INTERNAL ANATOMY OF INSECTS. 51 



1. To begin with thejirst. 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 wriggling 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 trachece^ 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 



* PLATE XIX. FIG. 9. . 

 E 2 



