136 Food Plants of Aphis rumicis 



the winged migrants had gone to Broad Beans, on which plant they 

 were reproducing rapidly. 



This brings us to the important question, What are the factors 

 underlying the production of winged forms and the consequent migra- 

 tory habits of Aphides ? The migratory instincts of the winged females 

 may underly the desire to escape from the original host plant, but it 

 may be that a change in the constitution of the cell sap of the host 

 plant, which affords the food of Aphides, has an influence on the pro- 

 duction of winged forms and the consequent migration of the Aphides 

 from the original host plant. 



In the case of the Euonymus bushes A and B (series A), when these 

 bushes became very heavily infested, being practically smothered with 

 aphids, both the winged viviparous females and the apterous forms 

 became surprisingly active and ran over the bushes and muslin cover 

 in a most restless manner as though wishing to escape. Moreover, 

 they became considerably reduced in size, both apterous and winged 

 forms being very much smaller than those produced on the Broad Beans 

 and Rumex plants. In the latter stages of the heavy infestation of the 

 Broad Bean plants, the aphids on these plants were also much smaller 

 than in the early stages of infection. It may be that owing to the 

 pathological condition of the plants induced by the heavy infestation, 

 the aphids were unable to obtain sufficient food, but there is the fourth 

 question of the change in the constitution of the cell sap brought about 

 by the heavy infestation of the plants. 



Woodworth, C. W. (1908)^, who made some observations on Aphis 

 brassicae, states that when a plant wilts the birth rate decreases and 

 suggests that the failure of plant lice to develop winged forms under 

 favourable conditions is due to the rapid development of the rest of 

 the body. He refers to a paper by Clark in the Journal of Technology, 

 Vol. I (which, unfortunately, I have been unable to consult), who 

 obtained winged forms of Macrosijphum rosae in the first generation, 

 by rearing the aphids on rose-cuttings grown in sand wetted with a 

 solution of magnesium salts. 



During the summer of 1913, I carried out a number of experiments 

 with Macrosiphum rosae, on rose-trees grown in soil treated with definite 

 quantities of various inorganic salts. I regret that owing to the extra- 

 ordinary numbers of aphids that Averc produced, and the confusion 

 resulting from this, that the experimental error is too great to allow 



1 Woodworth, C. W. (1908), Enlom. News, Philad. 1908 xix, pp. 122-3. 



