PARTHENOGENETIC PURE LINE OF APHIS AVENGE FAB. 93 



extensive as those in the seventy-third generation of the experi- 

 mental pure line. 



One individual was isolated from those obtained in Iowa, and 

 was started in the production of a check strain when the in- 

 dividual of the 1173 generation was selected for carrying on the 

 experimental strain obtained from Oregon. The means ob- 

 tained, and the lengths of the extreme variants of each fraternity 

 are plotted in Fig. 1 1 . The data for the Iowa check strain are 

 given in tabular form in Table X. What results do we find when 

 comparison is made between these two strains? 



1. In Regard to Size. By observing the plot showing the ex- 

 tremes and the mean of each of the fraternities we see that they 

 very nearly coincide. The parent individual of the Iowa strain 

 had a length very near that of the mean of the II 73 fraternity, 

 which was the fraternity I was rearing at the time I obtained this 

 stem parent. Of the five means obtained for the five fraternities 

 of the Iowa strain, three were slightly above and two slightly 

 below the means of the corresponding fraternities of the Oregon 

 strain. 



If we compare the average mean obtained from the fraternal 

 means of the 5 check generations of the two strains, we have: 



Average for the fraternal means: Oregon strain = 1.489 mm. 



Average for the fraternal means: Iowa strain = 1.479 mm. 



The difference between these two averages (one hundredth of 

 a millimeter) is an insignificant difference when we consider the 

 great range in variation found among individuals born of the 

 same parent. We find, therefore, that as far as size is con-: 

 cerned there has been no dwarfing or indication of lack of vigor 

 in growth due to the long continued parthenogenetic reproduction 

 in the Oregon strain (the main pure line of Aphis avence Fab.). 



2. In Regard to Color. When first found the Iowa strain of 

 Aphis avence showed a much deeper coloration than those in- 

 dividuals of the Oregon strain that had been bred for 73 genera- 

 tions in confinement. The individuals of the Iowa strain, how- 

 ever, lost these deeper colors immediately after being reared 

 under the same conditions as those of the Oregon strain, and were 

 undistinguishable from individuals of the latter strain. The 

 fact that the individuals of the Iowa strain were more deeply 



