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.
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.
The episterna and epimera are in certain groups, Neuroptera, etc., further subdivided each into two pieces (Fig. 102). The smaller pieces, hinging upon each other and forming the attachments of the muscles of flight, differ much in shape and size in insects of different orders. The difference in shape and degree of differentiation of these parts of the thorax is mentioned and illustrated under each order, and reference to the figures will obviate pages of tedious description. A glance, however, at the thorax of a moth, fly, or bee, where these numerous pieces are agglutinated into a globular mass, will show that the spherical shape of the thorax in these insects is due to the enlargement of one part at the expense of another; the prothoracic and metathoracic segments being more or less atrophied, while the mesothorax is greatly enlarged to support the powerful muscles of flight, the fore wings being much larger than those appended to the metathorax. In the Diptera, whose hinder pair of wings are reduced to the condition of halteres, the reduction of the metathorax as well as prothorax is especially marked (Fig. 91).
The patagia.—On each side of the pronotum of Lepidoptera are two transversely oval, movable, concavo-convex, erectile plates, called patagia (Fig. 92). On cutting those of a dry Catocala in two, they will be seen to be hollow. Cholodkowsky[[19]] states that they are filled with blood and tracheal branches; and he went so far as to regard them as rudimentary prothoracic wings, in which view he was corrected by Haase,[[20]] who compares them with the tegulæ, regarding them also as secondary or accessory structures.
The tegulæ.—On the mesothorax are the tegulæ of Kirby (pterygodes of Latreille, paraptera of McLeay, hypoptère or squamule), which cover the base of the fore wings, and are especially developed in the Lepidoptera (Fig. 90, A, t) and in certain Hymenoptera (Fig. 95, c).
The external opening of the spiracles just under the fore wings, is situated in a little plate called by Audouin the peritreme.