Because the pupa may be lodged with greater safety. This covering is in some composed of threads of silk. Sometimes only one or two threads are required to keep the pupa in a proper position ; in others, the silk is woven into cloth, or so matted together, as to resemble paper. The matter, of which these cases or cocoons are fabricated, is prepared by two long tubes, which take their rise in the abdomen, enlarge as they approach the head, and terminate by a duct, which opens under the labium, or lower lip. By pressing the orifice of this duct to one place, and then to another, the larva draws out the tenaceous threads. - Fleming.
Because the larva, when so inclosed, resembles an infant in swaddling hands. From the pupae of many of the butterflies appearing gilt as if with gold, the Greeks called them Chrysalides, and the Romans Au-relioe, and hence naturalists frequently call a pupa chrysalis, even when it is not gilt.
Because they derive their nourishment from their stores of fat.
Because their digestive organs are of much greater dimensions than when arrived at maturity; and in the condition of larvae, insects possess a variety of members, as legs, suckers, hairs, and even stigmata, (respiratory organs) which they do not possess in their maturity. - Fleming.
Because they may serve as breathing holes, through which the fluids of the animal become aerated.
The three portions of the body of insects, the trunk, head, and abdomen, in the different tribes, exhibit very remarkable combinations. In some of the crus-taceous animals, these portions are incorporated in the dorsal (or back) surface of the body. In some of the arachnidae, (or spider genus) the head and trunk are niched, while, in others, the head appears to be distinct, while the trunk and abdomen are incorporated. These modifications are extensively employed in the methodical distribution of the groups.
Because the young, being hatched, are destined to feed on those substances. Thus, the butterfly attaches her eggs to a leaf; the flesh-fly deposits her's upon carrion ; while others insert them into the young of other insects. - Fleming.
Because they may thus be protected from the destructive influence of rain, and other accidents.
Among other peculiarities of the propagation of insects, many, as the cochineal worm, the laced-flea, etc. become of an enormous size during pregnancy: thus, in the white ant, it has been calculated that the abdomen of the female, when about to lay her eggs, is 2000 times larger than previous to impregnation.