OF INSECTS. 
1G3 
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withdrawn ; they are to be found in nearly all the 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, ( 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 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 spe- 
cies which are longest and most frequently on the 
wing. When the tracheae, 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 
wiiat have been termed by Leon 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 branch! ets of the caterpillar of the 
Cossus, and he detected 236 longitudinal ones, 1336 
transverse, and 232 detached, so that the body of 
