic mouthparts have been highly modified for different feeding methods, including chewing, bloodsucking, sucking plant juices, and feeding on nectar. The sensory structures of the head include a pair of antennae, which typically bear numerous tactile and olfactory bristles, and eyes. Most higher insects have two kinds of eyes: simple eyes, which perceive only light, darkness, and movement; and compound eyes, which are made up of dozens to thousands of separate lenses and presumably form mosaic images. The thorax consists of three segments, which may be fused. Each segment bears a pair of legs; these may be modified for walking, jumping, grasping, burrowing, or swimming. In most insects, the rear two thoracic segments each carry a pair of wings. Some species, however, have only one pair of wings, and others lack wings entirely. The abdomen is segmented, having 11 segments (or fewer, if fusion has occurred). It is without appendages except at the hind end, where some species have external reproductive structures, sensory feelers, or both. Insects possess digestive, circulatory, respiratory, nervous, and reproductive systems. The digestive tract consists of the foregut in the head and thorax, and the midgut and hindgut in the abdomen. Digestive enzymes are secreted by salivary glands and by cells of the midgut; digested food is absorbed chiefly in the midgut. Undigested food residues are expelled from the hindgut through the anus. Insects have an open circulatory system. This means that the blood (hemolymph) is not confined exclusively to a network of vessels; at some point it is pumped into the interior body cavity where it directly bathes the internal tissues. In fact, insects have only one blood vessel, which runs dorsally the length of the body. The abdominal portion of this vessel is the heart, or pumping organ; the portion that extends through the thorax and head is called the aorta. Blood is discharged from the aorta into the head; it circulates through the head, thorax, and abdomen and is forced into the wings and legs by accessory pumps. As it circulates through the body spaces, the blood distributes nutrients, which it absorbs from the midgut or from the fat body, which is an important storage tissue in insects. The blood picks up wastes which are then removed from it by a system of malpighian tubules that open into the rear end of the midgut. Eventually the blood reenters the heart through small openings called ostia. Unlike most higher invertebrates (and all vertebrates), insects do not rely on the circulating blood to deliver oxygen to the body cells and to carry away the waste gas carbon dioxide. The respiratory system of insects consists of air tubes, or tracheae, which open at the surface of the abdomen and thorax through pairs of spiracles. The tracheae branch repeatedly, ending in minute, thin-walled tracheoles that reach every part of the body. Oxygen diffuses through the tracheole walls to enter the body cells. At the same time, carbon dioxide diffuses into the tracheoles from the cells. Insects have a central nervous system composed of double ventral nerve cords joined by paired ganglia (collections of nerve cell bodies and fibres). Each ganglion supplies nerves to one or more segments of the body. These nerves contain sensory nerve cells, which relay information from the senses to the central nervous system, and motor nerve cells, which carry directives from the central nervous system to the muscles and glands. The three main ganglia in the head are fused to form the brain, which sends nerves to the eyes, antennae, and other organs of the head. Besides having sensory organs on the head, most insects are covered with tactile hairs that may be sensitive enough to detect air vibrations. Some species also have well-developed hearing organs, such as the tympanic membrane found on the abdomen in grasshoppers. Reproduction in insects is sexual. The male reproductive system includes two testes, where sperm are produced; ducts that carry sperm from the testes; and a copulatory organ that deposits sperm in the female. The female system has two egg-producing ovaries and a pair of oviducts that unite to form a vagina. The vagina typically serves both for receiving sperm and for depositing eggs; females of some species have an ovipositor, an abdominal appendage used to deposit eggs. The class Insecta is divided into two subclasses: the Apterygota, which are primitive and wingless and include the silverfish and bristletails (order Thysanura); and the Pterygota, more advanced winged or secondarily wingless (having lost the wings found in earlier forms) creatures that exhibit some degree of metamorphosis in individuals. The Pterygota are divided into 27 living orders, including the beetles and weevils (Coleoptera), flies, midges, gnats, and mosquitoes (Diptera), mayflies (Ephemeroptera), the bugs (Heteroptera), aphids, leafhoppers, and cicadas (Homoptera), bees, wasps, and ants (Hymenoptera), termites (Isoptera), butterflies and moths (Lepidoptera), lacewings (Neuroptera), dragonflies and damselflies (Odonata), grasshoppers, locusts, crickets, and katydids (Orthoptera), fleas (Siphonaptera), thrips (Thysanoptera), and caddis flies (Trichoptera). Fossils indicate that primitive insects had developed by the Middle Devonian Period (about 380 million years ago). They probably evolved from a wingless terrestrial arthropod. It is believed that winged insects appeared very early in the Carboniferous Period (about 360 million years ago). Periods of explosive evolution, during which large numbers of new forms developed in relatively short spans of time, are thought to have occurred in the Carboniferous and Permian periods, giving rise to the orders known today as well as those known only as fossils. Insects' life cycle begins with the fertilized egg, from which hatches the larva. Because the rigid exoskeleton does not allow for growth, it must be shed periodically and replaced by a new and larger one produced by a layer of epidermal cells. In the Apterygota the succession of larval stages (called instars) between molts differs little from one to another or from the adult form. In the Pterygota metamorphosis generally occurs; it may be incomplete, meaning that the larva substantially changes