STRUCTMIE OF INSECTS. The insects are most closely related to the myriapods and to the spi ders and their allies (Arachnida). The body con palpi never more than four-jointed. The cpi pharynx forms the roof of the mouth and bears taste organs. The hypopharynx is usually well developed, and lies on the under side of the mouth just above she labium, and receives the end of the salivary duct. There is a pair of compound eyes, and usually from two to three simple eyes (ocelli). The thorax consists of three segments. There are three pairs of legs, each foot ending in a pair of claws; two pairs of wings, a pair to each of the two hinder thoracic segments. The wings are occasionally reduced or wanting in forms which, however, had winged ancestors. The abdomen consists at the most of from ten to twelve seg ments, and there are no functional abdominal legs except in the Thysanura and in the larva of Lepidoptera. The genital openings are usually single, but are paired in some orders.
The digestive canal is highly differentiated in the winged orders. In larva' it is a nearly straight tube extending from one end of the body to the other. In adult insects, however, it is usually much longer than the body, and there fore is more or less folded. It is situated below the dorsal vessel and above the ventral chain of nerve-ganglia. There are three main divisions. namely the anterior, middle, and posterior. The anterior and posterior divisions are morpho logically considered as invaginations of the ex tremities of the skin. The ersophagus is usually narrow. dilating behind to form the crop. Some times it dilates immediately behind the mouth, and this dilatation is known as 'pharynx.' It is followed by the 'gizzard' or proventrienlus. The true stomach or chylific ventricle is always pres ent, and is very variable in shape and size. Some times it is coiled like an intestine, and some times bears pouches varying in number from two to very many. The intestine varies greatly in length. being smaller anteriorly and frequently widening to form a rectum. There is sometimes a CreCt011 and odoriferous glands. The alimentary canal has two coats of muscles, a longitudinal and transverse coat. Salivary glands are pres ent in many insects, discharge into the mouth.
and vary greatly in their development. The silk spun by many larva collies from long silk-glands similar in form and situation to the simple tubes of the salivary glands. Alalpighian tubules are present in most insects in the hinder end of the !•ily, opening into the alimentary canal usually at the junction of the stomach and intestine. They vary greatly in length and number.
The nervous system consists of a well-developed Main, and no inure than thirteen pairs of ganglia may be more or less eonfused in tIk• spe cialized The cephalic ganglia are placed above the (esophagus, while the other ganglia are ventral. There is an accessory sympathetic sys
tem which is complex and difficult of dissection. Respiration is carried on by means of a system, of air-vessels called tracheae, which ramify through all parts of the body. These tracheae connect with the outer air through a series of spiracles which are orifices at the side of the body, there being usually one pair for each seg ment. The tracheae are elastic and consist of an outer cellular coat and an inner ehitinous layer, the latter strengthened by a spiral fibre. With some insects, as With certain bees and locusts, the tracheae expand in places to air-saes, and these sacs, as well as the finest capillary branches of the traehex, lack the spiral fibre.
Circulation is carried on through the dorsal vessel or heart, which is situated in the upper part of the body (just beneath the dorsal skin), extending fr3in the head or thorax to the pos terior extremity. It is an elongate tube consist ing of a number of united chambers, and is usual ly closed behind but open in front, having also several orifices at the side. There is a muscular layer with internal and external membranous layers. The blood is forced forward into the body-cavity, receiving oxygen from the terminal branches of the The muscular system is extensive, nearly four thousand muscles oc •u•ring in certain caterpillars.
CtAssi•icATioN. According to the generally accepted classification, there are nineteen orders of insects, is follows: Thysanura, springtails and bristletails. L'pheincrida, Al ay- fl ies.
eflonata, dragonflies.
Plceoptera, stone- flies.
I soptera, white ants.
Corrodentia, psocids and book-liee. .11a//ophapu, bird-lice.
Eaple.roptera, 'earwigs.
Orthoptera, grasshoppers, crickets, etc. Physopoda, thrips.
17cmiptera, true bugs, plant-lice, scale-insects. Neuroptcra, aphis-lions. ant-lions, etc. Mecoptera, scorpion-flies, etc.
Trieimptcra, caddis-flies.
Erpidoptera, butterflies and moths.
Diptrra, true flies.
Siphonopt•ra, fleas.
Colcoptera, beetles.
Ilymenovirra, bees, wasps, ants, saw-flies. etc.
Representatives of the principal of these groups are illustrated by typical forms on the accom panying colored plate, which are described in de tail elsewhere, under their respective names. See also colored plates of BEETLES, BUTTERFLIES, DRAGON-FLIES, and AIOTOS.
BIBLIOGRAPHY. Packard, Tr.rt-book of EntoBibliography. Packard, Tr.rt-book of Ento- mology (New York, 1898) : Comstock. Manual for the Study of Insects (Ithaca, ]S95) ; Pack ard, Uunic to the Study of /am cis (New York, 1880) ; Howard, The insect Book ( New York, 1901) ; Sharp, Cambridge 11 istory, cols. V. and Vi. (1.01)(1011, 1 S95.99