INFLUENCE OF TEMPERATURE ON INSECTS. 51 



sickly; the other by permitting the continued activity of 

 predaceous creatures. 



" These are very numerous. Moles continue at work in 

 mild winters, instead of burying themselves deep in the 

 ground ; and mice are constantly active. These small 

 mammalia destroy great numbers of Lepidopterous pupae, 

 and they abound in this district, as also do birds during 

 the winter in an extraordinary degree. As soon as severe 

 cold sets in to the north and east, the birds come down in 

 swarms to the open fields and sheltered hillsides of this 

 district, and it is hardly necessary to point them out as 

 most industrious and persevering destroyers of larvae. 

 Predaceous beetles and earwigs are generally on the alert 

 all through very mild winters; and although they probably 

 do not eat much at that time, and, indeed, are not very 

 plentiful in Pembrokeshire, they must destroy many larvae 

 and pupas, having little else to subsist upon. But I believe 

 that the mischief done by all these added together does not 

 equal that done by the nisei." * 



In his work on bark-beetles Eichhoff tells us that the 

 chief factors in the growth of these insects are good weather 

 and sufficient food. An uninterrupted dry, and hence hot, 

 summer checks the growth of the larvae, and retards their 

 speedy development, and more often prevents a repetition 

 of the broods than an uninterrupted wet and cold spring 

 and summer. Hence on account of the great heat and 

 drought many trees survive which otherwise would be in- 

 jured by the later broods of bark-beetles. The most favor- 

 able conditions for the increase of bark-beetles are doubtless 

 a warm early spring, a warm summer with frequent rains, 

 and a long mild autumn. 



It is well understood in central Europe that great num- 

 bers of may-beetles die during a cold wet May. After an 

 exceptionally warm and dry summer and autumn we may 



* Psyche, iv. 83; abstract from Ent. Month. Mag., June, 1882, 1. 



