The thorax, being composed of the three consecutive rings behind the head, falls naturally into three divisions—pro-, meso-, {100}and metathorax. These three segments differ greatly in their relative proportions in different Insects, and in different stages of the same Insect's life. In their more highly developed conditions each of the three divisions is of complex structure, and the sclerites of which it is externally made up are sufficiently constant in their numbers and relative positions to permit of their identification in a vast number of cases; hence the sclerites have received names, and their nomenclature is of practical importance, because some, if not all, of these parts are made use of in the classification of Insects. Each division of the thorax has an upper region, called synonymically dorsum, notum, or tergum; an inferior or ventral region, called sternum; and on each side a lateral region, the pleuron. These regions of each of the three thoracic divisions are further distinguished by joining to their name an indication of the segment spoken of, in the form of the prefixes pro-, meso-, and meta-; thus the pronotum, prosternum, and propleura make up the prothorax. The thoracic regions are each made up of sclerites whose nomenclature is due to Audouin.[21] He considered that every thoracic ring is composed of the pieces shown in Fig. 55, viz. (1) the sternum (B', _a_), an unpaired ventral piece; (2) the notum (A), composed of four pieces placed in consecutive longitudinal order (A'), and named praescutum (_a_), scutum (_b_), scutellum (_c_), and post-scutellum (_d_); (3) lateral pieces, of which he distinguished on each side an episternum (B', _c_), epimeron (_e_), and parapteron (_d_), these together forming the pleuron. We give Audouin's Figure, but we cannot enter on a full discussion of his views as to the thorax; they have become widely known, though the constancy of the parts is not so great as he supposed it would prove to be. Sometimes it is impossible to find all the elements he thought should be present in a thoracic ring, while in other cases too many sclerites exist. As a rule the notum of the meso- and metathoraces is in greater part composed of two pieces, the scutum and the scutellum; while in the pronotum only one dorsal piece can be satisfactorily distinguished, though a study of the development may show that really two are frequently, if not usually, present. On the other hand, one, or more, of the notal sclerites in some cases shows evidence of longitudinal division along the middle. The sternum or ventral piece, though varying greatly in form, is {101}the most constant element of a thoracic segment, but it has sometimes the appearance of consisting of two parts, an anterior and a posterior. The pleuron nearly always consists quite evidently of two parts, the episternum, the more anterior and inferior, and the epimeron.[22] The relations between these two parts vary much; in some cases the episternum