scutellum sometimes extends over the anterior edge of the metathorax; in some instances it entirely conceals its surface. An impressed line sometimes separates an anterior portion from the rest, and in such a case this is called the Postscutellum. As the internal cavity of the mesothorax is separated from that of the metathorax, by a partition denominated the Mesophragma, so the latter is separated from the abdomen by a similar parietal wall, called the Metaphragma, a small space only being left for the passage of the necessary organs. The ventral plate of the metathorax is the metasternum, a portion which most commonly assumes a shape similar to that of its counterpart above, but it varies greatly in some of the orders. On each side of the thoracic segment now under consideration, are found two horny pieces, which Kirby and Spence have named plura and periplura. In the latter, the halteres or poisers of Dipterous insects are situated.
We have now to consider the organs attached to the thorax, by which motion is effected either on the land or in the air, viz. the legs and wings. The former are never wanting, and their number is invariably the same; the latter are often absent, and when present, their number varies from two to four. Such kinds as possess four wings, are called Tetrapterous insects; those with two, Dipterous; and when altogether wanting, the species are said to be Apterous. The point at which the wings are inserted, has been already indicated; they articulate to the thorax by means of numerous small pieces. Viewed relatively to each other, the
