J. Davidson 139 



no nourishment from the host plant. This was seen in the case of the 

 aphids on Euonymus, and on Broad Beans. It would be interesting 

 to know if the patholofjjical condition of the plants had an effect on the 

 surface tension of the cell sap. 



In the paper on the mouth parts of ScMzoneura lanigera, referred 

 to on the prccedin<j; page, there is described a " taste organ," which is 

 situated near the entrance of the suction canal into the pharynx. It 

 is very probable that by means of this organ the aphids are able to 

 appreciate changes in the quality of the cell sap, although owing to the 

 extreme minuteness of these mouth parts it is almost impossible to 

 demonstrate its function practically. That the aphids become restless 

 and dissatisfied with their hosts at different periods of the plant's growth, 

 or when the plant is heavily infested, has been seen throughout these 

 experiments as is indicated in the observations given. 



As is seen in the case of the tent experiments, Aphis rumicis, under 

 favourable circumstances, will select its host plant. Thus in the Com- 

 partment A, Broad Beans became first infected, then the Shirley Poppies 

 and the other plants only became infected to a very slight degree. 



Davis (1909), found that Aphis maidis Fetch, showed a decided 

 preference for broom corn plants over Indian corn and Sorghum^. 



It is an interesting feature of aphid habits, that one species may 

 infest a number of different species of plants. This is the case with 

 the species at present under consideration {Aphis rumicis). It is readily 

 seen, however, that although this particular species of Aphis may live 

 on a number of difEerent species of plants, some of these plants are 

 subject to a greater infestation than others. 



It was observed that while Broad Beans infected with Aphis euonymi 

 from Euonymus europaeus became rapidly infested by the reproduction 

 of the aphids, Red Beet infected from Broad Beans did not become so 

 heavily infested, although the few aphids produced appeared to be 

 healthy. 



Again, in all cases where Broad Beans and Shirley Poppies were 

 infected with this species, the former plants became heavily infested 

 sooner than the latter. 



Further, it was noted that certain plants in the garden of Acton 

 Lodge were infected with Aphis rumicis. On many plants, such as 

 Broad Beans, Rumex sp. and Chenopodium album, the colonies repro- 

 duced in great numbers and in most cases gave rise to a more or less 



^ Davis, J. J. (1909), "Biological studies of three species of Aphididae," U.S.D.A. 

 Bur. Ent. Techn. Ser. No. 12, pt. vm, p. 146. 



Ann. Biol, i 10 



