When an insect first issues from the egg, it is called by naturalists _larva_, and, popularly, a caterpillar, a grub, or a maggot. The distinction, in popular language, seems to be, that _caterpillars_ are produced from the eggs of moths or butterflies; _grubs_ from the eggs of beetles, bees, wasps, &c.; and _maggots_ (which are without feet) from blow-flies, house-flies, cheese-flies, &c., though this is not very rigidly adhered to in common parlance. Maggots are also sometimes called _worms_, as in the instance of the meal-worm; but the common earth-worm is not a larva, nor is it by modern naturalists ranked among insects. [Illustration: _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. [Illustration: _a_, Pupa of a Water-Beetle (_Hydrophilus_); _b_, pupa of _Sphinx Ligustri_.]