128 ANIMAL ECOLOGY 



examples of sudden variations in numbers. But the variations 

 are usually less spectacular, although hardly less important. If 

 we take any animal about which we possess a reasonable amount 

 of information, we shall find that its numbers vary greatly 

 from year to year. The common wasps (Vespa vulgaris and 

 V. germanica) are very abundant in some summers, and very 

 scarce in others. It is well known to collecting entomologists 

 that there are very good and very bad years for butterflies. 

 Although the data are scattered and have not so far been 

 properly correlated, a study of the existing literature leaves 

 no doubt that there is enormous annual variation in the numbers 

 of insects. A typical example is recorded by Coward ,^2 who 

 noted that the mottled umber moth {Hyhernia defoliaria) was 

 particularly abundant on oak trees in Cheshire in 191 8 and 19 19, 

 and especially in 1920, and caused much defoliation. At the 

 same time leaf-galls, especially the spangle (caused by Neuro- 

 terus lenticularis , a species of the Cynipidce), were unusually 

 plentiful in 1920. He also noted that the acorn-crop was 

 average in 191 8 and 1919, and failed entirely in 1920. These 

 changes in the trees and the insects must have had an appre- 

 ciable effect upon the other animals in the oak woods, since 

 jays and wood-pigeons eat the acorns ; starlings, chaffinches, 

 tits, and armies of warblers eat the caterpillars of the moth ; 

 while pheasants and other birds eat the spangle galls. This 

 example will give some idea of the way in which the numbers 

 of insects are always shifting from year to year, and how the 

 changes must inevitably affect a number of other animals 

 associated with them, often causing the latter in turn to vary 

 in numbers. 



2. In other countries similar variations in numbers occur. 

 The sugar-cane froghopper of Trinidad fluctuates com- 

 paratively regularly, with a period of four or five years. -^^^ 

 The bad " cotton- worm " (weevil) outbreaks in the United 

 States occur at intervals of about twenty-one years.^^^ Aphids 

 occur very numerously in some summers, e.g. in 1836 there 

 was a very big maximum of numbers in Cheshire, Derbyshire, 

 and South Lancashire and Yorkshire, and in one place the 

 swarms occurred over an area of twelve by five miles. ^^'^ 



