VI 



THE ALDER FLY 



^75 



carrying along colourless corpuscles, can be seen by 

 the microscope to flow 

 at each pulsation of the 

 dorsal vessel into the 

 space which surrounds 

 the trachea. These ap- 

 pendages are tracheal 

 gills, very like those 

 of many other aquatic 

 larvae. There are no 

 tracheal gills to the 

 eighth and ninth seg- 

 ments. To the ninth 

 abdominal segment is 

 attached the long tail, 

 which appears to repre- 

 sent a tenth segment. It 

 is fringed, and contains 

 two tracheal tubes, much 

 resembling indeed two 

 ordinary tracheal gills 

 completely fu.sed to- 

 gether. In the usual 

 position the tracheal 

 gills curve upwards and 

 backwards, and it is 

 only in the dead larva 

 that they lie confusedly 



as in Fig. 86. It is 

 said in some books that 

 they can be employed 

 as fins for swimming, 



Fig. 86. — Larva of Sialis. The jointed re- 

 spiratory appendages of the abdomen 

 are in the living larva curved upwards 

 and backwards. 



T 2 



