FLIES. 57 



of the blue dun, there is a succession of dif- 

 ferent tints, or species, or varieties, which ap- 

 pear 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 ge- 

 nerations of ephemerae of the same species. 

 The excess of heat seems equally unfavour- 

 able, as the excess of cold, to the existence 

 of the smaller species of water-insects, which, 

 during the intensity of sunshine, seldom ap- 

 pear in summer, but rise morning and evening 

 only. The blue dun has in June and July a 

 yellow body ; and there is a water-fly which in 

 the evening is generally found before the moths 

 appear, called the red spinner. Towards the 

 end of August, the ephemerae appear again 

 in the middle of the day : a very pale small 



