MORPHOLOGICAL EFFECTS OF CROWDING 313 



the sexual female is wingless but the male is winged. The problem 

 with which we are especially concerned here is the relation of crowd- 

 ing to the production of the winged (alate) parthenogenetic forms 

 from apterous mothers. 



Grassi (1907; vide Shull, 1929) first reported that crowding of 

 aphids of the genus Phylloxera on their food plant was followed by 

 wing production. Davidson (i 914), in speaking of his own work with 

 the aphid Schizoneura and other species, attributes the appearance of 

 winged forms to some change in the constitution of the cell-sap 

 products of the overpopulated plant. He summarizes his work by 

 saying: "Throughout my experiments it was observed that when 

 the plant had finished its active growth, or became heavily infested 

 with aphids, the changes resulting either in the quality or the quan- 

 tity of the cell sap (or both) seemed to induce the production of 

 winged forms." 



Wadley (1923), using RopJialosiphum, reports in detail experi- 

 ments which show that when aphids were allowed to multiply for 

 two or three generations and to overcrowd the plants in their experi- 

 mental cage, a high percentage of alate aphids were invariably pro- 

 duced. Even when apterous aphids were removed from these crowd- 

 ed plants and placed on new and flourishing host plants, their prog- 

 eny, reared with abundance of food, gave from 30 to 86 per cent 

 alate individuals, while the control gave from 7 to 20 per cent. 

 Wadley attributes these results to the efi'ect of limited nutrition, 

 since he was able to obtain similar data from starvation without 

 crowding. In both instances, alate forms tend to give rise to apter- 

 ous forms, as they do normally under optimum conditions. 



Ackerman (1926) confirmed the results obtained by Wadley, using 

 the same species of aphid. In one experiment with 776 aphids reared 

 under crowded conditions, 34 per cent were winged; while with no 

 overcrowding, all of 280 aphids were wingless. Ackerman also found 

 that partial starvation tended to produce a high percentage of winged 

 forms in the offspring. 



Reinhard (1927), using another species of Aphis, has undertaken 

 the most extensive set of experiments to date dealing with the effect 

 of crowding upon wing production in aphids. He first tested the 



