I OO THORAX CHAP. 



and inetathorax. 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 con- 

 ditions 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 prpnotum, 

 prosternum, and propleura make up the prothorax. The thoracic 

 regions are each made up of sclerites whose nomenclature is due 

 to Audouin. 1 He considered that every thoracic ring is com- 

 posed of the pieces shown in Fig. 55, viz. (1) the sternum (B r , 

 a), an unpaired ventral piece ; (2) the notum (A), composed of 

 four pieces placed in consecutive longitudinal order (A r ), and 

 named praescutum (a), scutum (&), scutellum (c), and post-scutel- 

 lum (d) ; (3) lateral pieces, of which he distinguished on each 

 side an episternum (B r , c), epimeron (e), and parapteron (d}, these 

 together forming the pleuron. We give Audouiii'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. Some- 

 times 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 

 1 Ann, Sci. Nat. I. 1824, p. 97, etc. 