form only in the last molt, emerging as an adult with fully formed wings and genitalia, or it may be complete, as in the butterflies, whose larvae molt into a pupal phase and then again into the adult form. The adult insect exists largely to reproduce. Indeed, in some adult insects this singleness of purpose supersedes even eating. Mating behaviour varies widely among insects but frequently involves the production by male or female of powerful chemical attractants known as pheromones. Insects are found in almost every terrestrial and freshwater habitat. By sheer numbers as well as by diversity of habit they fulfill many important ecological functions. They promote the decay of organic material and the formation of soil; they are instrumental in the pollination of many plants (the insects and the flowering plants, in fact, evolved together); and they are a major part of many food chains. Their interaction with humans is also various. Frequently they are competitors for food, either in the field or in storage; some insects are carriers of crop, livestock, or human disease; some are nuisances; some can infest and destroy buildings, garments, or carpets. On the other hand, insects produce such valuable commodities as honey, silk, wax, and dyes; some are used for human food; some are valuable in controlling populations of other, potentially harmful, species; and some have proved useful in scientific and medical research. Additional reading General and classification A.D. Imms, Imms' General Textbook of Entomology, 10th ed. rev. by O.W. Richards and R.G. Davies, 2 vol. (1977); V.B. Wigglesworth, The Life of Insects (1964); C.T. Brues, A.L. Melander, and F.M. Carpenter, Classification of Insects, 2nd rev. ed. (1954). Insect form R.E. Snodgrass, Principles of Insect Morphology (1935, reissued 1993); J.S. Kennedy (ed.), Insect Polymorphism (1961, reissued 1980). Insect physiology and biochemistry M. Rockstein (ed.), The Physiology of Insecta, 2nd ed., 6 vol. (197374); V.B. Wigglesworth, The Physiology of Insect Metamorphosis (1954), The Principles of Insect Physiology, 7th ed. (1972, reissued 1982), Insect Hormones, 2nd ed. rev. (1983); V.G. Dethier, The Physiology of Insect Senses (1963); D. Gilmour, The Biochemistry of Insects (1961); V.J.A. Novak, Insect Hormones: The Physiology, Morphology, and Phylogeny of the Insect Endocrines (1966); W. Mordue et al., Insect Physiology (1980). Insect behaviour and communications C.G. Johnson, Migration and Dispersal of Insects by Flight (1969); C.T. Brues, Insect Dietary: An Account of the Food Habits of Insects (1946); J.W.S. Pringle, Insect Flight, 2nd ed. rev. (1983); P.T. Haskell, Insect Sounds (1961); Lorus J. Milne and Margery Milne, Insect Worlds (1980), a popularly written description of insect adaptability and use of the environment; Michael D. Atkins, Introduction to Insect Behavior (1980). Economic and ecological importance Robert L. Metcalf and Robert A. Metcalf, Destructive and Useful Insects: Their Habits and Control, 5th ed. (1993); J.R. Busvine, Insects and Hygiene: The Biology and Control of Insect Pests of Medical and Domestic Importance, 3rd ed. (1980); R.M. Gordon and M.M.J. Lavoipierre, Entomology for Students of Medicine (1969); T.R.E. Southwood, Ecological Methods: With Particular Reference to the Study of Insect Populations, 2nd ed. (1987, reissued 1991). Sir Vincent Brian Wigglesworth Classification Distinguishing taxonomic features The class Insecta is divided into orders on the basis of the structure of the head, including eyes, mouthparts, and antennae; the thorax, including legs and wings; and the abdomen, including segmentation, spiracles, and appendages (cerci, styli, furcula). The genitalia and their accessory structures, usually located in the ninth abdominal segment, are important in classification. Other taxonomic criteria include bristles (their form and arrangement are known as chaetotaxy), sensory receptors (spines, hairs, sensilla, tympanal organs), pattern of wing venation, and position of mouthparts. In addition, type of metamorphosis and form of larva and pupa are used to distinguish insects. Critical appraisal The classification into orders presented above is acceptable to most entomologists. Although the apterygotes are classified here as four unrelated orders of the subclass Apterygota, three of these groups (proturans, collembolans, diplurans) are considered by some as separate classes equivalent to the class Insecta. In such a classification the subclass Apterygota includes only the thysanurans and relatives. The order Orthoptera infrequently includes, in addition to the grasshoppers and crickets, three other closely related groups (classified above as distinct orders): dictyopterans (cockroaches and mantids), grylloblattids, and phasmids (stick and leaf insects). The term orthopteran often is used as a common name for these four groups. The Mallophaga (chewing lice) and Anoplura (sucking lice), classified here as orders, sometimes are grouped as suborders of an order Phthiraptera, a group of obligate permanent ectoparasites of birds and mammals. The homopterans and heteropterans, here classified as separate orders, sometimes are considered as suborders of an order Hemiptera. Both groups have piercing-sucking mouthparts; for this reason they are believed to be related closely to each other. Some entomologists, however, consider distinguishing features other than the mouthparts sufficiently important to accord full ordinal status to each group. The term neuropteran frequently is used to describe three closely related groups, classified here as three distinct orders: Neuroptera (lacewings), Raphidiodea (snakeflies), and Megaloptera (dobsonflies and alderflies). Although the tendency has been to classify these groups as distinct orders, they sometimes are placed in the order Neuroptera. Among the lepidopterans, members of the family Micropterigidae are more primitive than existing trichopterans (caddisflies). Although some entomologists treat them as a distinct order (Zeugloptera), others place them in the order Lepidoptera. The aberrant parasitic Stylops and its allies have been treated as the order Strepsiptera. The tendency now, however, is to include them in the order Coleoptera. Sir Vincent Brian Wigglesworth