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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being superseded by another kind of mechanism.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 GUIs 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 
