FLIES. G3 



nution of the quantity of fly. There were 

 always some seasons in which the tempera- 

 ture was favourable to a quantity of fly; for 

 instance, fine warm days in spring for the 

 grannam, or brown fly; and like days in 

 May and June for the alder fly, May fly, 

 and stone fly ; but I should say that within 

 these last twenty years I have observed a 

 general diminution of the spring and autum- 

 nal flies, except in those rivers which are fed 

 from sources that run from chalk, and which 

 are almost perennial — such as the Wandle, 

 and the Hampshire and Buckinghamshire 

 rivers; in these streams the temperature is 

 more uniform, and the quantity of water 

 does not vary much. I attribute the change 

 of the quantity of flies in the rivers to the 

 cultivation of the country. Most of the 

 bogs or marshes which fed many consider- 

 able streams are drained ; and the conse- 

 quence is that they are more likely to be 

 affected by severe droughts and great floods 

 — the first killing, and the second washing 

 away the larvae and aurelias. May flies 

 thirty years ago were abundant in the upper 



