SUMMER AND WINTER EGGS. 123 



no further, the fact tliat many animals lay different eggs in the 

 summer from those they lay in the autumn may be dii'ectly re- 

 ferred to the difference of temperature at the two seasons, and 

 they are accordingly known as summer eggs and winter eggs. 

 But the formation, of eggs certainly does not only depend on 

 warmth, but also on food, both as to quantity and quality, as 

 well as on the chemical conditions of the surrounding medium, 

 the moisture of the air, and other conditions ; so that it must not 

 be assumed out of hand and as certain that this classification 

 (according to the seasons) of the two forms of eggs, which occur 

 among Crustacea, Insects, Rotatoria, and others, is invariably 

 appropriate j on the contrary, it needs direct experimental 



Fig. ZO.— Aphis Bcccahunga, a PlsLnt-Lonso. To the Irft the Aviiigless, and to tho right tho 

 winged, form of the female. 



proof in each separate case, and this, as I have said, has not 

 hitherto been obtained on any comprehensive method. 



In a few cases only have we experimental evidence that the 

 formation of a particular form of eggs (and mode of reproduction) 

 directly results from variations in temperature. Thus, for 

 instance, we know that the Aphis (fig. 30), a family of tho 

 Hemiptera well known as the Plant-Louse, sometimes in a 

 favourable summer produces as many as fourteen generations 

 by parthenogenesis, as the eggs then produced do not need ferti- 

 lisation by the male. At the beginning of the cold season males 

 first make their appearance, and the eggs fertiUsed by them live 

 through the winter, being known as winter eggs, till the next 

 spring, when the rising temperature developes and hatches the 

 embryo. Now, if the summer Aphides that multiply by par- 



