20 ON THE ANATOMY OF THE FLY. 



insect reaches maturity, this layer becomes extremely attenuated, 

 and forms a structureless membrane very difficult to trace ; it is 

 best seen in the tracheae of the larva. 



The spiral fibre has been described as a distinct coat between 

 the other coats, but it is really part of the internal. M. 

 Blanchard* even supposed that a circulation of the nutritive fluid 

 takes place in insects, in a channel between the coils of the spiral 

 and the two layers of membrane, which he supposed to be con- 

 nected with the dorsal vessel ; but no such channel exists. f 



It is extremely difficult to say at what point in the division of the 

 tracheal vessels the spiral disappears. Most of the vessels which 

 are as small as the 10,000th part of an inch, exhibit no trace of 

 transverse marking, but a very distinct double contour is observ- 

 able in their walls ; whilst in others of the same magnitude dis- 

 tinct transverse markings are visible with high powers, at irreg- 

 ular intervals. The smallest divisions have probably 110 spiral, 

 and consist, I believe, entirely of the external coat, which 

 assumes the form of a structureless membrane. j 



The larger tracheaB of the head and thorax of the perfect fly 

 differ very considerably in structure from those already described, 

 and form large reservoirs of air. Numerous sacculi which lie 

 between the muscles, mere dilatations of these tracheae, open into 

 them, and the main lateral trachea) terminate in a pair of large sacs 

 at the base of the abdomen, the pulmonary sacs c f the French 

 naturalists. The walls of these vessels and their sacculi are 

 composed entirely of a brittle membrane, corresponding to the 



* Comp. Rend. xxiv. 



t It is needless to repeat the arguments against such a circulation, which 

 even if the spiral were a distinct layer, are fully given in thexxviii. and 

 xxix. vols. of the Comp. Rend. 



* In the stag beetle the irregularity of the spiral in the smaller tracheal 

 tubes is easily seen. 



