624 EXTERNAL ANATOMY OF INSECTS. 
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 Andrena, 
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 
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 
structure, which it resumes immediately after the forma- 
tion of the bulla*.” 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» and some others, as 
Psocus, where the folds traverse the nervures. I shall next 
make a few 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 
of insects and those of birds, in which M. Chabrier seems 
to agree with him, has named the nervures in the anterior 
* Jurine Hymenopt. 19. and 7. v. > PratE X, Fie. 4. 
