INTERNAL ANATOMY OF INSECTS. 55 
respiratory horns of aquatic pupe are usually situated,— 
they appear to terminate in a transparent point?. The 
pupa of a Tipula observed by Reaumur, instead of two 
has only one of these respiratory organs, in the form of a 
very fine hair proceeding from the anterior end of the 
trunk, and considerably longer than the animal itself. 
It is observable that aquatic insects that come to the 
surface of the water for air, receive it at the anus, often 
carrying it down with them as a brilliant bubble of quick- 
silver. ‘This is generally done by means of spiracles in 
perfect insects, but in the water-scorpion tribe in that 
state respiration is by means of a long hollow tube, con- 
sisting of two concavo-convex pieces which apply exactly 
to each other. ‘This is found in both sexes, and there- 
fore cannot be an ovipositor, as some have thought¢. 
These respiratory organs, however, are not invariably 
confined to aquatic larvee and pupze, tor those of some 
aphidivorous flies have anal ones, and the pupa of Dol:- 
chopus nobilitatus, or a fly nearly related to it, which is 
terrestrial, has likewise a pair of long sigmoidal ones on 
the back of the trunk?. The pupa also of the rat-tailed 
larvee just noticed as having four horns, resides under 
the earth, the insect being only aquatic in its grub state. 
2. J am next to consider those respiratory appendages 
by which aquatic insects, since they do not come to the 
surface for that purpose, appear to extract air for respi- 
ration from the water; so that they may be looked upon 
im some degree as analogous to the gills of fishes: there 
is, however, this difference between them-—in fishes, the 
* De Geer vi. 395—. t. xxiv. f. 16, 18. d. Vat. Vinfol; 2. 
© De Geer iii. 367. t. xviii. f. 1, 2, 9. 
“ [bid. vi. 36. 194—. tii. f. 2, 3. s. 
