EXTERNAL ANATOMY OF THE THORAX 



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In the more gener- 

 alized orders, notably 

 the Orthoptera, the 

 three segments are 

 distinct and readily 

 identified. 



Each segment con- 

 sists of the tergum, 

 pleurum, and sternum. 

 In the prothorax these 

 pieces are not sub- 

 divided, except the 

 pleural ; in such case 

 the tergum is called 

 the pronotum. The 

 prothorax is very 

 large in the Orthop- 

 tera and other gener- 

 alized forms, as also 

 in the Coleoptera, but 

 small and reduced in 

 the Diptera and Hy- 

 menoptera. In the 

 winged forms the 

 tergum of the meso- 

 thorax is differenti- 

 ated into four pieces 

 or plates (sclerites). 

 These pieces were 

 named by Audouin, 

 passing from before 

 backwards, the prca- 

 scutum, scutum, scutel- 

 lum,&u<lpostscuteUum. 

 In the nymph stage 

 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 night, of which they 



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