EXTERNAL ANATOMY OF INSECTS. 625 



brachial, here called the costal and mediastinal nervures, 

 covered by a supple membrane, which in a state of repose 

 becomes flaccid a : it is easily detected, being of a paler 

 colour than the nervures between which it lies ; this is 

 what I call the Phi alum ; we have before seen that it 

 exists also in Elytra and some Hemelytra b ,- but I have 

 not detected it in any other wings. 



I have before given you a sufficiently full account of 

 the alulce or winglets of Diptera c ,- and shall here only 

 observe that they are not confined to one particular tribe, 

 as has been usually imagined; but though sometimes 

 extremely minute, simple, and not easily detected, are an 

 universal distinction of the Order. 



Having thus endeavoured to elucidate the larger Areas 

 into which wings appear to be divided ; I shall next 

 say something on the smaller ones produced by the in- 

 tersection or ramification of the nervures ; these had 

 been named areolets {areola) several years before M. 

 Jurine's work, in which he calls them, I think improper- 

 ly, cellules [cellulce\ was published ; I therefore retain 

 the prior term. The general structure of the nervures 

 of the wings of insects having been before explained 11 , I 

 shall not here repeat what I then said ; but there is a curious 

 circumstance connected with it, particularly visible in the 

 wings of certain Hymenoptera, that I must not pass with- 

 out notice. If you examine attentively with a microscope 

 against the light the wing of any Nomada or A?idrena, 

 you will discover little transparent points in some of the 

 smaller transverse nervures that form the middle areolets, 

 in which the nervure becomes white and looks as if it 



a Sur le Vol. des his. c. i. 428. b See above, p, 600, 616. 



Vol. II. p. 358-, See above, p. 559. d Vol. II. p. 346— 



VOL. III. 2 S 



