ii6 NATURAL HISTORY OF AQUATIC INSECTS CH. 



composed of a row of feathered bristles. This no 

 doubt increases the sculling effect of the rapid 

 strokes of the tail, by which the larva executes its 

 sudden movements. Each feathered bristle is in- 

 serted by a forked base, and the whole row is con- 

 nected by the cuticle, which can be thrown into 

 folds like the valves of a concertina, or separated 

 at pleasure. The folds are independent in their 

 action, and some of them may be closed while others 

 remain open. This arrangement was first described 

 to me by Mr. Hammond. There is a similar vertical 



FIG. 32. Details of fin of larva of Corethra. 



fin on the tail of the Gnat-larva : both have evidently 

 a common origin, and are used in the same way. 



The head of the Corethra larva narrows forward. 

 When the larva first emerges from the egg it is 

 provided with a single pair of minute eyes, which are 

 little more than pigment-spots. Subsequently a 

 larger and many-facetted eye forms by the side of 

 this. Weismann believes that this eye is directly con- 

 verted into that of the fly. The antennae project in 

 front, and the mouth-parts lie beneath. The antennae 

 are not, as in most Insects, merely sensory organs, 



