PARTHENOGENETIC PURE LINE OF APHIS AVENGE FAB. 95 



colored when first found was probably due to the fact that they 

 had been feeding on the leaves of the oat plants which contained 

 much more chlorophyll than the tender shoots of wheat, upon 

 which the individuals of the main pure line were being fed. At 

 any rate the check experiment showed that they were not de- 

 pendent upon a greater vigor or virility. 



3. Fecundity.- From the data here obtained it is not easy to 

 compare satisfactorily the fecundity of the individuals of the 

 two strains. Six days was always the minimum time allowed 

 for the development and production of young of each individual. 

 Using this six-day life cycle schedule, however, we find consider- 

 able difference between the total number of individuals obtained 

 from the five parents concerned. In the case of the Oregon 

 strain 55 young were produced. In the case of the Iowa strain 

 kept under identical conditions as a check, only 46 individuals 

 were obtained. This gives for the average size of the frater- 

 nities of the Oregon strain 9.93 individuals, and for the frater- 

 nities of the Iowa strain an average of 7.44 individuals. We 

 find, however, that one other factor enters into the results to such 

 an extent as to make these findings of little value for comparing 

 the relative fecundity of the two strains. This factor is the 

 length of the period of developtnent r 



4. Length of Period of Development. Of the 55 individuals 

 obtained in the Oregon strain during the 5 test generations, only 

 2 failed to reach the paternity age in the 6 day period allowed. 

 In the 46 individuals obtained at the same time in the 5 genera- 

 tions of the Iowa strain 8 individuals failed to reach the age of 

 paternity in the 6 days allowed. 



We find, then, that the Iowa strain developed much more 

 slowly under the same conditions than the Oregon strain. I 

 believe that the Oregon strain had changed its developmental 

 period, since it was started about 2 years previously 



W T hat then was the effect of continued parthenogenetic re- 

 production for 73 generations in Aphis avence upon the virility 

 of the strain? We find that by the means of comparison just 

 reported no evidence of racial deterioration or loss of virility in 

 any respect. We do find, however, strong evidences of adapta- 

 tion in regard to the shortening of the growth period. Yet in 



