600 EXTERNAL ANATOMY OF INSECTS. 



is either rectangular or rounded, with a prominence of 

 the ely trum within it. The sutural and anal angles exist 

 only where the elytra are truncated at the apex. In this 

 case the sutural is generally rectangular, and the anal 

 rather obtusangular or rounded. The Hypoderma is the 

 fine soft membrane before noticed a that lines the under- 

 side of the elytra, the use of which is probably to prevent 

 injury to the wings from friction with their usually hard 

 substance; this membrane is commonly of either a 

 pallid or brownish colour ; but in some insects, as Sta- 

 phylinus hybridus, rrmrinus, &c, Buprestis Gigas, it is of 

 a beautiful green or blue ; and it exhibits the puncla, 

 stria, and other modes of sculpture of the elytra very 

 distinctly, the pores of which usually perforate this mem- 

 brane b . Just under the shoulders of these organs you 

 may observe an oblong and sometimes roundish spot, 

 occasioned by the hypoderma in that part being parti- 

 cularly tense, and covering a cavity or pocket which ap- 

 pears to be connected with the axis by the hollow part, 

 which I regard as representing the Costal Area ; this 

 pocket is evidently the analogue of a part in the wings 

 noticed by M. Chabrier c , and named by me the phi- 

 alum .- from its connexion with the axis by a channel, 

 this part in elytra should also seem destined to receive a 

 fluid to add to the weight of the margin and its means of 

 resistance. 



5. Shape. The shape of elytra is various; taken to- 

 gether, in which case, in describing insects, they are de- 

 nominated coleoptra, their most common form is more 

 or less oblong, or forming more or less a considerable 



a See above, p. 402—. *> ibid. 399. 



c Sun le Vol dcs Ins. i. c. 4X8—. c. ii. 325. 



