The metamorphoses or transformations of insects have always been regarded with great admiration. A worm inhabiting a muddy pool becomes a winged creature that sports in the air. A crawling caterpillar that ravenously devours some kind of herbage with its horny jaws, eating vastly more in proportion to its size than an ox, is converted into a splendid butterfly, flitting from flower to flower and feeding only on nectareous juices. The intermediate or pupa state only adds to the wonder. Time caterpillar, after several moltings, or changes of skin, and when it has attained its utmost size, ceases from eating. perhaps fixes itself under a leaf, becomes incased in a horny covering. as in a second egg, and from this it finally breaks forth a moth or a butterfly. Many larvfe also, when about to change into the pupa state, spin cocoons (q.v.) in which they envelop themselves by means of spinnerets on the under-lip. through which a viscid secretion passes in fine threads which harden into silk. But whilst the pupae of many insects are motionless, or nearly so, and eat no food whatever, the pupae of other insects, as dragon flies, are active and voracious. The intermediate or pupa state often differs little from the larva state, except in the of wings, or from the perfect state, except in the wings being merely rudimentary and still mnfit, for flight.
An opinion at one time prevailed that the successive envelopes of the larva were all contained from time beginning within the first, within them the covering of the pupa, and. within it the perfect insect. This extraordinary fancy has given place to the belief, established on sufficient observation, that the envelopes which time growing larva succes sively casts off are merely a hard, thick. extravascular, and unextensile epidermis; that the jaws, claws, etc., of the larva, with which it parts when it becomes a pupa, in the. ease of insects undergoing complete metamorphosis are connected with the epidermis; and that the covering of pupa is a new secretion. Discoveries, however, do not render less marvelous, but only more admirable, the changes which take place. Of these, some of the most important are in the organs of the mouth, the digestive organs, and the nervous system, It is not certain that any insect has a voice or cry, although the origin of the sounds produced by some of them, as the plaintive, squeaking note of the death's-head moth, i4 not known. The sounds of which we'do know the origin are not produced by the mouth or throat. See GRASSHOPPER, DEATII-WATCH, and GiCADA.—The ktininZiny or buzzing of insects during flight has been commonly ascribed to the extremely rapid vibrations of their wings. Burmeister, however, supposes it to be produced by vibratory in the respiratory spiracles of the thorax, acted upon by the forcible emission of air during the violent muscular action necessary for flight.
Insects are all animals of small size, and many of them are minute. The largest species are tropical, and insects of all sizes abound in warm far more than in cold climates. The insects of the polar regions are comparatively few, and are to be seen only during summer; those of them whose whole existence is .not comprised within a ' single year spending the winter, as very many insects of temperate climates also do, in a state of torpidity. All insects are very fond of heat, and many which do not become completely torpid in cold weather become partially so. It is only in warm weather that insects display their greatest activity. As to their geographical distribution, insects are found in all countries, to the utmost alpine and polar limits of vegetable life. Many kinds are peculiar to particular climates and countries. The insects of the Malayan archipelago and of Australia, like their other natural productions, are generally very different from those of other parts of the world. The insects of elevated mountainous
regions within the tropics generally resemble those of the temperate and frigid zones, but are seldom the same. The multitude of species of insects is very great. The species of coleopterous insects alone, or beetles, are more numerous than all those of vertebrated animals together.
A few insects are important for their usefulness to man, and a greater number for the injuries which they inflict. Of the former, bees and silkworms deserve to be first named; and after them the cochineal insect and cantharides or blistering-flies. There are a few others to which we are indebted for substances useful in medicine and the arts, cs kermes, lac, galls, etc. Of the injuries inflicted by insects, the most serious arc those caused by the destruction of herbage and crops, as by the ravages of locusts, of some kinds of cater pillars, and of numerous tribes of coleopterous and dipterous insects. See CORN-FLY, TURNIP7FLY, etc.
The primary division of insects into those which do not and those which do undergo metamorphosis (ametab.ilia and metabolia of Leach), has been . already noticed. The former are divided into the orders thysanura (q v.) and parasita (q.v.i or anoplura, and are all included in the order aptera (see AprEnous INSECTS) or wingless insects of Unmet's. The insects undergoing metamorphosis, which are far more numerous, are divided into two great groups, mandibulata and haustellata, the former having the mouth fitted for mastication, the latter for suction. The mandOndata form the univer sally recognized orders coleoptera, orthoptera, including dermuptera of some entomolo gists, neuroptera, and hymenoptera; the haustellata form the orders hemiptera, including homoptera of some, lepidoptera, strepsiptera, diptera, and suctoria (aphamptera of sonic). See these heads.
Fossil causes conspire to make the remains of insects in the strati fied rocks comparatively rare, such as their possession of the power of flight, their soft and speedily decomposing bodies, and the extent to which they are preyed upon by other animals. That they were abundant during some periods is, however, very evident. In the lower lias several bands of limestone occur, which, from the abun. dance of insect remains contained in them, have been called "insect limestone." They are crowded with the wing-cases of several genera of coleoptera,•and insects almost entire are frequently found. The strongly nerved wings of some neuroptera are beau tifully perfect. In the cocene strata at Auvergne, a considerable thickness of limestone is formed entirely of the indusia or cases of the aquatic larva of a neuropterous insect. Amber from tertiary strata often abounds in insects captured and inclosed while this petrified gum was in its primitive fluid condition, and now made permanent in the transparent stone, with every minute detail of structure beautifully preserved.
The oldest strata in which insects remains have been observed belong to the carbon iferous period. The remains consist of fragments of neuroptera, orthoptera, and cole optera.
The lower has insects belong to various orders; they are generally of a small size, apparently indicating a temperate climate. In the upper lias they are not infrequent; a few specimens have been found in the oolite proper; and in the wealden both land and water forms occur. None have been noticed as yet in the deep-sea rocks of the cretaceous period, but in the newer tertiary strata they are common, especially in the -; amber from the lignite beds of Germany and in the cavern deposits. It is worthy of remark that no new forms have been observed; all are either referred to living genera or placed in new yet nearly allied genera.