316 Scientific Proceedings, Royal Dublin Society. 



With individuals of some species, however, the parthenogenetic generations 

 may be continued under favoura'ole conditions for a long period, as has been 

 shown in many cases. For example, Klodnitzld (1912) carried on agamic 

 generations of Sijylionophora rosae var. glmica, Buckton, for more than a year. 

 Ewing (1916) carried on Aphis avenae for seventy-three parthenogenetic 

 generations. 



It would appear that in some species the sexual forms develop after a 

 definite number of agamic generations have been passed through ; but this is 

 not the case in all species, as is seen in my experiments and in those of Davis 

 (1914) with Calli^iterus trifolii, Monell. 



A further consideration is that, while certain individuals of the later 

 summer agamic generations close the life cycle by producing only sexual 

 forms, other individuals may also produce agamic females which continue the 

 agamic generations under favourable conditions for a very long period. 

 Klodnitzld carried on three cultures of Aphis hederae Kalt. over a number of 

 generations. In two lines sexual forms appeared in the thirty-first generation, 

 but no sexual forms appeared in. the other line, even after forty-two genera- 

 tions had been passed through. Davis (1915) observed a similar agamic 

 series in Macrosiphum pisi Kalt. 



The continuation of parthenogenetic reproduction throughout winter 

 under favourable conditions of food and temperature has been obtained in my 

 experiments with Aphis rumicis. With the advent of unfavourable winter 

 conditions in the field, the agamic generations would die off, but under mild 

 conditions, and given a suitable food-plant, they may survive the winter, and 

 slowly carry on the parthenogenetic reproduction. 



The finding of a colony of agamic females of Aijhis rumicis by Davidson 

 (1914) on January 30th, 191-3, on Eiwnymus japionicus, which, when brought 

 into the green-house, continued agamic reproduction throughout 1913, would 

 seem to support the latter view. This is an important consideration from 

 the economic standpoint. 



It is possible, of course, that some late parthenogenetic generations, delayed 

 in their development on the approach of winter conditions, would normally 

 under favourable conditions lead to sexual forms. 



It appears that in Aphids the development of viviparous parthenogenetic 

 generations, between the ova and the sexual forms, is an expression of 

 adaptation and selection to environmental conditions, extending over the 

 favourable seasons of the year. 



Certainly in nature the approach of winter conditions would normally be 

 the factor limiting the extent to which the parthenogenetic phase could be 

 extended. 



