Page:Popular Science Monthly Volume 39.djvu/236

224 place. The female takes care to place her eggs in conditions which will permit her progeny to find food for themselves from birth; but her solicitude usually ends at this point, while she leaves it to the rays of the sun to do the rest—a happy arrangement, for in most cases the parents die before the hatching. The embryo is developed rapidly under the influence of the ambient heat, and in a short time breaks the shell or springs the cover.

From the egg issues a being very little or not at all like its parents. Insects, indeed, undergo, before attaining full development, a series of transformations which are designated as metamorphoses. These metamorphoses are complete or incomplete; and there are even what we might call hypermetamorphoses, as in the case of the cantharides, the evolutional life of which, only recently well understood, is much more complicated than that of most other insects. The metamorphosis is said to be incomplete when the forms of the young insects, on coming out from the egg, are like those of the adult. Insects whose metamorphoses are complete come from the egg in the form of larvæ.

The growth of the insect all takes place during the larval state, and is very rapid. The superficial envelope soon becomes too small for the body it contains. So, at determinate periods of their existence, larvæ change their skin, or, to speak more accurately, burst the integument which envelops them, and shed it. This transformation constitutes the molting, which is repeated from three to eight times, according to the species. During these periods of transition, the larva, as if ill, loses its insatiable appetite, ceases to eat, and becomes stationary.

Insects of incomplete metamorphosis likewise acquire their full development through successive moltings. Each of the moltings is attended by a corresponding perfectionment of some part of the organism.

Larvæ of insects often have a horned head, with jaws that permit them to crush food. This conformation also occurs in the larvæ of insects which in the adult state have the mouth organized for suction. Some larvæ of Dipteræ, however, have the fore part of the body terminated by a pointed and retractile appendage; they might be spoken of as acephalous.

After the head come the rings, very like one another, and not exceeding a dozen in number. In these there are three varieties of structure: larvæ having only articulated legs (Fig. 2, upper); those having both articulated and membranous legs; and apodous larvae, or those having no legs (Fig. 2, lower).

In the first variety the articulated legs, which end in one or two claws, are attached to the three rings immediately following the head, each ring supporting a pair of legs. The same rings form, at a later stage, the thoracic casing of the perfect insect.