Influence of Weather on Insects. 123 



of such flies as are then in existence. The dates 

 already given for the appearance of the natural 

 insects will hold good in ordinary seasons, but it 

 must be remembered that the times of their devel- 

 opment and the terms of their existence are alike 

 subject to the influence of heat and cold. Speak- 

 ing generally, and in view of the whole season, I 

 may say that an early spring and a warm summer 

 will accelerate their advent and shorten their stay ; 

 and that a backward season will retard their de- 

 velopment and prolong the period of their exist- 

 ence. But all the flies of a particular month are 

 not alike influenced by the same weather. What 

 may be favourable for one class may be quite the 

 reverse for another. The characteristic flies of 

 spring and autumn come out with heat and sun- 

 shine, but this cannot be said for all the flies of 

 summer. Some of them including the whole 

 family of duns require a comparatively cold day 

 for their appearance ; while others, as the sand-fly, 

 the black mote, the green-tail and May-fly, do not 

 appear but on hot bright days ; and even in the case 

 of these latter there is a " distinction with a differ- 

 ence." 



It is not enough, therefore, that the angler, in 

 selecting his cast for the day, should merely con- 

 sult his season's list of flies ; he should ascertain 

 for himself, by actual observation, what insects are 



