INSECTS MODIFIED BY CIRCUMSTANCES. 77 



vary considerably. Insects, more than any other 

 creatures, are immediately influenced by climate and 

 locality, and are specially modified in species, numbers, 

 brilliancy of colouring, etc., and time of appearance, 

 by external circumstances. In certain localities peculiar 

 species may abound, while some of those tribes in- 

 habiting other districts in countless hosts may be 

 entirely wanting. I would not expect to meet with the 

 green drake fly on the cold streams of the mountains, 

 nor the sooty moorland dun fly in our warm and sheltered 

 valleys : while the same species which are common to 

 both will appear probably a month earlier in the latter 

 locality than the former. Moreover, I have frequently 

 observed the natural fly of the same species vary con- 

 siderably in size and colour on different rivers in different 

 parts of the country : while in some cases, different 

 individuals of the same tribe will be found to differ 

 much among themselves, in regard to size and colour, 

 even in the same locality. For instance, individuals 

 of the yellow sally, and little yellow Mayfly, may be 

 found exhibiting not only great difference in size, but 

 ranging in all shades of colour, from a dirty white or 

 light buff, to a brilliant yellow, on the same day. The 

 general rule is, that those insects which appear earliest 

 in the season, and those which have most recently 

 emerged from the chrysalis state, are generally of a 

 soberer colour than those succeeding them at a later 

 period, or have been longer in the winged state. Under 

 this consideration, the angler should be careful to provide 

 himself with artificial representatives that largely vary 



