468 



ANIMAL RESPIRATION 



portion to the depth at which they are able to exist, while at the 

 same time their air-tubes are more and more feebly developed. 

 Some of them, on the other hand, are altogether destitute of 

 red pigment and live near the surface, being at the same time 

 well provided with air-tubes. The larvse may breathe entirely 

 through the skin, but are usually provided with thread-like gills 

 on the eleventh and twelfth rings of the body. After a time 

 the midge-larva passes into a quiescent pupa-stage, during which 

 breathing is effected by means of a pair of tufted gills situated at 

 the front end of the body. These contrast markedly with the two 

 breathing trumpets which the pupa of an ordinary gnat bears in 

 a similar position (see p. 442), and which admit ordinary air into 

 the breathing-tubes. 



Sand-Midges i^SiimiliadcE). — These notorious blood-suckers are 

 found in most parts of the world, sometimes occurring in vast 



swarms, and bestowing their 

 unwelcome attentions on both 

 human beings and cattle. The 

 cylindrical larva (fig. 573) 

 possesses a sucker at the hin- 

 der end of its body, by means 

 of which it holds on to a 

 stone or other firm body, and 

 is maintained in a vertical 

 position. Two beautiful tufted 

 gills are borne upon the head 

 and are kept in constant move- 

 ment, which serves a double 

 purpose, promoting respira- 

 tion and also bringing a supply 

 of food within reach of their 

 owner. Later on the larva 

 becomes a pupa, which is 

 contained in a sort of open 

 case, from which the head end freely projects. Upon this are 

 carried two bunches of long tubular gills (fig. 573), by which 

 breathing is carried on until the perfect insect is fully formed and 

 ready to make its escape into the air. 



Fig. 573. — Stages in life-history of a Sand-Midge [Sininlid], 

 enlarged (actual size shown by short lines), a, Larva, B, Pupa. 

 c, Pupa- case. 



