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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



from the abdomen and, together with the precosta, becomes trans- 

 ferred to the metathorax, where the precosta forms a distinct post- 

 notal plate united laterally with the epimera. Thus it is usually found 

 that the segment which assumes the leading role in the flight mechan- 

 ism is provided with a phragma at both its anterior and its posterior 

 end. 



Since the tergal plates of the mesothorax and metathorax are the 

 intermediary elements in the wing mechanism between the dorsal mus- 



FiG. 4. — Diagram of the typical relation of the head and the prothorax in 



pterygote insects. 



Cv, cervix ; lev, 2cv, first and second lateral cervical sclerites ; DMcl, dorsal 

 longitudinal muscles ; H , head ; ilsg, 2lsg, first and second primary interseg- 

 mental lines ; iPh, first thoracic phragma ; Poc, postocciput ; PoR, postoccipital 

 ridge; PT , posterior arm of tentorium; ^1, ^"2, thoracic sterna; SA, sternal 

 apophyses ; Ss, spinisternites ; Ti, T2, thoracic terga ; VMcl, ventral longitudinal 

 muscles. 



cles of the segments and the bases of the wings themselves, it is clear 

 that a proper execution of their function depends upon the ability of 

 each to respond to the muscle tension at its ends with a dorsal curva- 

 ture reaching its maximum at the transverse line between the wing 

 bases. For this reason, as Weber (1924, 1925) has pointed out, the 

 terga of the wing segments are provided with "internal ridges so ar- 

 ranged that the force of the muscles will not merely deflect the an- 

 terior and posterior parts of the plates, but will be distributed gradu- 

 ally toward the iniddle from each extremity, and thus produce an even 

 dorsal flexion with its apex between the fulcra of the wings. 



