GENERAL DISCUSSION. 19 



reproduction and some unknown internal factor determine which sex 

 character shall control the development of each egg. The whole 

 question of sex determination will be seen to be far more complex in 

 the case of the aphids than in insects where no parthenogenetic gener- 

 ations intervene between the fertilized egg and the production of 

 males. 



In collecting aphids this year it was found that the sexual genera- 

 tion appeared in many species from two to three weeks earlier than in 

 1903 and 1904, though the weather continued warm much later than 

 during the two preceding autumns. This seemed to be related to the 

 earlier ripening up of the various host plants. The earlier fruiting 

 and drying up was especially conspicuous in Oenothera biennis and 

 some of the rose bushes. Sexual forms of both the brown and green 

 rose aphid were found on young growing tips of these same bushes, 

 but they may have been affected by the general sap conditions of the 

 plant. Finding sexual forms among the willow aphids collected from 

 young shoots at Harpswell, Maine, on June 29, suggested that in 

 some species there may be definite cycles of generations not directly 

 subject to external conditions, as Weismann maintains for the Daph- 

 nids ('76, '79). This species of aphids deserves further investigation. 



One is also somewhat mystified by the fact that in some species 

 there is a sudden and complete change from the parthenogenetic to 

 the sexual mode of reproduction, while in others, as for example the 

 brown and the green rose aphids, parthenogenetic generations continue 

 to multiply until destroyed by the cold of winter. The complete 

 change seems to be the rule in the majority of the species in this cli- 

 mate, and is much more easily explained than the mixed condition. 



So far it has not been possible to produce the sexual generations 

 by subjecting the aphids to artificial changes of conditions. This 

 may be due to the fact that the brown and green rose aphids, which 

 have been used for most of the experiments, may be the most unfavor- 

 able for such work, as they produce scattered sexual forms while con- 

 tinuing the parthenogenetic generations, indicating either that they 

 are less sensitive to changes in environment or that they are less well 

 adapted to this climate. 



This is evidently a case where experimental work must be based on 

 rather extensive observation and cytological research, and the results 

 recorded in this paper give a far better basis for experiment than we 

 had a year ago. 



BRYN MAWR COLLEGE, January 75, 1906. 



