188 REPRODUCTION 



In insects there are still further complications. In the case 

 of the plant lice Aphis we have a winter egg which produces 

 the female in the spring. This will continue to produce 

 generation after generation parthenogenetically. These are 

 wingless; but quite unexpectedly and suddenly a generation 

 will appear which has wings, and these wings undoubtedly 

 aid in spreading and scattering these most devastating 

 enemies to plant life. In an allied form of Phylloxera, which 

 has done so much to destroy the vines in France, we even have 

 three different kinds of female instead of two (the wingless 

 and the winged), as in Aphis ; these females reproduce partheno- 

 genetically before the male at last turns up. 



Fig. 70. The Apple Aphis, Aphis pomi, virgin females. 

 a, wingless; b, winged. Magnified 20. From Carpenter. 



Plant lice increase at a perfectly appalling rate. I once 

 heard a distinguished lecturer in America tell his class that 

 if all the young of a plant louse survived, and all the young 

 of the survivors survived, and so on, in the course of one year 

 there would be a column of plant lice ^vith a cross-section of 

 one square mile advancing into space with the velocity of light! 



In somcAvhat higher insects, such as the saw-flies (Ten- 

 thredinidae), the gall-flies (Cynipidae), and the scale-insects 

 or mealy-bugs (Coccidae), no male has ever been found. It is 

 a curious fact that in some cases of parthenogenesis amongst 

 insects the product is always of one sex, some clutches of 

 eggs producing always females, the other always males. 



There is no doubt that amongst the classes mentioned 

 in the foregoing paragraphs, the females seem to get on 

 perfectly well without any male, and it may here be stated 



