FLIES. 65 



form, 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 considerable streams are drained ; 

 . and the consequence 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 part of the Teme river in Hereford- 

 shire, where it receives the Clun: they are 

 now rarely seen. Most of the rivers of that- 

 part of England, as well as of the west, with; 

 the exception of those that rise in the still 

 uncultivated parts of Dartmoor and Exmoor, 

 are rapid and unfordable torrents after rain, 

 and, in dry summers, little more than scanty 

 rills; and Exmoor and Dartmoor, almost 

 the only considerable remains of those moist, 

 spongy, or peaty soils, which once covered 

 the greatest part of the high lands of Eng- 

 land, are becoming cultivated, and their 

 sources will gradually gain the same cha- 

 racter as those of our midland and highly- 

 F 



