6*26 EXTERNAL ANATOMY OF INSECTS. 



was interrupted, though in substance it seems continued: 

 these little points, somewhat resembling minute air bub- 

 bles detained in the tubes, are what M. Jurine, who first 

 discovered them, has, on that account, named bulla, which 

 he thus further describes: — " When the tube (of the ner- 

 vure) arrives at the spot where a bulla is to be formed, it 

 extends itself on all -sides in minute threads in the upper 

 membrane of the wing, losing its colour and tubular struc- 

 ture, which it resumes immediately after the formation 

 of the bulla 3 ." But if you look closely at them you will 

 find that there is always a slight fold of the wing that 

 cuts the nervure exactly at the bulla, and if the fold 

 changes its direction they accompany it ; their object, 

 therefore, is clearly to relax the tension so as to admit a 

 little motion where the fold is; consequently, rather than 

 bulla (bubbles), they should be denominated articulations. 

 A similar construction, but on a larger scale, may be ob- 

 served in the wings of Coleoptera b and some others, as 

 Psocus, where thefolds traverse the nervures. I shall next 

 make afew observations on the principal nervures; and first 

 a word upon their names. M. Jurine, being of opinion 

 that a striking analogy exists between the wings o{ insects 

 and those of birds, in which M. Chabrier seems to agree 

 with him, has named the nervures in the anterior margin 

 of the wings of the former, radius and cubitus, as corre- 

 sponding with the bones so named in the fore-arm of the 

 latter, and the plate which often terminates these ner- 

 vures in Hymenoptera, he names the carpus ; it may look 

 like presumption to differ from two such weighty autho- 

 rities, but as their observations seem to have been too 



» Jurine Hymenopt. 19. and t. v. b Plate X. Fig. 4. 



