FLIES. 55 



metamorphosis and quit the bottoms of the 

 rivers, and the mud and stones, for the sur- 

 face, and the light and air. The brown fly 

 usually disappears before the end of April, 

 likewise the grannam; but of the blue dun, 

 there is a succession of different tints, or 

 species, or varieties, which appear in the 

 middle of the day all the summer and 

 autumn long. These are the principal flies 

 on the Wandle — the best and clearest 

 stream near London. In early spring these 

 flies have dark olive bodies; in the end of 

 April and the beginning of May they are 

 found yellow; and in the summer they be- 

 come cinnamon coloured; and again, as the 

 winter approaches, gain a darker hue. I 

 do not, however, mean to say that they are 

 the same flies, but more probably successive 

 generations of Ephemerae of the same spe- 

 cies. The excess of heat seems equally 

 unfavourable, as the excess of cold, to the 

 existence of the smaller species of water in- 

 sects, which during the intensity of sunshine 

 seldom appear in summer, but rise morning 

 and evening only. The blue dun has in 



