FIG. 91.—Thorax of the house-fly: _prn_, pronotum; _prsc_, præscutum; _sc′_, mesoscutum; _sct′_, mesoscutellum; _psct′_, postscutellum; _al_, insertion of squama, extending to the insertion of the wings, which have been removed; _msphr_, mesophragma; _h_, balancer (halter); _pt_, tegula; _mtn_, metanotum; _epis_, _epis′_, _epis″_, episternum of pro-, meso-, and metathorax; _epm′_, _epm″_, meso- and meta-epimerum; _st′_, _st″_, meso- and metasternum; _cx′_, _cx″_, _cx‴_, coxæ; _tr′_, _tr″_, _tr‴_, trochanters of the three pairs of legs; _sp′_, _sp″_, _sp‴_, _sp‴′_, _sp‴″_, first to fifth spiracles; _tg′_, _tg″_, tergites of first and second abdominal segments; _u′_, _u″_, urites. ] The metathorax is usually smaller and shorter than the mesothorax, being proportioned to the size of the wings. In certain Neuroptera and in Hepialidæ and some tineoid moths, where the hind wings are nearly as large as those of the anterior pair, the metathorax is more than half or nearly two-thirds as large as the mesothorax. In Hepialidæ the præscutum is large and distinct, while the scutum is divided into two widely separated pieces. The postscutellum is nearly or quite obsolete. The pleurum in each of the three thoracic segments is divided into two pieces; the one in front is called the _episternum_, since it rests upon the sternum; the other is the _epimerum_. To these pieces, with the sternum in part, the legs are articulated (Fig. 89). Between the episterna is situated the breastplate or _sternum_, which is very large in the more primitive forms, as the Orthoptera, and is small in the Diptera and Hymenoptera. [Illustration: FIG. 92.—Prothorax of _Geometra papilionaria_: _n_, notum; _p_, pleura; _st_, sternum; _pt_, patagia; _m_, membrane; _f_, femur; _h_, a hook bent backwards and beneath, and connecting the pro- with the mesothorax.—After Cholodkowsky. ]