The mouths of mandibulate insects are sometimes called petfeet, and those which exhibit a different character, imperfect. The terms, however, are improper—each kind is perfect, according to the purposes for which it is to be used. Yet a correspondence of structure maybe traced, so that the parts of the mandibulate mouth may be recognized under various and very remarkable modifications in the mouths of insects which ,ive by suction. Thus the filaments which form the proboscis of butterflies are the maxillae excessively lengthened, and the cutting parts of the mouth of the flea arc the mandibles and maxillm. The proboscis of flies represents the lower lip.
The alimentary canal of insects is usually more or less convoluted. Between the mouth and the proper digestive stomach, it sometimes exhibits a crop (honey-bag of bees) in insects which live by suction, and this is either a dilatation of the lower part of the gullet or a lateral vesicle; sometimes a gizzard, with muscular walls, often armed with horny pieces, for trituration of food. The stomach is of a very elongated form. The liver is represented by long slender bile-tubes, four or more in number. which wind around the intestine, and pour their secretion into it, where it originates from the stom ach. The salivary glands are generally similar tubes.
The eyes of insects are of two kinds—simple or stemmata, and compound or composite. See EYE. Some insects have only simple eyes (ocelli), some have only compound eyes; but the greater number have two large compound eyes on the sides of the head, and three small simple eyes between them. Compound eyes occur in insects only in their mature or perfect state; the eyes of larvae are simple.
The antennae (q.v.) are generally regarded as organs of touch. They are attached to the head in front of the eyes, and are always present, and always two in number. They exhibit a vast variety of different forms. Insects make much use of their antennae to investigate surrounding objects by contact, although, if this is their sole use, it is not very easy to assign any probable reason for some of their forms; but there is not mitch plausibility in the conjectures which assign to them a part in the exercise of the senses of hearing and smell, although these senses and taste are evidently enjoyed by insects, or at 'mast by insects in great perfection, and their particular seat and organs are not well ascertained. The sense of smell appears to be of great importance to insects in guiding them to their food. The sexes are distinct in all insects, and very remarkable differences are often exhibited by the males and females of the same species, in size, color, and the form and structure of .parts that have no immediate connection with time reproductive system. What are called neuters in some tribes are imperfectly developed females. The connection of the sexes takes place only once in the lives of insects, and a remarkable provision is made in the female for the consequent fertilization of eggs that in some—as bees—continue for a long time afterwards to be successively developed.
Insects are generally oviparous; a few are ovoviviparous. The aphides afford an instance of what has been called the alternation of generations. The greater number of insects take no care of their eggs after depositing them, and many themselves pass out of existence before the eggs are hatched; the chief part of the lives of insects being gen erally spent in their immature states, and their brief existence in a perfect state serving mainly for the propagation of their species. Thus many insect tribes disappear entirely on the approach of winter, their eggs awaiting the warmth of spring or summer to be hatched. The case is very different, however, with bees, ants, earwigs, and some others, which carefully tend and rear their young.—The number of eggs laid by insects is very various, but often very great. The flea, indeed, only lays about 12, and many dipter ous and coleopterous insects about 50; but the silkworm produces from 500 to 2,000; a single queen bee is supposed to lay 40,000 or 50,000 in a season; and the female termite or white ant, laying about 60 eggs in a minute and for a period of very considerable though unknown duration, exceeds as to the number of her eggs any other known animal in the world.
The eggs of insects are generally white, yellow, or green; they are of very various shapes—round, cylindrical, conical, lenticular, etc.; they are sometimes smooth, some times beautifully sculptured.
The stage of development at which insects come forth from the egg is very different in different tribes• in some they appear as footless worms; in others have rudiment ary feet, but still with very little power of locomotion; in others, besides little claws representing the six feet of the perfect insect, there are on the abdominal segments of the wormlike body fleshy tubercles serving as feet ; in others still, the legs are well developed, and the insect on issuing from the egg differs little from the perfect insect, except in the want of wings; whilst, finally, in a comparatively small number (lice, etc.), there is no obvious difference except in size. Similar differences of time degree of development appear in the mouth, eyes, and other organs. Hence the subsequent changes by which the mature state is reached are very different in degree; and insects being primarily divided into those which undergo and those which do not undergo metamorphosis, some of the former are commonly spoken of as undergoing complete and others incomplete metamorphosis. In the first state of insect life the insect is called a larva (q.v.). Grubs, caterpillars, and maggots are the larvie of different orders of insects. From this state it passes into that of a pupa (q.v.), or nymph—a chrysalis or aurelia is the pupa of a lepi dopterous insect—and finally it becomes an imago, or perfect insect.