598 EXTERNAL ANATOMY OF INSECTS. 
minence of the elytrum within it. The swtwral and anal 
angles exist only where the elytra are truncated at the 
apex. In this case the swtural is generally rectangular, 
and the anal rather obtusangular or rounded. The Hy- 
poderma is the fine soft membrane before noticed? that 
lines the underside of the elytra, the use of which is pro- 
bably to prevent injury to the wings from friction with 
their usually hard substance; this membrane is com- 
monly of either a pallid or brownish colour; but in some 
insects, as Staphylinus hybridus, murinus, &c., Buprestis 
Gigas, it is of a beautiful green or blue; and it exhibits 
the puncta, strig, and other modes of sculpture of the 
elytra very distinctly, the pores of which usually perfo- 
rate this membrane’. Just under the shoulders of these 
organs you may observe an oblong and sometimes round- 
ish spot, occasioned by the hypoderma in that part being 
particularly tense, and covering a cavity or pocket which 
appears 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‘, and named by me the pfz- 
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 
portion of an ellipse; taken separately, it inclines to that 
* See above, p. 401—. > Ibid. 397. 
© Sur le Vol des Ins. c. i. A28—. ¢. ii. 325, 
