42 



THE SCOLYTID BEETLES. 



scutellar lobe, postscutellar processes, prephragma, and postphragma ; 

 also the pleural clavicula, clavicle, coracoidal, tergal, prescutal, scu- 

 tellar, and postscutellar processes, and connecting ligaments. 



Mesothoracic and. Metathoracic Wings. 



While there is a wide difference in the appearance and structural 

 details of the elytra and the hind wings of beetles, they are evidently 

 homologous and differ only in their modification in structure and 

 function. 



VENTRAL 

 Fig. 28.— Dendroctonus: Left tibiae, dorsal and ventral aspects. 12, simplex; IS, pseudotsugas; I4, piceaperda; 

 15, engelmanni; 16,borealis; 17,obcsus; 18,rufipennis. (Original.) 



Structure. — The wing consists of two layers of integument, the dor- 

 sal evidently rising from the tergites and the ventral from the epi- 

 pleurum (epimerum). Between these layers there is a system of tra- 

 cheation and circulation. The integument of the mesothoracic wing 

 or elytron is chitinous throughout, while that of the metathoracic 

 or hind wing, with the exception of the veins and basal pieces, is 

 membranous. 



Traclieation. — The same system of primary tracheae prevails in 

 both the elytra and the hind wings. In the former it corresponds in 



