is conspicuously the more anterior, while in others the epimeron is placed much above it, and may extend nearly as far forwards as it. It may be said, as a rule, that when the sternum extends farther backwards than the notum, the epimeron is above the episternum, as in many Coleoptera; but if the sternum be anterior to the notum, then the episternum is superior to the epimeron, as in dragon-flies. We would here again reiterate the fact that these "pieces" are really not separate parts, but are more or less indurated portions of a continuous integument, which is frequently entirely occupied by them; hence a portion of a sclerite that in one species is hard, may in an allied form be wholly or partly membranous, and in such case its delimitation may be very evident on some of its sides, and quite obscure on another. [Illustration: FIG. 55.—Mesothorax of _Dytiscus_, after Audouin. A, notum; A', pieces of the notum separated: _a_, praescutum; _b_, scutum; _c_, scutellum; _d_, post-scutellum: B, the sternum and pleura united; B', their parts separated: _a_, sternum; _c_, episternum; _d_, parapteron; _e_, epimeron.] {102}The parapteron of Audouin does not appear to be really a distinct portion of the pleuron; in the case of _Dytiscus_ it is apparently merely a thickening of an edge. Audouin supposed this part to be specially connected with the wing-articulation, and the term has been subsequently used by other writers in connexion with several little pieces that exist in the pleural region of winged Insects. The prothorax is even more subject to variation in its development than the other divisions of the thorax are. In the Hymenoptera the prosternum is disconnected from the pronotum and is capable, together with the first pair of legs, of movement independent of its corresponding dorsal part, the pronotum, which in this Order is always more or less completely united with the meso-thorax; in the Diptera the rule is that the three thoracic segments are closely consolidated into one mass. In the majority of Insects the prothorax is comparatively free, that is to say, it is not so closely united with the other two thoracic segments as they are with one another. The three thoracic rings are seen in a comparatively uniform state of development in a great number of larvae; also in the adult stages of some Aptera, and among winged insects in some Neuroptera such as the Embiidae, Termitidae, and Perlidae. In Lepidoptera the pronotum bears a pair of erectile processes called patagia; though frequently of moderately large size, they escape observation, being covered with scales and usually closely adpressed to the sides of the pronotum.