EXTERNAL ANATOMY OF THE THORAX 



87 



In the more gener- 

 alized orders, notably 



the Orthoptera, the ^ 

 three segments are 



distinct and readily ', 



identified. 



o 



Each segment con- 5 



sists of the tergum, s 



pleurum, and sternum. | 



In the prothorax these f 



pieces are not sub- ~. 



divided, except the * 



pleural ; in such case J 



the tergum is called | 



f 



the pronotum. The 5 



prothorax is very g 



large in the Orthop- 3 



tera and other gener- | 

 alized forms, as also 



CD 



in the Coleoptera, but ^ 



small and reduced in | 



the Diptera and Hy- g 



menoptera. In the I 



winged forms the g 



tergum of the meso- g 



thorax is different!- | 



ated into four pieces I 



or plates (sclerites). | 



These pieces were 



named by Audouin, 2 



passing from before | 



backwards, the prce- z 



scutum, scutum, scutel- | 



lum, andpostscutellum. 



In the nymph stage g 



and in the wingless ? 



adults of insects such 



as the Mallophaga, 



the true lice, the 



wingless Diptera, ants, etc., these parts by disuse and loss of the 



wings are not differentiated. It is therefore apparent that their 



development depends on that of the muscles of flight, of which they 



