142 ANIMAL LIFE AND SOCIAL GROWTH 



boring host plants and start a new colony of 

 wingless, parthenogenetic, female aphids. After 

 one or more such transfers, males and sexual 

 females appear as in the Cladocera and resistant 

 eggs are formed which serve to carry the colony 

 over such adverse conditions as the winter season 

 and the like. 



The cause of the appearance of truly sexual 

 forms in these aphids is not yet fully known, but 

 there is much good evidence that the appearance 

 of winged forms may be influenced by decreased 

 temperature and by decreased illumination but 

 in some species at least, the production of wings 

 follows over-crowding closer than it does any 

 other environmental factor. Many investigators 

 have reported wing production following crowding 

 in different species of aphids. In one case 59 

 generations of Aphis were reared without any 

 winged forms except in three cases when the 

 aphids became crowded. This experiment lasted 

 a full year. In the species used, decrease in tem- 

 perature or in light intensity would not produce 

 wings but in every case they apparently followed 

 directly upon the crowding of the wingless mothers. 

 At various times sub-cultures from the uncrowded 

 experiments were allowed to become crowded and 

 winged forms invariably resulted. Crowding ap- 

 pears to be proven to be a potent and perhaps the 



