806 



INSECTS 



What They Are. An Insect is an animal which, in 

 the adult stage, has its body divided into three distinct 

 regions : the head, the 

 thorax and the abdomen 

 (Fig. 1134). The head 

 bears one pair of anten- 

 nae, 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 

 world are the cray-fish, 

 sow-bugs and crabs, but 

 these are mostly aquatic 

 animals, breathing by 



1134. A beetle. 

 Showing the different parts. 



true gills ; they have two 

 pairs of antennae, and at 

 least five pairs of legs. 

 Centipedes, or "hundred-legged worms," and millipedes, 

 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 antenna, and have four pairs of legs. 



How They Are Constructed. Insects are constructed 

 on an entirely different plan from the higher animals. 

 Their supporting skeleton is outside, it being simply 

 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, nerves, 

 and other organs within. The mouth-parts, antennae 

 and eyes of an Insect are attached to its head, and all 

 are exceedingly useful organs, as will be shown later in 

 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- 

 fied for catching other Insects for 

 food ; others have hind legs fitted 



1135. 



Head of grasshopper. 

 Showing the great eye. 



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

 the surface of the has little "pockets" on its hind 

 ampound eye is also legs for carrying pollen to feed 



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 slightjy greenish blood. There is no 

 system of tubes, like oxtr 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 many suppose, but 



its young. 



The arrangement of the internal 

 organs in Insects is somewhat pe- 

 culiar. The alimentary or food- 

 canal in larvae is a nearly straight 

 tube, occupying the central portion 

 of the body; in adult Insects it is 

 usually much longer than the body 

 and is more or less folded; from 

 the mouth the food passes through 

 a pharynx, an esophagus, some* 

 times a crop and a gizzard, a stom- 1137. The four stages 

 ach, and a small and large intes- 

 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 cor"ds extend from this around the food canal to 

 another ganglion or nerve center in the lower part of the 

 head; two nerve cords then extend longitudinally along 



1136. Fossil dragon-fly, Petalia longialata (X 1-5). 



through a series of holes along the sides of the body. 

 These openings, or spiracles, lead into a system of air 

 tubes, called tracheae. These tracheae 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 tracheae 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 entangled under their wings, or they may be 

 provided with organs known as tracheal gills; these are 

 usually plate-like expansions of the body that are abun- 

 dantly supplied with tracheae, in which the air is brought 

 practically in contact with the air in water, and may thus 

 be purified. More than 4,000 different muscles have been 

 found in a single caterpillar. Notwithstanding their deli- 

 cate appearance, these muscles are really very strong and 

 their rapidity of action is wonderful ; in certain gnats the 



in an insect's life egg, larva, pupa, imago. The codling-moth 

 Egg much enlarged; others X 1/4. 



muscles move or vibrate the wings 15,000 times per second. 

 Their Sensations. Insects can see, feel, hear, taste 

 and smell, and they may also possess other senses, as A 

 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 



