very difficult to say at what precise point of time the transition from the one to the other takes place; and in such species as are perfectly apterous, (such as Cimex Lectularius and many Phasmidæ,) a like difficulty may he experienced in distinguishing the pupa from the imago.
The second division comprises the pupæ of all the orders not enumerated above, and constitutes, therefore, by far the most extensive of the two. Although they agree in the general property of being wholly unlike the larvæ, and in being incapable of eating and walking, they yet offer not unimportant distinctions among themselves in several particulars. Some have all the limbs encased in separate membranous envelopes, and therefore lie free, although closely ad-pressed to the body; others are covered with a hard skin or horny case, on which the different parts can be traced by their forming projecting lines; while in others the integument is opaque and uniform, concealing every thing within it. To the first of these belong the entire orders Coleoptera and Hymenoptera, as well as certain tribes among the Neuroptera and Diptera. These were called by Linnæus incomplete pupæ, by Lamark mumiæ coarctatæ, and by Burmeister pupæ exaratæ. The second form the obtected pupæ of Linnæus (pupæ larvatæ, Burm.) and comprehend all those of the order Lepidoptera, which are usually known by the term chrysalis. The third subdivision includes the larger proportion of the order Diptera, which were the pupæ coarctatæ of Linn., the case being nothing more than the dried skin of
