CHAPTER X

ARTHROPODS (Continued). CLASS INSECTS

111. Their numbers.-It has been estimated that upward of three hundred thousand named species of insects are known to the zoologist, and that these represent a fifth, or possibly a tenth, of those living throughout the world. Many of these species, as the may-flies and locusts, are represented by millions of individuals, which sometimes travel in such great swarms that they darken the sky. With nearly all of these the struggle for existence is fierce and unrelenting, and it is little wonder that such plastic animals have changed in past times and are now becoming modified in order to adapt themselves to new situations where food is more abundant and the conditions less severe. Owing to such modifications we find some species fitted for flying, others for running and leaping, or for a life underground, and many for a part or all of their lives are aquatic in their habits.

112. External features.-The body of an insect-the grasshopper, for example-consists of a number of rings arranged end to end, as we have seen them in the Crustacea and the segmented worms. In the abdomen these are clearly distinct, but in the thorax, and especially the head, they have become so intimately united that their number is a matter of uncertainty. These three regions-head, thorax, and abdomen—are usually clearly defined in most insects, but they are modified in innumerable ways in accordance with the animal's mode of life.

The head usually carries the eyes, a pair of feelers (antennæ), and three pairs of mouth-parts which may be fashioned into a long, slender tube to be used in sucking, and frequently as a piercing organ; or they may be constructed for cutting and biting. The thorax bears three pairs of legs and often one or two pairs of wings. The appendages of the abdomen are usually small and few in number, or even absent.

113. Internal anatomy. The restless activity of insects is proverbial. Some appear to be incessantly moving about, either on the wing or afoot, and are endowed with comparatively great strength. Ants and beetles lift many times their own weight. Numerous insects are able to leap many times their own length, and others perform different kinds of work with a vigor and rapidity unsurpassed by any other class of animals. As is to be expected, the muscular system is well developed, and exhibits a surprising degree of complexity. Over five hundred muscles are required for the various movements of our own bodies, but in some of the insects more than seven times this number exist. The amount of food necessary to supply this relatively immense system with the required nourishment is correspondingly large. Many insects, especially in an immature or larval condition, devour several times their own weight each day. Their food may consist of the juices of animals or plants, which they suck out, or of the firmer tissues, which are bitten or gnawed off.

Not only do the mouth-parts stand in direct relation to the habits of the animal and to its food, but, as we have often noticed before, the internal organization is also adapted for the digestion and distribution of the nutritive substances in the most economical way. For this reason we find the alimentary canal differing widely in the various forms of insects. In each case it extends from the mouth to the opposite end of the animal, and ordinarily consists of a number of different parts. In the insect shown in

Fig. 69 the mouth soon leads into the esophagus, which in turn leads into the crop that serves to store up the food until ready for its entry into the stomach; or in some of the ants, bees, and wasps it may contain material which

may be disgorged and fed

to the young. In many cases the stomach is small and ill-defined as in Fig. 69, and again it may reach enormous dimensions, nearal C ly filling the body. It may also bear numerous lobes or delicate hair-like processes, which afford a greater surface for the absorption of food. Behind the stomach are a number of slender outgrowths that are believed to act as kidneys. Beyond their insertion lies the intestine, which, like the stomach, is the subject of many modifications in the different kinds of insects. The digested food is rap

idly absorbed through the coats of the stomach and intestine and enters a circulatory system which reminds us of what exists in many of the Crustacea. The heart is situ ated above the digestive tract, and from it arteries pass out to different parts of the body. Here the blood leaves the vessels and is poured directly into the spaces among the viscera, whence it is finally conducted through irregular channels to the heart by its pulsations.

In the Crustacea the blood is made to pass through a respiratory system usually in the form of definite gills, and the oxygen with which it is charged is distributed to all

parts of the body. In the insects the blood serves almost entirely to carry the food, and the oxygen is conveyed. through the animal by a remarkable contrivance found only in the insects, the spiders, and a few related forms.

114. Respiratory system.-If we examine an insect, the grasshopper for example, we find a number of small brown spots on each side of the abdomen, each of which under a magnifying-glass is seen to be perforated by a narrow slit. Carefully opening the body, we find that each slit is in communication with a white, glistening tube that rapidly branches and penetrates to all parts of the animal. When the body is expanded the air rushes into the outer openings, on through the open tubes, and is distributed with great rapidity to all the tissues of the body. In many insects some of these tubes connect with air-sacs which probably serve to buoy up the insect during its flights through the air.

115. Wingless insects (Thysanura).-The simplest of all insects are the fishmoths and springtails, relatively small organisms covered with shining scales or hairs. The first of these is occasionally seen running about in houses feeding upon cloth and other substances, while the latter live in damp places under stones and logs. They are without wings, but are able to run rapidly and to leap considerable distances. In addition to the ordinary appendages, the abdomen bears what are perhaps rudimentary legs, a fact which, together with their relatively simple structure, strengthens the belief that the insects have descended from centiped-like ancestors.

116. Grasshoppers, crickets, katydids, etc. (Orthoptera).Rising higher in the scale of insect life, we arrive at the group of the cockroaches, crickets, grasshoppers, locusts, and other related insects. Four wings are present, the first pair thickened and overlapping the second thinner pair. The latter are folded lengthwise like a fan, which is said to have given the name Orthoptera (meaning straight-winged) to

this group of insects. These extend all over the world, being particularly abundant in the warmer countries, and their strong biting mouth-parts and voracious appetites render many of them dreaded pests to the farmer. The cockroaches are nocturnal in their habits, racing about at night, devouring victuals in the pantry and gnawing the bindings of books. During the day their flat bodies enable them to secrete themselves in crevices wherever there is sufficient moisture.

In the grasshoppers, locusts, katydids, and crickets the body is more cylindrical, and the hind pair of legs are often greatly lengthened for leaping.

FIG. 70.-The Rocky Mountain locust.-
After RILEY, from The Insect World.

The crickets and katydids are nocturnal, the former remaining by day in burrows which they construct in the earth, the latter resting quietly in the trees. At night they feast upon vegetable matter principally, though

some species are known to prey on small animals. Those insects we usually term grasshoppers (properly called locusts) are specially destructive to vegetation. Some species are strong fliers, and this, connected with their ability to multiply rapidly, renders them greatly dreaded pests. They have been described as flying in great swarms, forming black clouds, even hiding the sun as far as the eye could reach. The noise made by their wings resembled the roar of a torrent, and when they settled upon the earth every vestige of leaf and delicate twig soon disappeared.

The eggs of the majority of Orthoptera are laid in the ground, where they frequently remain through the winter. When hatched the young quite closely resemble the parents, and, after a relatively slight metamorphosis, assume the adult form.

117. Dragon-flies, may-flies, white ants, etc. (Pseudoneuroptera). The dragon-flies, caddis-flies, may-flies, and white ants