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_. [Illustration: FIG. 93.—Transformation of the bumble bee, Bombus, showing the transfer of the 1st abdominal larval segment (_c_) to the thorax, forming the propodeum of the pupa (_D_) and imago; _n_, spiracle of the propodeum. _A_, larva; _a_, head; _b_, 1st thoracic; _c_, 1st abdominal segment. _B_, semipupa; _g_, antenna; _h_, maxillæ; _i_, 1st; _j_, 2d leg; _k_, mesoscutum; _l_, mesoscutellum; _m_, metathorax; _d_, urite (sternite of abdomen); _e_, pleurite; _f_, tergite; _o_, ovipositor; _r_, lingua; _q_, maxilla. ]