THE BREEDING SEASON 11 



animal may produce as many as fourteen consecutive generations of 

 young by parthenogenesis, the ova undergoing development without 

 being fertilised by the male. At the beginning of the winter male 

 plant-lice make their appearance and fertilise the eggs, which develop 

 in the succeeding spring. Reaumur, however, by artificially main- 

 taining a constant summer temperature, succeeded in producing 

 more than fifty parthenogenetic generations of plant-lice, all descended 

 from a single mother. 1 



Morgan, however, describes some observations which seem to 

 indicate that the change is not merely due to temperature. He 

 shows, for example, that the sexual forms of Aphis may appear in 

 the autumn before the onset of the cold weather, and conversely that 

 many individuals may continue to reproduce parthenogenetically, 

 until finally they perish from the cold. Morgan suggests that the 

 alternation in the mode of reproduction may depend upon changes 

 which take place in the food-plant in the autumn, instead of being 

 solely a temperature effect. He shows also that there is evidence for 

 the conclusion that in the genus Chermes, in which the alternation of 

 generations occurs between the fir-tree and the larch, the conditions 

 existing on the larch are those that call forth the sexual forms. 2 



It has been supposed that the change in the environment is also 

 responsible for determining the sexes in aphids. Miss Stevens, 

 however, has recently shown that what appears to be a change in sex 

 should rather be regarded as a change from the parthenogenetic to 

 the sexual mode of reproduction. 3 According to this view the sex of 

 each individual is determined by the character of the gamete or 

 gametes by which it is developed. The supposed influence of food 

 and external conditions upon sex-determination in various kinds of 

 insects, and other animals, is discussed at some length in a future 

 chapter of this work (Chapter XV.). 



Semper pointed out long ago that the occurrence of reproduction 

 (or of the particular mode of reproduction), with insects as with 

 other animals, depends, among other things, upon the nature of the 

 diet, upon the chemical conditions of the surrounding medium, upon 

 the moisture of the air, or upon other circumstances which are 

 often unknown. Thus, failure to breed in a new environment 

 is experienced by many Lepidoptera. For example, Death's-Head 

 hawk moths, which are commonly blown over to this country from 

 the Continent, but do not breed here continuously, deposit their eggs 

 on young potato plants, and these develop into moths which emerge 

 in the autumn. These moths, however, are quite infertile, so that, 



1 Semper, loc. cit. 2 Morgan, loc. cit. 



3 Stevens, "Studies in the Germ-Cells of Aphids," Carnegie Institution 

 Report, Washington, 1906. 



