XIX. INSECTS

Insects and Crustaceans Compared. Both crustaceans and insects belong to a great group of animals which agree in that they have jointed appendages and bodies, and that they possess an exoskeleton. This group is known as the Arthropoda.

Insects differ structurally from crustaceans in having three regions in the body instead of two. The number of legs (three pairs) is definite in the insects; in the crustaceans the number is not always fixed (as in the Entomostraca), but is always more than three pairs. The exoskeleton, composed wholly of chitin in the insects, is usually strengthened with lime in the crustaceans. Both groups have compound eyes, but those of the crustacea are stalked and movable. The crustaceans receive sensations of touch by means of sensory hairs which protrude through the exoskeleton. The other sense organs do not differ greatly. The most marked differences are physiological. The crustaceans take in oxygen from the water by means of gills, while the insects are air breathers, using for this purpose air tubes called trachea.

The young of both insects and crustaceans usually undergo several changes in form before the adult stage is reached. They are thus said to pass through a metamorphosis. Both insects and crustaceans, because of their exoskeleton, must molt in order to increase in bulk.

The insects are divided into a number of large groups called orders. The insects found in each order possess certain characters in common. We shall now examine several representatives from some of the different orders of insects commonly met with.

The Locust. type of the class

THE ORDER ORTHOPTERA

The locust or short-horned grasshopper is a
Insecta, which is characterized by possessing a

body made up of segments, having jointed appendages, three pairs of legs, and breathing through a system of air tubes called

trachea. It also belongs to the order Orthoptera (straight wings) because the wings, when at rest, lie straight along on the body.

External Structure.1 - Any common form, as Melanoplus femur-rubrum, may be used. Examine the body of the grasshopper. The anterior region is the head, the middle part the thorax, the posterior the abdomen. To which region are wings and legs attached? Which region is the stoutest?

Notice that the body is covered with an exoskeleton. This is composed of chitin, a substance chemically akin to that of a cow's horn.

The grasshopper makes its home in fields of grass. Some species live in vacant lots where there is considerable earth exposed. Do such grasshoppers ever have the color of their surroundings? How might this be of service to them?

Examine the legs of a living grasshopper, with a view to finding out their position when at rest. Examine the hind legs. Do you find any adaptations present which fit the legs for jumping? Examine the hooks and pads on the last segment or tarsus. Look for other adaptations. Besides flying and hopping, the grasshopper also crawls. In a resting position, it clings by means of the hooks and pads on the foot or tarsus.

Spread out the wings. Note their position. Note any differences between the two pairs. Which pair would be most useful in flight? Notice the delicate lacelike underwings, the supporting veins of which are composed of tubes that carry blood and air.

Notice the abdomen carefully. The most anterior segment is incomplete, and bears an oval structure, the tympanum, or ear drum. Count the number of complete segments in the abdomen. The female grasshopper has the free end of the abdomen modified for the purpose of egg laying. Note the two-parted structures making up the ovipositor or egg layer. The male has a more rounded abdomen.

Observation of the abdomen of a living grasshopper shows a frequent movement of the abdomen. Count the number of movements in a minute. This is the breathing of the grasshopper. Along the side of the abdomen in eight of the segments (in the red-legged grasshopper) are found tiny openings called spiracles. A large spiracle may easily be found in the middle segment of the thorax. These spiracles open into little tubes called trachea. The trachea carry air to all parts of the body. By the movements of the abdomen just noted, air is drawn into and forced out of the trachea.

Tracheæ.

The tracheæ divide and subdivide like branches of

a tree so that all the body cavity is reached by their fine endings.

Some even pass outward into the

veins of the wings.

Each of

these tubes contains air. The blood of an insect does not circulate through a system of closed blood tubes as in man, but instead it more or less completely fills that part of the body cavity which is not filled with other organs. A heart (a hollow muscular organ containing several openings, situated on the dorsal side of the insect) simply stirs up the blood inside the body cavity in much the same way that a rubber bulb would circulate water if squeezed inside a pail of

Spiracle with its tracheæ removed from an insect; 8, spiracle. Three times natural size. Photographed by Davison.

water, so that sooner or later the blood comes in contact with the oxygen passed in through the trachea.

