FLIES. 63 



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 



