502 



THE STUDY OF INSECTS. 



The tergum or dorsal wall of each thoracic segment is composed 

 typhically of four sclerities. These are ar- 

 ranged in a linear series (Figs. 608, 609). 

 They are named, beginning with the first or 

 most cephalic, prccscutum (prae-scu'tum) (a), 

 scutum (scu'tum) {b), scutellum (scu-tel'lum) (c), 

 and postscutellum (post-scii-tel'lum) {d). In 

 the prothorax of beetles the sutures between 

 these four sclerites are obsolete, the pronotum 

 appearing to be composed of a single sclerite (Fig. 609, 14). The 

 mesonotum and metanotum are usually covered by the wings and 



Fig. 608.— Diagram of the 

 fixed parts of a thoracic 

 segment. 



Fig. 609 — Dorsal aspect of the thorax of a 

 beetle; 14, pronotum; 15, mesonotum; 16, 

 metanotum. Dysticus, dissected. (After 

 Audouin.) 



Fig. 610.— Figure of a beetle, 

 showing the scutellum. 



elytra, excepting the scutellum of the mesothorax, which is usually 

 quite conspicuous, appearing as a more or less triangular piece be- 

 tween the elytra at their base (Fig. 610). As this is the only one of 

 the three scutella that is usually seen, it is termed the scutellum. 



Each pleurum or lateral part of each thoracic segment is composed 

 chiefly or entirely of two sclerites, arranged more or less obliquely. 

 The first of these (Fig. 608, e) is the episternum (ep-i-ster'num), the 

 second (Fig. 608,/) is the epimeron (ep-i-me'ron). A third sclerite 

 (Fig. 608,^) is sometimes present near the dorsal end of the epister- 

 num; this is the parapteron (pa-rap'te-ron.) Paraptera have not been 

 found in the prothorax. In beetles the paraptera of the mesothorax 



