J. Davidson 110 



Broad Beans, which plants they heavily infest throughout the Summer. 

 In Autumn the winged migrants from the Broad Beans return to Rumex, 

 sexuparae being })roduced in late Autumn, and subsequently ova being 

 laid on the Rumex plants. 



In the second life-cycle, ova are produced by the sexuparae on 

 Euonymus in late Autumn or Winter, which hatch out in Spring. The 

 winged migrants subsequently produced migrate to Poppies in June, 

 which plants they heavily invest as Aphis papaveris. In some years, 

 when the number of Apliids produced is abnormal, some of the migrants 

 go from the Poppies to Mangolds and many plants of the Chenopodi- 

 aceae family. In Autumn the winged migrants return to Euonymus 

 where sexuparae are produced and ova laid. 



The aphids taken from these different plants showed no structural 

 differences, although they differed slightly in size or colour on the 

 different host plants. 



It seemed to the present author that, if these two parallel life- 

 histories for Aphis rumicis were stable, the question of the influence 

 of the host plants on aphids is an important factor. The two life- 

 cycles seemed to show that the preceding host plant upon which a 

 generation of aphids is produced, has a determining influence on the 

 species of plant subsequently selected by the winged migrants. 



Theobald found that winged viviparous females taken from Euony- 

 mus lived on Broad Beans, and gave rise to the " bean black fly," but 

 from field observations he was unable to trace whether the winged 

 migrants from Euonymus went to Broad Beans. 



This seemed to the present author to be a very important question 

 in connection with the two life-cycles described for Aphis rumicis. It 

 intimated that the two parallel life-cycles might be merged into one by 

 crossing from Euonymus to Broad Beans, and from Rumex to Poppies. 

 If the two life-cycles proved to be absolutely constant and separate, 

 a very important feature would be established, namely the establish- 

 ment of two biological species (A. euonymi and A. rumicis), both 

 resembling each other in structure but differing physiologically in habits. 



As the results obtained in these experiments will show. Aphis euonymi 

 will heavily infest Broad Beans, and Aphis euonyyni reared on Rumex 

 will heavily infest both Broad Beans and Poppies. Thus the two life- 

 cycles may be merged into one. The life-history, however, has not 

 been completed, as owing to leaving England in September, I have 

 been unable to trace the history of the sexuparae. However, the plants 

 are still under observation, and the ova will be looked for in due course. 



