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tubes, assuming the form of pneumatic bags, inflating when the air is admitted to them, and becoming flaccid when it is withdrawn; they are to be found in nearly all the orders, but are most numerous and conspicuous among the Orthoptera.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 than twenty exist in the abdomen, of an ovoid shape, and lying transversely, ten on each side. They are so much developed in certain Diptera, (Syrphidæ, Tabanidæ, &c.) as to make the abdomen appear transparent, and as it were vitreous. Their principal use is probably to diminish the specific gravity of the body, and thereby increase the power of flight; at all events they will be attended with this effect, and they have, in fact, been chiefly observed in the species which are longest and most frequently on the wing. When the tracheæ, 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 Léon Dufour parenchymatous tracheæ. In Nepa these are oblong bodies, placed immediately beneath the scutellum, (Pl. 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
