a, Ametabolous pupa of Cicada; b, caterpillar of tussock moth (Laria fascelina); c, larva of the poplar beetle (Chrysomela populi); d, larva of Sinex; e, larva of the common gnat.
There are, however, certain larvæ, as those of the Cicada, the crickets, the water-boatman (Notonecta), the cockroach, &c., which resemble the perfect insects in form, excepting that they are destitute of wings; but in the pupa state these appear in a rudimentary condition, at least in such species as have wings in the mature stage of existence. The pupæ are active and eat. Insects, the larvæ and pupæ of which are so similar to the adults, are termed Ametabolous (a, without, [Greek: metabolê], change); those the larvæ of which undergo changes of a marked character, Metabolous (Insecta ametabola and Insecta metabola, Burmeister).
Larvæ are remarkably small at first, but grow rapidly. The full-grown caterpillar of the goat-moth (Cossus ligniperda) is thus seventy-two thousand times heavier than when it issues from the egg; and the maggot of the blow-fly is, in twenty-four hours, one hundred and fifty-five times heavier than at its birth. Some larvæ have feet, others are without; none have wings. They cannot propagate. They feed voraciously on coarse substances; and as they increase in size, which they do very rapidly, they cast their skins three or four times. In defending themselves from injury, and in preparing for their change by the construction of secure abodes, they manifest great ingenuity and mechanical skill. The figures on the preceding page exemplify various forms of insects in this stage of their existence.
a, Pupa of a Water-Beetle (Hydrophilus); b, pupa of Sphinx Ligustri.
When larvæ are full grown, they cast their skins for the last time, undergo a complete change of form, excepting in the case of ametabolous larvæ, cease to eat, and remain nearly motionless. The inner skin of the larva now becomes converted into a membranous or leathery covering, which wraps the insect closely up like a mummy: in this condition it is termed Pupa, from its resemblance to an infant in swaddling bands. Nympha, or nymph, is another term given to insects in this stage;[M] moreover from the pupæ of many of the butterflies appearing gilt as if with gold, the Greeks called them Chrysalides, and the Romans Aureliæ, and hence naturalists frequently call a pupa chrysalis, even when it is not gilt. We shall see, as we proceed, the curious contrivances resorted to for protecting insects in this helpless state. The following are examples of insects in the imago, or perfect state.
Insects in the Imago or perfect state.
a, Nemopteryx coa, Leach.—b, Myrmeleon formicalynx, Fabricius.—c, Hesperia comma, Fabricius.—d, Nepa cinerea, Linnæus.
After a certain time, the insect which has remained in its pupa-case, like a mass of jelly without shape, is gradually preparing for its final change, when it takes the form of a perfect insect. This state was called by Linnæus Imago, because the insect, having thrown off its mask, becomes a perfect image of its species. Of some, this last portion of their existence is very short, others live through a year, and some exist for longer periods. They feed lightly, and never increase in size. The chief object of all is to perpetuate their species, after which the greater number quickly die. It is in this state that they exercise those remarkable instincts for the preservation of their race, which are exhibited in their preparations for the shelter of their eggs, and the nourishment of their larvæ.