104 



GNATS OR MOSQUITOES — CHAPTER VI 



calls for especial notice ; the " aspiratory vesicle," or, as I 

 prefer to speak of them, the pneumatic sacs. This structure 

 is not, as has been stated, in any way peculiar to the gnats, 

 but is, I find, often even better developed in the midges, and 

 other allied insects ; moreover it is not a median, but a paired 

 structure and I believe that its size, which has hitherto 



Pig. 24. — To Illustrate Ceetain Points in the Anatomy op the 

 Respiratory System, and of the Pneumatic Vesicle. 



A, camera lucida tracing of the distribution of the main tracheae in the 

 abdomen of C. fatigans (Wiod.) x 15 diams ; B, one of the abdominal stig- 

 mata and the tracheis springing from it, x 75 diams ; C, dissection of a 

 midge, to show the relations of the pneumatic vesicle; p, pneumatic vesicle; 

 V, mid-gut, X 15 diams ; D, base of one of the pneumatic vesicles nearly 

 emptied of air, x 200 diams ; E, air bubbles contained in fibres of the 

 pneumatic vesicles as seen under -t^th apochromat. objective, x about 1,000 

 diams. It will be observed that the air bubbles are of irregular and varied 

 forms, and not uniformly spherical, as they would be if simply suspended in 

 a mucilaginous fluid. 



been absurdly understated, is inversely proportional to the 

 size and power of the wings in the different species in 

 which it is found. 



The reason it has been hitherto mistaken for a single 

 median sac is that, owing to the pressure of the contained 

 air, the two sacs come to lie one behind the other. They 

 have no true organic connection with the oesophagus and 

 the only reason why they have been supposed to be 

 diverticula of the digestive canal is that they are often 



