60 Biological Studies of Aphis rumicis 



Colonies of Aphis rumicis have been recorded on scarlet runners 

 (Davidson (2)), on climbing haricot beans (Malanquin and Moitie(6)) and 

 many other plants, where the infestation is not heavy as compared with 

 beans. There is a wide distribution of the species on different kinds of 

 plants which are less or more favourable as indicated by the degree of 

 infestation. 



The food of aphids is the sap derived from certain cells of the host 

 plant. Sections of plant tissues fixed with aphids in situ, show that 

 the phloem of the vascular bundles is particularly sought after, though 

 cells of the cortex, parenchyma and mesophyll of the leaf, are also 

 tapped by the stylets. This question is being further studied, but it is 

 evident that while the phloem is very important other cells of the plant 

 must be regarded as a source of nourishment. 



The constitution of the cell sap may differ considerably in different 

 species, and the hydrogen-ion concentration of cell sap should be in- 

 vestigated in relation to the relative susceptibihty of plants to aphid 

 attacks. In this respect Comes' (i) view that the degree of acidity of the 

 cell sap of American vines is in direct relation to the resistance to 

 Phylloxera is important, as is also the recent work on the hydrogen-ion 

 concentration of plant juices and the factors affecting it. 



Since physiological characters of plants may be specific and follow 

 Mendehan laws of heredity, this line of investigation, mth the possi- 

 bihty of breeding resistant strains, would seem to be a profitable one, 

 and research as to the relative intensity of reproduction on different 

 val-ieties of the same species is being continued. 



Since the chemical composition, or physiological characteristics of 

 the cell sap, is an important factor, one is inchned to the view that the 

 various species of aphids, at any rate in the viviparous parthenogenetic 

 generations, have adapted themselves to many different plants. 



In the wide distribution of winged migrants, and the fortuitous 

 adventures of these individuals, they may alight on many different 

 plants — especially in the case of a marked polyphagous species like 

 Aphis rumicis — and produce young. Certain plants (more susceptible) 

 provide the necessary stimulus for rapid reproduction. On other plants 

 on the contrary only a moderate reproduction occurs, and on others 

 the colonies may soon die out. 



The adaptabihty of the aphis to its food plants is an important con- 

 sideration, and is doubtless influenced by the nature of the food plants 

 available. This is a local consideration and probably accounts for the 

 widely different food plants (cultivated or otherwise) to which species 



