THE ENTOMOLOGIST. 277 



that the cause of this variableness in abundance is to be 

 found in the rapid power of increase which insects possess, 

 and in the great vicissitudes to which they are exposed. 

 Thus, taking two hundred as the average number of eggs 

 produced by a single pair of moths or butterflies (I have 

 known Amphydasis prodroniaria lay nearly a thousand), it is 

 obvious that, if all lived, a single season would suffice to 

 make a scarce insect common. Upon the average of years 

 doubtless only one in the hundred would in general survive ; 

 but the margin which the great number of the progeny 

 allows for variation in abundance is enormous, viz., a hun- 

 dred-fold in one season, ten thousand-fold in two seasons 

 (representing, in double-brooded insects, one year only). Of 

 course the actual numbers produced from a single pair are 

 always far within this limit ; but the causes which determine 

 what proportion shall finally arrive at maturity are so variable 

 that the result must be variable too. Let me enumerate 

 some of these causes. 1. Suitable laying and pairing time. 

 — Most insects are summer insects. Now a summer insect, 

 in its perfect state, does not in general live many days; if 

 therefore the weather be unfavourable during its brief 

 existence, the insect has not much chance of continuing its 

 species. As to moths, I know it is almost impossible to get 

 some of them to pair, except at a particular time of the 

 night, and on many nights (such as cold and dry ones) not at 

 all. Few butterflies will fly at all, except in sunshine ; hence 

 I am not surprised at the undoubted fact that the wet and 

 almost sunless summer of I860 caused for years afterwards a 

 sensible scarcity among many species of diurnal Lepidoptera. 

 A butterfly may be said to be born in a muslin dress, and to 

 have "nothing else to wear;" so that if the weather prove 

 wet all its life, it is natural that it should stay at home, and 

 " grow, live and die in single blessedness." 2. Frosts in 

 Spring. — I suppose there are more larvae feeding at the end 

 of May than at any other time of year. The slaughter 

 among them must be prodigious when snow and frost, with 

 biting north-east winds, come on, as they did in the Epsom 

 week, 1867, blackening and killing the tender leaves and 

 twigs of the oak trees. Minor causes, such as an increase in 

 the number of Ichneumons, birds, or other foes to insects, 

 floods in winter, &c., will occur to every one. Now imagine 



