Each segment consists of the _tergum_, _pleurum_, and _sternum_. In the prothorax these pieces are not subdivided, except the pleural; in such case the tergum is called the _pronotum_. The prothorax is very large in the Orthoptera and other generalized forms, as also in the Coleoptera, but small and reduced in the Diptera and Hymenoptera. In the winged forms the tergum of the mesothorax is differentiated into four pieces or plates (sclerites). These pieces were named by Audouin, passing from before backwards, the _præscutum_, _scutum_, _scutellum_, and _postscutellum_. In the nymph stage and in the wingless adults of insects such as the Mallophaga, the true lice, the wingless Diptera, ants, etc., these parts by disuse and loss of the wings are not differentiated. It is therefore apparent that their development depends on that of the muscles of flight, of which they form the base of attachment. The scutum is invariably present, as is the scutellum. The former in nearly all insects constitutes the larger part of the tergum, while the latter is, as its name implies, the small shield-shaped piece directly behind the scutum. [Illustration: FIG. 90.—Thorax of _Telea polyphemus_, side view, pronotum not represented: _em_, epimerum of prothorax, the narrow piece above being the prothoracic episternum; _ms_, mesoscutum; _scm_, mesoscutellum; _ms″_, metascutum; _scm‴_, metascutellum; _pt_, a supplementary piece near the insertion of tegulæ; _w_, pieces situated at the insertion of the wings, and surrounded by membrane; _epm″_, episternum of the mesothorax; _em″_, epimerum of the same; _epm‴_, episternum of the metathorax; _em‴_ epimerum of the same, divided into two pieces; _c′_, _c″_, _c‴_, coxæ; _te′_, _te″_, _te‴_, trochantines; _tr_, _tr_, _tr_, trochanters. _A_, tergal view of the mesothorax of the same; _prm_, præscutum; _ms_, scutum; _scm_, scutellum; _ptm_, postscutellum; _t_, tegula. ] The præscutum and postscutellum are usually minute and crowded down out of sight between the opposing segments. As seen in Fig. 90, the præscutum of most moths (Telea) is a small rounded piece, bent vertically down so as not to be seen from above. In Polystœchotes and also in Hepialus the præscutum is large, well-developed, triangular, and wedged in between the two halves of the scutum. The postscutellum is still smaller, usually forming a transverse ridge, and is rarely used in taxonomy. [Illustration: