What They Are.— An Insect is an animal wliich, in 

 the adult stage, has its body divided into three distinct 

 regions : the head, the 

 thorax and the abdomen 

 (I'^iK. li:U). The head 

 hears one pair of anten- 

 nie, and there are always 

 three pairs of legs and 

 usually either one or two 

 pairs of wings attached 

 to the thorax. By these 

 characteristics one can 

 usually readily distin- 

 guish an adult Insect 

 from any other animal. 

 Among the near relatives 

 of Insects in the animal 



least five pairs of Ieg> 

 Centipedes, or "hundred-legged worms," and millipede'-, 

 or "thousand-legged worms," are also nearly related to 

 Insects, but they have the thorax and abdomen forming a 

 continuous region, with from 6 to 200 segments, each bear 

 ing one or two pairs of legs ; they have one pair of anten 

 nae. The layman usually classes such animals as the spi 

 ders, mites and daddy-long-legs among the Insects, but 

 they form a distinct class, as they have the head and thorax 

 grown together, no antennce, and have four pairs of legs 

 ITow They Are Constructed.— Insects are constructed 

 on an entirely different plan from the higher animals 

 Their supporting skeleton is outside, it being simplj 

 the skin hardened more or less by a horny substance 

 known as chitin. This firm outer wall, or skeleton, sup 

 ports and protects the muscles, blood-vessels ner\es, 

 and other organs within. The mouth-parts, antennae 

 and eyes of an Insect are attached to its he vd and all 

 are exceedingly useful organs, as will be shown later m 

 discussing the feeling and the other sensations of an 

 Insect. An Insect's wings and 

 legs are always borne by the 

 thorax. The wings are primarily 

 organs of flight, but are used as 

 musical organs by some of the 

 ' grasshoppers and crickets. Fe- 

 male canker-worm moths, bed- 

 bugs, and some other Insects 

 have practically no wings, and the 

 house-flies, mosquitoes, male 

 bark lice, and similar Insects 

 have but one pair of wings. In- 

 sects use their legs primarily for 

 walking, running or climbing ; 

 some have their front legs modi- 

 Head of erasshopper. fied for catching other Insects for 

 Showing the great eye. food; others have hind legs fitted 

 A detail of a part of for jumping, while the honey-bee 

 "pockets" on its hind 



1135. 



the surface of the 



eye : 



legs 





shown 



The arrangement of tlj> 

 organs in Insects is soim 

 culiar. The alimentary . , ; 

 canal in larvae is a nearly srnu-iu 

 tube, occupying the central portion 

 of the body; in adult Insects it is 

 usually much longer than the bndv 

 and is more or h-ss f„],]ot]; fi-nin 

 the mouth the foml nn-^rv tlnMUL'li 

 a pharynx, an r-..|.!i ilj u ~ , ,,,Mir- 

 times a crop and ^i _■! .n-l .i -i^.m iiJ:. Thr' [nur stages 

 ach, and a small :mh1 I.iil''' iinr-- 

 tine. The nervous system of an 



Insect is similar to that in the higher animals, but it 

 extends along the venter instead of the back. There is 

 a little brain in the upper part of the head, and two 

 nerve cords extend from this around the food canal to 

 another ganglion or nerve center in the lower part of the 

 wo nerve cords then extend longitudinally along 



INSECTS 



the venter and connect a series of nerve centers or 

 ganglia, typically one for each segment of the body. 

 From each of these ganglia or little brains nerves arise, 

 which supply the adjacent organs and ramify through- 

 out the body. In Insects, all parts of the body cavity that 

 are not occupied by the internal organs are filled with a 

 rich, colorless or slightly greenish blood. There is no 

 system of tubes, like our arteries and veins, in which 

 the blood is confined and through which it flows 

 There is a so-called "heart" above the food-canal, along 

 the middle line of the back; it is a tube consisting of 

 several chambers communicating with each other and 

 with the body cavity by valvular openings. The blood 

 is forced through this heart into the head, where it es- 

 capes into the body cavity. It then flows to all parts of 

 the body, even out into the appendages, in regular 

 streams which have definite directions, but which are 

 not confined in tubes They, like the ocean currents, 

 are definite streams with liquid shores Insects do not 

 breathe through the mouth, as manj suppose, but 



1136 Fossil dragon fly Petaha longialata (X 1 

 through a series i f holes along the sides of the body 

 These openings, or spuacles, lead into a sjstem of air 

 tubes, called trachete. These trachete branch and finally 

 ramify all through the Insect. Insects have no lungs, 

 but the tracheae sometimes connect with air-sacs or 

 bladders in the body, which help to buoy up the Insect 

 when flying. Thus the relation between the circulation 

 of the blood and respiration is not nearly so intimate in 

 Insects as in man. In Insects the air is carried to all 

 the tissues of the body in the trachefe and the blood 

 simply bathes these tissues. Just how the blood is 

 purified and how the waste matter is disposed of in In- 

 sects are not yet clearly understood. Aquatic Insects 

 breathe by either carrying down bubbles of air from the 

 surface e'ntanLrlf.l iin'drr their wings, or they may be 

 provided witli -iririiti- kiM>\vn as tracheal gills; these are 

 usually platr likr . \i,.in-i..iis of the body that are abun- 

 dantly suppliid w iili tr;i'-!n!P. in which the air is brought 

 practically in luntatt with tli.- nir in wntcr. and may thus 

 be purified. More than i.OOii .lnl.-. m niu. '. - liave been 

 found in a single caterpillar. N .M heir deli- 

 cate appearance, these musi'li- : ii -trongand 



their rapidity of action is won. In 1 III ; in ., ri:,iTi tjnats the 



.^„ 



?o. — The codlin 



muscles! 



head; 



iiir:it.ili.. wiiiL's ir.jiontimes per second. 

 Their 6'<ll^./^ ■/.... ~ In-, .ts .nn -.-i-, feel, hear, taste 

 and smell, an.l th.-v niav aN.. ]...ss,.ss other senses, as » 

 sense of direction. Many Insects have two kinds of eyes. 

 On each side of the head the large compound eye is easily 

 recognized (Fig. 1135); each compound eye is composed 



