11 



Feb. On the causes regulating the abundance of 

 insects at different times and places, Mr F. 

 Men-ifield. At the tropics and on Continents, 

 species are more numerous than in islands, 

 becaiise there ■v\^as more space, and the food 

 plants, and heat and moistiu-e were more 

 varied. In England there vras greater 

 variety in the number of individuals than of 

 species. lu some seasons, and at some 

 places, great numbers of particular species of 

 butteriiies or moths appeared, and under 

 apparently similar conditions of food, scarcely 

 any of the same kind were found for years 

 afterwards. Hence arose the enquiry, why ? 

 In the case of butterflies, some only appear 

 during a short period of the year and a part 

 of tlie day ; if then, the weather were un- 

 fa voiu-able or wet, it would prevent their 

 coming out, especially as five-sixths of the 

 butteriiies appeared only in sunshine. This 

 could not be said of moths in the same 

 degree, for the gi'eater number came out at 

 night ; but then they only appeared for a few 

 hours during a period of 10 to 14 days. It 

 could bo understood that if the weather were 

 cold or unfavoui'able at then' time of coming 

 out, but few would bo encountered. The 

 enemies of insects fluctuated. Cold winters 

 killed off large numbers of birds which preyed 

 on insects ; this might account for greater 

 numbers in some years. At present we knew 

 but little of the diseases of insects. There 

 was a provision made to keep up species, 

 both hj the great number of eggs and the 

 power by which pupae are able to wait over a 

 season. It was considered insect existence 



