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. [Illustration: 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. _CHAPTER II._ STRUCTURES FOR PROTECTING EGGS.--MASON-WASPS; MASON-BEES; MINING-BEES.