STRUCTURE OF VVFM. 303 



found three pairs of conspicuous spiracles, occupying 

 the second, third, and fourth rings, and placed on 

 cylindrical tubes. It is worthy of remark that the 

 caterpillar spins a double cocoon, the outer of a thin, 

 and the inner of a close texture ; and when the re- 

 spiratory gills of the larva are cast off with the old 

 skin, the insect knows how to surround itself with an 

 atmosphere of air in the midst of the water where 

 it resides, the inner cocoon being impervious to 

 moisture. How it contrives to renew this air when 

 vitiated is not yet known, but that it derives it from 

 the water is proved by its always dying if removed 

 into the air.* 



We have formerly described the beautiful appara- 

 tus for respiration in the larvse of the gnat family 

 {CulicidcPy Latr.) ;t but this is rivalled by the or- 

 gans destined for the same purpose in their pupse, 

 which organs are situated in the head instead of the 

 tail. The pupa, in the same way, changes its po- 

 sition in the water, swimming with its head upwards 

 instead of downwards, as before. To enable it to 

 maintain this position, the pupa is further provided 

 with a fin-tail, like a fish, by which it can move itself 

 at pleasure in the water. It no longer, indeed, re- 

 quires to take food, but air is indispensable to it ; and 

 water being so unstable, were the pupa incapable of 

 swimming, it would seldom escape being drowned. Its 

 respiratory apparatus consists of two tubes, situated 

 behind the head, on what may be called the shoulders. 

 They are of a funnel shape, and project very sensibly, 

 though we should not be disposed to compare them, as 

 Reaumur does, to asses' ears. The transparency of 

 this pupa renders it easy to see in it the parts of the 

 gnat. 



An apparently more simple, though no less re- 

 markable apparatus for respiration, was observed by 



* De Gecr, Mem. i. 531. f See page 156. 



