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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.