JNTERNAL ANATOMY OF INSECTS. 47 



and becomes ventral. Generally there is a pair of spi- 

 racles to each segment, and in those insects that have a 

 hypochondriack joint a there is often a spiracle in it. The 

 last segment of the abdomen is always without these 

 orifices, as is the basal one in Velia, Ranatra, and some 

 other bugs. A singular anomaly distinguishes the Li- 

 bellulina : they appear to have no abdominal spiracles b , 

 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 larvae in this tribe have seven or nine 

 on each side c , and Reaumur speaks of them as disco- 

 verable in the pupa d . 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 e . M. Latreille observed one on each 

 side of the base of the scale on the footstalk of the ab- 

 domen in ants f . 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 

 sometimes separate the dorsal from the ventral segments : 

 these folds allow of a considerable distention of the ab- 

 domen, which is probably necessary when all the air- 

 vessels are full. In a gravid Ichneumon I once saw it en- 

 larged to more than twice its natural size by means of 

 this membrane, through which the eggs were distinctly 



* VOL. III. p. 708. b Sprengel, Comment. 3, 



c Ibid. d vi. 398. 



' De Geer, ii. 635. f Fourmis, 22. 



