VARIATIONS IN THE NUMBERS OF ANIMALS 129 



There were similar aphid swarms in parts of England in the 

 year 1869,22 and doubtless many other ones which have 

 escaped being recorded. The aphid increase sometimes 

 affects the numbers of their normal enemies such as ladybirds 

 and syrphid flies, which may attain vast numbers at an aphid 

 maximum.22 



In some cases the fluctuations in numbers of insects are 

 extraordinarily regular in their rhythm. The most remarkable 

 of these are the cycles in numbers of cockchafers {Melolontha 

 vulgaris and M. hippocastani) in Central Europe. Every few 

 years these beetles appear in countless numbers, and the 

 heavy damage which they do to crops and forest trees has 

 directed much attention to the phenomenon, so that we possess 

 very good records for a number of years back. In some 

 districts there is a regular three-year cycle, in others a four- 

 year cycle. The maximum numbers have in some places 

 occurred exactly every three years for more than sixty years 

 at a time.^^ There is also a fifteen-year period in the size of 

 the maxima. 



3. Marine animals also are subject to considerable variations 

 in numbers from year to year, and these are well shown by the 

 careful notes made by Allee ^^ upon the marine littoral species 

 in the neighbourhood of Woods Hole, Massachusetts. An 

 echinoderm, Arhacia punctulata, was particularly abundant in 

 191 7, but there were practically none in 1918. By 1919 the 

 numbers had recovered again. The hydroid Tubularia, which 

 usually dominates the wharf- pile association in early June, 

 and dies away by early August, was so much influenced by the 

 early season of 1919 that it had entirely gone by early July 

 (instead of August) ; and in consequence many animals such 

 as the molluscs Columhella and Lacuna, which feed upon 

 Tuhulariay were very scarce that year, while the other members 

 of the July Tubularia association were also scarce or absent 

 (e.g. polychaetes of various genera). The failure of Tubularia 

 to be present in the right month affected the numbers of all 

 the other animals associated with it. 1920 was normal, but a 

 similar upset took place again in 1921. 



4. The work which has been done upon soil protozoa at 



K 



