ANATOMY | 45 



\\hiili in turn is proportionate to th< gfc \\ 1 



t >as in Thysaniira and ( 'ollemlmla ) or tin- tWO p 

 in area a> in Termil ila-, ()<l(na!a, Trii liopt <-ra and most Lepidoj 

 the im-so- and metathorax arc equal. 1 1 the !' .< c<-d tin- 



hind ones (EphemeiicUe, Hymenoptera) the iin-solhora\ i> proportion 

 ately larger than the inetall in Diptera, where no hind 



win^s oceur. If the fore win^s are small < ( 'oleopt era ) or a! 

 absent (Stylopid;e) the mesothorax i> . .rre^pondin.L'ly smaller than the 

 metuthorax. The prothorax, which never I.e. . may he enlarged 



!ly to form a protective shield, as in Orthnptera. Hemiptera and 



Cbleoptera;or, on the contrary, may be greatly p $ si 



reduced, as in Ephemerida, Odonata, Lepi- 

 doptera and Hymenoptera. In the primitive 

 A])terygota the prothorax may become re- 

 duced (many Collembola) or slightly enlarged P r ' 

 (Lepismd) . 



The dorsal wall of a thoracic segment is 

 termed the notum, or tergum; the ventral wall, FIG. 58. Diagram of the 

 the sternum, and each lateral wall, a pleuron; ^^2^^^ 

 the restriction of these terms to particular . episternum; p, praescutum; 



..-i i j. j t ,1 Pr, parapteron; ps, postscutel- 



segments of the thorax being indicated by the lum . Si SC utum; */. scuteiium; 

 prefixes pro-, meso- or meta-. These parts are " sternum.- After COMSTOCK. 

 usually divided by sutures into distinct pieces, or sclerites, as represented 

 diagrammatically in Fig. 58. Thus the tergum of a wing-bearing 

 segment is regarded as being composed of four sclerites (tergites, Fig. 

 59), namely and in order, preescutum, scutum, scuteiium and postscu- 

 tcllum. The scutum and scuteiium are commonly evident, but the 

 two other sclerites are usually small and may be absent. According 

 to Snodgrass, the tergum consists primitively of a single sclerite, the 

 notum; the four sclerites having arisen as specializations; being not 

 always homologous in different orders of insects. Each pleuron con- 

 sists chiefly of two sclerites (pleurites, Figs. 58 and 60), separated from 

 each other by a more or less oblique suture. The anterior of these 

 two. which joins the sternum, is termed the episternum; the other, the 

 cpimcron. The former is divided into two sclerites in Odonata and 

 both are so divided in Neuroptera. 



The sternum, though usually a single plate, is in some instar 

 divided into halves, as in the cockroach, or even into five sclerites 

 1 Forliculidae). 



To these should be addvd the patagia of Lepidoptera a pair of 