Muscular activity. Insects have the most powerful muscles of any animals of their size. Relatively, an enormous amount of energy is released during the jumping or flying of a grasshopper. The trachea pass directly into the muscles, where oxidation takes place when the muscles are exercised. This oxygen is taken directly to the parts of the body where it is to be used. The body temperature of the grasshopper is slightly higher than the surrounding air. Why?

Mouth Parts. - Feed a grasshopper with a blade of grass. Note how the animal holds the grass. What appendages are used? Note the upper lip and lower lips, the latter biparted. The dark brown jaws (mandibles) may be seen underneath. Below them are a pair of smaller pointed parts, the maxillæ, to which are attached a pair of jointed palps. Note also that palps are attached to the lower lip.

Food Taking and Blood Making. The plant food taken by the grasshopper is held in place in the mouth by means of the little jaws or maxilla while it is cut into small pieces by the mandibles.

Just behind the mouth is a large crop into which empty the contents of the salivary glands. It is this fluid mixed with digested food that we call the "grasshopper's molasses." After the food is digested by the action of the saliva and other juices, it passes in a fluid state through the walls of the intestine where most of it becomes part of the blood. As blood it is passed on to tissues, such as muscle, to be used in repairing that which is used up during the flight of the insect. Some of the foods are doubtless

at once oxidized to release energy for the active insect.

Eyes. A considerable part of the surface of the head of the grasshopper is taken up by the compound eyes. Compare them with your own in position. Examination with a lens shows the whole surface to be composed of tiny hexagonal spaces called facets. Each facet is believed to be a single eye, with perhaps distinct vision from its neighbor. The grasshopper also has three simple eyes on the front of the head. Find them.

Cov

OTHER SENSE ORGANS.-The segmented feelers or antenna have to do with the sense of touch and smell. The ear of the grasshopper is found under the wing on the first segment of the abdomen as before noted. Longitudinal section ering the body and on the appendages, are found hairs of part of the (sensory hairs) which appear to be sensitive to touch. compound eye of NERVOUS SYSTEM. The nerve chain, as in the crayan insect; a, facets; c, nerves. fish, is on the ventral side of the body. As in the crayfish, it passes around the gullet near the head to the dorsal side, where a collection of ganglia forms the brain. Nerves leave the central system as outgoing fibers which bear motor impulses. Other nerve fibers pass inward, and produce sensations.

Make a careful drawing of the locust, showing as many of the above parts as you can, and label them neatly.

Life History. The female red-legged locust lays its eggs by digging a hole in the ground with its ovipositor or egg layer, the modified end of the abdomen. From twenty to thirty eggs are laid in the fall; these hatch out in the spring as tiny wingless grasshoppers, otherwise like the adult. As in the crayfish, the young molt in order to grow larger, each grasshopper undergoing several molts before reaching the adult state. In the fall most of the adults die, only a few surviving the winter.

Economic Importance of the Grasshopper. As far back as Biblical times, the grasshopper was noted for its destructive

ness. Frequent mention has been made of damage wrought by them in the early history of this country. In recent times, the damage has been appalling, especially in the central West. In 1874-1876 the damage to crops by the Rocky Mountain locust. has been estimated at $200,000,000. At certain times, these locusts migrate from Colorado, Wyoming, and Dakota, where they breed during the summer, and descend in countless millions upon the grain fields to the eastward. Fortunately these invasions have been rare.

Relatives of the Locust. One member of this group that we associate with the grasshopper is the cricket. In structure and life habits it resembles the locust. Crickets live most of the time under logs or stones, and seem to prefer darkness to light. The cheerful chirp of the male house cricket is made by rubbing the thickened edge of one wing against a rasplike projection on the opposite wing. The rate of the chirp seems to depend upon the temperature of the surrounding air. Another musician known to all is the katydid. This insect, with its green body and wings, can scarcely be distinguished from the leaves on which it rests. This affords the katydid immunity from attack by many enemies. The protection thus received illustrates what is called protective resemblance. The walking stick, which resembles the twigs on which it is found, and the walking leaf insect of the tropics, are other examples of protective resemblance.

resemblance.

The mantis, shown in the illustration on the following page, is provided with strongly built forelegs, with which it seizes and