ANATOMY AND PHYSIOLOGY 45 



which in turn is proportionate to the size of the wings. When wings 

 are absent (as in Thysanura and Collembola) or the two pairs equal 

 in area (as in Termitidae, Odonata, Trichoptera and most Lepidoptera) 

 the meso- and metathorax are equal. If the fore wings exceed the 

 hind ones (Ephemeridae, Hymenoptera) the mesothorax is proportion- 

 ately larger than the metathorax; as also in Diptera, where no hind 

 wings occur. If the fore wings are small (Coleoptera) or almost 

 absent (Stylopidae) the mesothorax is correspondingly smaller than the 

 metathorax. The prothorax, which never bears wings, may be enlarged 

 dorsally to form a protective shield, as in Orthoptera, Hemiptera and 

 Coleoptera; or, on the contrary, may be greatly PCS/ 



reduced, as in Ephemerida, Odonata, Lepi- 

 doptera and Hymenoptera. In the primitive 

 Apterygota 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; P rinci P al scierites of a tho- 



' racic segment, em, epimeron; 



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



r ,T .-, i . T i i ,1 Pr, parapteron ; ps, postscutel- 



segments of the thorax being indicated by the lum . St scutum; sli scute iium; 

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

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

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

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

 59), namely and in order, pr&scutum, scutum, scuteiium and postscu- 

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

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

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

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

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

 sists chiefly of two scierites (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 

 epimeron. The former is divided into two scierites in Odonata and 

 both are so divided in Neuroptera. 



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

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

 (Forficulidaa). 



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



