animals are barely visible to the naked eye. They are found in almost every part of the world, from the arctic seas to those of the tropics, and in fresh as well as salt water. They are so numerous that the sea in places is colored by their bodies. So prolific are they that it is estimated that one copepod may produce in a single year four billion five hundred million offspring. These animals form a large part of the food supply of many of our most important food fishes as well as the food of many other aquatic animals. The whale, for example, subsists largely on this kind of food. They are, then, in an indirect way, of immense economic value.

Degenerate Crustaceans. - One of the most interesting forms to a zoölogist is the goose barnacle. This crustacean, like all others of the group, is free-swimming during its early life. Later, however, after passing through several changes in form during its development, the barnacle settles down on a rock or some floating object, fastens itself along the dorsal surface, and remains fastened during the rest of its life. Food comes to it in a current of water, which is set in motion by the rhythmical beating of the appendages. Thus food particles are carried along the ventral side of the body to the mouth. Such animals, having lost the power of locomotion, are said to be degenerate.

Parasitic Crustaceans. Other crustaceans have become even more helpless and have come to take their living from other animals. In some cases they become simply a bag for absorbing nourishment from the host on which they are fastened. Such is the sacculina, a degenerate crustacean that lives attached to the body of the crab. Others attach themselves to fishes and are known to fishermen as fish lice.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

Burnet, School Zoology, pages 67-73. American Book Company.

Davison, Practical Zoology, pages 133–141. American Book Company.

Herrick, Textbook in General Zoology, Chap. XIII. American Book Company. Jordan and Kellogg, Animal Life, Chap. VIII. D. Appleton and Company. Jordan, Kellogg, and Heath, Animal Studies, Chap. IX. D. Appleton and Com

pany.

ADVANCED

Herrick, The American Lobster, Report of U.S. Fish Commission, 1895.

Huxley, The Crayfish. D. Appleton and Company.

Mead, Reports of the R.I. Inland Fisheries Commission.

Parker, Elementary Biology. The Macmillan Company.

Parker and Haswell, Textbook of Zoology. The Macmillan Company.

XIX. THE INSECTS

Problem XXX. A study of some animal likenesses and differences, and the classification of insects (optional). (Laboratory Manual, Prob. XXX)

(a) Grasshopper—a straight-winged insect.

(b) Butterfly or moth—a scale-winged insect.
(c) The typhoid fly- a two-winged insect.
(d) A beetle-a sheath-winged insect.
(e) A bug-a half-winged insect.

(f) The dragon fly-a nerve-winged insect.
(g) The bee-a membrane-winged insect.
(h) Summary of differences between orders.
(i) Making a logical definition.

Insects the Winners in Life's Race. We are all familiar with common examples of insect life. Bees and butterflies we have already studied in connection with their work in the cross-pollination of flowers. Mosquitoes and flies all too often come to our notice as pests; the common household insects sometimes annoy us, while we often hear and see in a small way the harm done by insects in the field and garden. Insects are a successful group. They outnumber all the other species of animals on the face of the earth. They hold their own in the air, in the water, and on land. Fitted in many ways to lead the successful life, they have become winners in life's race.

We have already, from our study of a bee, formed some idea of what an insect is. But it would be unfair to expect to know all insects from our slight knowledge of one form. Our object in the study of this chapter will be to get some first-hand knowledge of some common insects so that we may classify them and distinguish one from another. This great group, containing more than half of the known representatives of animal life on the earth, is made up of a number of groups called orders. The insects contained in these orders have certain characters of structure and

life history in common, yet each differs somewhat from the other orders. The characters which all the groups contain in common give us a working definition of an insect.

One of the most common insects in the United States is the locust or grasshopper, as it is commonly called. The study of a living specimen (or if it cannot be obtained, a dead locust) will, better than any other insect, give us insight into the structure and life processes of this great group.

The Locust (Red-legged Grasshopper). The segmented body is divided into a head, a middle portion (the thorax), and a posterior part, the abdomen. The legs, six in number, and two pairs of wings are attached to the thorax. The animal lives a rather active life in the fields, the hind pair of legs being adapted by shape, position, and in structure. for leaping. Careful examination of the foot of the animal shows a num

Locust (lubber grasshopper): AB, abdomen; ANT, antennæ; E, eye; M, mouth; P, pads on feet; T, thorax; OV, ovipositor.

ber of tiny hooks and pads, by means of which the foot is fitted for clinging to the swaying grass stalks.

Wings. The membranelike wings, when spread out, show differences in structure. Notice the many veins. The outer pair, strong and narrower than the inner pair, serve to protect the inner wings, used for flying, which when at rest fold up like a fan. The animal, when in its natural habitat, is nearly the color of the grass on which it lives. The tough exoskeleton covering the body is formed largely of chitin, a substance somewhat like that which forms the horns of a cow.

Thorax. Three segments form the thorax, each bearing a pair of jointed legs, the two posterior segments bearing wings also. The Abdomen. The segmented abdomen does not bear appendages, but at the posterior end of the abdomen of the female are found paired movable pieces which together form the egg layer or ovipositor. The male grasshopper has a rounded abdomen.

h

Breathing Organs. Observation of the abdomen of a living grasshopper shows a frequent movement of the abdomen. 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 tracheæ 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 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

Cross section through the body of an insect: a, food tube; h, heart; n, nerve cord; t, tracheæ opening at t by spiracle.

part of the body cavity which is not filled with other organs. Oxygen is thus brought in contact with the blood by means of the tracheæ.

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 tracheæ pass directly into the muscles and other tissues. Here oxygen is passed into the tissues, and oxidation takes place when work is done.

Food-Taking and Blood-Making. The grasshopper is provided with two pairs of jaws, a forklike ventral-lying pair, the maxillæ, and a pair of hard cutting jaws, the mandibles. These parts are covered when not in use by two flaps, the upper and lower lips. The plant food taken by the grasshopper is held in place in the mouth by means of the little jaws, or the maxillæ, 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 it becomes part of the blood. As blood it is passed on to tissues, such as muscle, to make new material to be used in repairing that which is used up during the flight of the insect or to be 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. 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. The simple eyes probably are only able to perceive light and darkness. The separate units of the compound eye probably each give a separate impression of light and color. Thus a compound eye is most favorable for perceiving the movement of objects.

Other Sense Organs. The segmented feelers, or antennæ, have to do with the sense of touch and smell. The eardrum, or tympanum, of the grasshopper is found under the wing on the first segment of the abdomen. Covering the body and on the appendages, are found hairs (sensory hairs) which appear to make the insect sensitive to touch. Thus the armor-covered animal is put in touch with its surroundings.

Nervous System.

The nerve chain, as in the crayfish, 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. These are called sensory fibers.

Life History. The female red-legged locust lays its eggs by digging a hole in the ground with her 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. No great change in form occurs, the metamorphosis being said to be incomplete. In the fall most of the adults die, only a few surviving the winter. Relatives of the Locust. Among the near relatives are the brown or black crickets, cockroaches and "waterbugs," the katydid,