INTERNAL ANATOMY OF INSECTS. 51 
1 . To begin with, the Jirst. These are often found in 
insects which, during their two first states, live in the 
water. No better example, nor one more easy to be ex- 
amined, of this structure, can be selected, than the gnat 
(Culcx). You must have occasionally observed in tubs 
of rain-water, numerous little wriggline worm-like ani- 
mals, which frequently ascend to the surface ; there re- 
main a while, and then bending their head under the 
body rapidly sink to the bottom again. These are the 
larvae of some species of the genus just named ; and if 
you take one out of the water and examine it, you will 
perceive that it is furnished near the end of its body with 
a singular organ, which varies in length according to the 
species, and forms an angle with the last segment but 
one a . The mouth of this organ is tunnel-shaped, and 
terminates in five points like a star ; and by this it is 
usually suspended at the surface of the water, and pre- 
serves its communication with the atmosphere : in its in- 
terior is a tube which is connected with the trachea, and 
terminates in several openings, visible under a micro- 
scope, at the mouth of the organ. The points or rays of 
the mouth when the animal is disposed to sink in the 
water, are used to close it, and cut off its communication 
with the atmosphere. When the animal is immersed, a 
globule of air remains attached to the end of the tube, 
so that it is in fact of less specific gravity than that ele- 
ment, and it is not without some effort that it descends 
to the bottom ; but when it wishes to rise again, it has 
only to unclose the tube, and it rises without an effort 
to the surface, and remains suspended for any length of 
time. Its anal extremity is clothed with bunches of 
1 Plate XIX. Fig. 9. a. 
E 2 
