OP INSECTS. 103 



These are, properly, only distended tubes, assuming 

 the form of pneumatic bags, inflating when the air is 

 admitted to them, and becoming flaccid when it is 

 withdrawn ; they are to be found in nearly all the 

 orders, but are most numerous and conspicuous among 

 the Orthoptera. In Truxalis nasutus no fewer thar 

 twenty exist in the abdomen, of an ovoid shape, and 

 lying transversely, ten on each side. They are so 

 much developed in certain Diptera, {Syrphidce, Taba- 

 nidce, c.) as to make the abdomen appear transpa- 

 rent, and as it were vitreous. Their principal use 

 is probably to diminish the specific gravity of tflfc 

 body, and thereby increase the power of night ; at 

 all events they will be attended with this effect, and 

 they have, in fact, been chiefly observed in the spe- 

 cies which are longest and most frequently on the 

 wing. When the trachese, instead of ramifying in 

 the usual manner, interlace each other, and unite 

 into matted bundles, as they have been observed to 

 do in certain Coleoptera and Hemiptera, they form 

 what have been termed by L6on Dufour parenchy- 

 matous tracheae. In Nepa these are oblong bodies, 

 placed immediately beneath the scutellum, (PI. III. 

 fig. 9, g, g^) free in the middle and fixed only at the 

 ends, and may almost be supposed, when taken in 

 connection with some accessory parts, to be a faint 

 representation of an incipient pulmonary organ. 



The indefatigable Lyonnet has had the patience to 

 count the tracheal branchlets of the caterpillar of the 

 Cossus, and he detected 236 longitudinal ones, 1336 

 transverse, and 232 detached, so that the body of 



