DISEASES AND INSECTS 



DISEASES AND INSECTS 1035 



1293. A beetle, showing the 

 different parts. 



fish, sow-bugs, and crabs, but these are mostly aquatic 

 animals, breathing by true gills; they have two pairs 

 of antennae, and at least five pairs of legs. Centipedes, 

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

 sand-legged worms," are 

 also nearly related to in- 

 sects, but they have the 

 thorax and abdomen form- 

 ing a continuous region, 

 and with six to 200 seg- 

 ments, each bearing one 

 or two pairs of legs; they 

 have one pan- of antennae. 

 The layman usually 

 classes such animals as 

 the spiders, mites and 

 daddy - long - legs among 

 the insects, but they form 

 a distinct class, as they 

 have the head and thorax 

 grown together, no an- 

 tennae, 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, 

 supports 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. Female canker-worm 

 moths, bed-bugs, and some other insects have prac- 

 tically no wings, and the house-flies, mosquitos, male 

 bark lice, and similar insects have but one pair of 

 wings. Insects use their legs primarily for locomo- 

 tion; some have their front legs modified for catching 

 other insects for food; others have hind legs fitted for 

 jumping, while the honey-bee has little "pockets" on 

 its hind legs for carrying pollen to feed its young. 



The arrangement of the internal organs in insects is 

 interesting and somewhat peculiar. The alimentary 

 or food canal in larvae is a nearly straight tube, occupy- 

 ing 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 

 stomach, and a small and large 

 intestine. 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 head; two 

 nerve cords then extend longi- 

 tudinally along 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 throughout the body. In insects, all 

 parts of the body cavity that are not occupied by the 

 internal organs are fifled with a rich, colorless or 

 slightly greenish blood. There is no system of tubes 

 like our arteries and veins, in which the blood is con- 



66 



1294. 



Head of grasshopper. 



Showing the great eye. 

 A detail of a part of the 

 surface of the compound 

 eye is also shown. 



fined 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 escapes 

 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 

 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 

 insects 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 



1295. Fossil dragon-fly, Petalia longialata. ( X K) 



are usually plate-like expansions of the body that are 

 abundantly supplied with tracheae, in which the air ia 

 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. Not- 

 withstanding their delicate appearance, these muscles 

 are really very strong and their rapidity of action is 

 wonderful; in certain gnats the muscles move or 

 vibrate the wings 15,000 times a 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. 1294) ; each compound eye 

 is composed of many small eyes, from fifty in some 

 ants to many thousands in a butterfly or dragon-fly. 

 Between these compound eyes, from one to four sim- 

 ple eyes are to be found in many adult insects. Cater- 

 pillars and other larvae possess only simple eyes. It is 

 thought that each facet of the compound eye sees a 

 part of an object; thus the whole eye would form a 

 mosaic picture on the insect's brain. The simple 

 eyes doubtless see as our eyes do, and seem to be 

 best adapted for use in dark places and for near vision. 

 Insects do not see the form of objects distinctly, but 

 their eyes are doubtless superior to ours in distin- 

 guishing the smallest movements of an object. It is 

 now supposed that no insects can distinctly see objects 

 at a greater distance than 6 feet. It must be a sixth 



