INTERNAL ANATOMY OF INSECTs. 47 
letter*, and becomes ventral. Generally there is a pair 
of spiracles to each segment, and in those insects that 
have a hypochondriack joint® there is often a spiracle in 
it. The last segment of the abdomen is always without 
these orifices, as is the basal one in Vela, Ranatra, and 
some other bugs. A singular anomaly distinguishes the 
Libellulina: they appear to have no abdominal spiracles *, 
yet I have seen the abdomen of Libellula depressa when 
reposing, contract and dilate alternately, from whence it 
follows that this part is concerned in respiration, Spren- 
gel says that the larvee in this tribe have seven or nine 
on each side4, and Reaumur speaks of them as disco- 
verable in the pupa®. I have carefully examined the 
pupa-skin of most of the genera of Libellulina, under a 
powerful magnifier, but have not succeeded in discover- 
ing any thing like these organs in the abdomen. ‘The 
Ephemera and probably the other Neuroptera have ab- 
dominal spiracles‘. M. Latreille observed one on each 
side of the base of the scale on the footstalk of the abdo- 
men in ants’. Generally the abdominal spiracles may 
be described as planted in the crust of the insect; but in 
many cases their station is in the membranous folds, 
which I have therefore named the pulmonarium, that some- 
times separate the dorsal from the ventral segments: these 
folds allow of a considerable distention of the abdomen, 
which is probably necessary when all the air-vessels are 
full. Ina gravid Ichneumon I once saw it enlarged to 
more than twice its natural size by means of this mem- 
brane, through which the eggs were distinctly visible.— 
* Vor. ILI. p. 706—. > Ibid. p. 709. 
© Sprengel, Comment. 3. i " Tbid. 
© vi. 398, f De Geer, ii. 635. & Fourmis, 22. 
