158 ANATOMY AND PHYSIOLOGY 



atory tubes, which were not properly understood till 

 explained by M. Leon Dufour. Externally they have 

 the appearance of an ovipositor, and have often been 

 mistaken for such, although they exist in both sexes. 

 Each is composed of two pieces, grooved internally, 

 and meeting each other very closely at the edges, 

 thus forming an elongated siphon, which introduces 

 the air to two spiracles placed at the termination of 

 the two principal air-tubes running along each side 

 of the body. In this and analogous instances, the 

 other spiracles have become obsolete, their functions 

 being superseded by another kind of mechanism. 



In the cases hitherto alluded to, whether relating to 

 terrestrial or aquatic insects, the pneumatic mechanism 

 is adapted for the admission of air from the atmos- 

 phere ; it is next to be considered as fitted to separate 

 and imbibe the air which is mechanically mixed with 

 water. The species possessed of such an apparatus 

 are as exclusively aquatic as fishes, and the apparatus 

 itself is quite analogous to the respiratory mechan- 

 ism of these animals. The Gills or Branchiae are pro- 

 cesses of the epidermis, projecting from the body in 

 various forms, commonly resembling hairs or leaves, 

 and completely filled with delicate tracheae. The 

 exterior membrane is so fine as to allow these to be 

 seen through it. The manner in which the air is 

 extracted by their means is founded on the following 

 law of organic chemistry, recently discovered by M. 

 Dutrochet. If we enclose a liquid or a gas in a 

 cavity with permeable coats, a bladder for example, 

 and plunge this bladder into another liquid or gas 



