various organs are, of course, greatly modified, some being more fully developed, and others becoming almost or wholly obsolete. Three principal types of form in the construction of the mouth prevail among haustellate insects, which have received the names of Promuscis, Proboscis, and Haustellum. Each of these is characteristic of a particular order, and will therefore demand a detailed examination in the subsequent exposition of the peculiar characters of these orders; at present it may be merely remarked, that the tube for the transmission of the fluids is generally formed by the elongation of the maxillæ, accompanied with some accessory parts, sometimes the upper, at other times the under lip forming a lengthened sheath to support and protect the other parts.
The next primary division of an insect which requires attention is that which may be described generally as lying between the head and abdomen, viz. the Thorax. It is a highly important part, in as much as it bears all the organs of motion, and determines in some degree the whole shape of the insect; the variety of its forms and the multitude of pieces entering into its composition, render the study of it rather difficult, and this difficulty has been increased by the complex nomenclature which has been assigned to its parts. A slight inspection suffices, for the most part, to shew that it is made up of three principal parts; the anterior of these, bearing the fore-legs, is termed the prothorax; the next, which supports the middle-legs and the fore-wings, (in such species as are provided with four wings,) is the mesothorax;
