How Insects Eat. The jaws of insects (Fig. 4) are horny processes situated on each side of the mouth. They are variously toothed, so as to tear the food, and move horizontally instead of up and down as in the horse. The act of taking the food, especially if the insect be carnivorous in its habits, is quite complex, as not only the true jaws, but the accessory jaws (maxillæ, Fig. 5, a, upper, b, under side of the head of a young beetle; at, antennæ, md, mandible, mx, maxillæ, mx[1], labium) and the feelers (palpi) attached to the maxillæ, and the under lip (labium) are of great service in enabling the insect to detect its food both by the senses of touch and smell. The maxillæ are in the fully grown beetle (Fig. 6) divided into three lobes, the outermost forming the palpus, and the two others forming sharp teeth, often provided with hairs and minute brushes for cleansing the adjoining parts; these strong curved teeth are used in seizing the food and placing it between the grinders, where it is crushed, prepared for digestion and swallowed. Fig. 7 represents the mouth parts of the humble bee. (b, upper lip; d, mandible; e, maxilla; f, maxillary palpus; g, tongue; ih, labium and tabial palpi; k, eye.)
7. Mouth parts of a Humble Bee.
The alimentary canal passes through the middle of the body, the stomach forming usually a simple enlargement. Just before the stomach in certain insects, as the grasshopper, is a gizzard armed with rows of powerful horny teeth for finely crushing grass.
Insects eat almost incredible quantities of food when young and growing rapidly. Mr. Trouvelot tells us in the "American Naturalist" that the food taken by a single American Silk-worm in fifty-six days is equal to eighty-six thousand times its primitive weight! On the other hand, after the insect has finished its transformations, it either takes no food at all, as in the May fly, or merely sips the honey of flowers, as in the butterfly, while the June beetle and many others like it eat the leaves of trees, and the tiger and ground beetles feed voraciously on other insects.
How Insects Walk. In man and his allies, the vertebrates, the process of walking is a most difficult and apparently dangerous feat. To describe the mechanics of walking, the wonderful adaptation of the muscles and bones for the performance of this most ordinary action of life, would require a volume. The process is scarcely less complex in insects. Lyonnet found 3,993 muscles in a caterpillar, and while a large proportion belong to the internal organs, over a thousand assist in locomotion. Hence the muscular power of insects is enormous. A flea will leap two hundred times its own height, and certain large, solid beetles will move enormous weights as compared to the bulk of their bodies.
8. Larva of a beetle (Photuris).
In walking, as seen in the accompanying figure (Fig. 8), three legs are thrown forward at a time, two on one side and one on the other.
Flies and many other insects can walk upside down, or on glass, as easily as on a level surface. A fly's foot, as in most other insects, consists of five joints (tarsal joints), to the last one of which is appended a pair of stout claws, beneath which is a flat, soft, fleshy cushion or pad, split into two (sometimes three) flaps, beset on the under surface with fine hairs. A part of these hairs are swollen at the end, which is covered with "an elastic membranous expansion, capable of close contact with a highly polished surface, from which a minute quantity of a clear, transparent fluid is emitted when the fly is actively moving." (T. West.) These hairs are hence called holding, or tenent, hairs. With the aid of these, but mainly, as Mr. West insists, by the pressure of the atmosphere, a fly is enabled to adhere to perfectly smooth surfaces. His studies show the following curious facts. "That atmospheric pressure, if the area of the flaps be alone considered, is equal to just one-half the weight of a fly. If the area covered by the tenent hairs be added, an increase of pressure is gained, equal to about one-fourth the weight of a fly. This leaves one-fourth to be accounted for by slight viscidity of the fluid, by the action I have so often alluded to, which may be called 'grasping,' by molecular attraction, and, doubtless, by other agents still more subtle, with which we have at present scarcely any acquaintance."