FLIES. 55 



of eggs, and their size, probably affecting the 

 health of the animal, and compressing ge- 

 nerally the vital organs in the abdomen. They 

 are at least six weeks or two months after 

 they have spawned before they recover their 

 flesh : and the time when these fish are at the 

 worst is likewise the worst time for fly-fishing, 

 both on account of the cold weather and be- 

 cause there are fewer flies on the water than 

 at any other season. Even in December and 

 January there are a few small gnats or water- 

 flies on the water in the middle of the day, in 

 bright days, or when there is sunshine. These 

 are generally black ; and they escape the in- 

 fluence of the frost by the effects of light on 

 their black bodies, and probably by the ex- 

 treme rapidity of the motions of their fluids, 

 and generally of their organs. They are found 

 only at the surface of the water, where the 

 temperature must be above the freezing point. 

 In February a few double-winged water-flies 

 which swim down the stream are usually found 

 in the middle of the day, — such as the willow- 

 fly; and the cow-dung-fly is sometimes carried 

 on the water by winds. In March there 

 E 4 



