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 



