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. 6o8, 609). 
They are named, beginning with the first or 
most cephalic, prccscutum (prae-scu'tum) (a), 
scutum (scu'tum) ( b) % scutellum (scu-tei'lum) (r), 
and postscutellum (post-scu-teriuin) (< 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.) 
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 
