120 THE VARIATION OF ANIMALS IN NATURE 



differences are likely to be present as well, even if requiring 

 refined methods for their detection. Food- or host-selection 

 is the feature in which physiological differentiation has been 

 most studied, but Thorpe (1930) also notes differences in 

 the susceptibility of scale-insects to fumigation, and differences 

 in song may also be mentioned. Owing to the difficulty of 

 the investigation not very many examples have been really 

 exhaustively examined, but it is clear that various stages can 

 be traced from forms which differ only in physiology to those 

 which also differ morphologically, eventually to such an 

 extent that they are regarded as closely allied species. 



Hachfeld (1926) records that in the bee, Trachusa byssina, 

 different individuals use different plant-leaves with which to 

 build their nests. In different localities different plants are 

 the main source of material. 



Hackett and Missiroli (1931) have investigated factors 

 leading to the reduction of malaria in various areas in Europe. 

 It is practically certain that the disappearance of this disease 

 in some localities [e.g. parts of Italy) is due not to preventive 

 measures but to the establishment of definite zootrophic races 

 of Anopheles which attack domestic animals but not human 

 beings. Another instance of purely physiological races may 

 be found in the wasp Tiphia popilliavora. This is being im- 

 ported into the United States from the East to control the 

 introduced Japanese Beetle {Popillia japonica) , which has proved 

 a serious pest. Hollo way (193 1) finds that the forms of this 

 wasp found in Korea, China and Japan respectively cannot 

 be separated into geographical races on the basis of their 

 structure, but that they are so different physiologically that 

 three strains must be recognised if economic measures are 

 to be successful. The strains differ principally in their tem- 

 perature-relations and their consequent fitness to survive in 

 the climate of the United States. The strains differ, for 

 instance, in their length of life, developmental period and in 

 the minimum temperature for mating. As a result of such 

 differences the Chinese race is able to maintain itself only at 

 the extreme southern border of the area now infested. For 

 control in the greater part of eastern U.S.A. the Japanese race 

 is alone suitable. 



Fulton (1925) finds races of tree-crickets, Oecanthus, which 

 differ in song, method of oviposition and habitat, but not in 



