240 



SOME ASPECTS OF ECOLOGY 



divorced from physiology. Recent research has been directed 

 towards analyses of the influence of individual constituents of 

 the food, and the part played by vitamins or substances allied to 

 them. Increasing attention is also being given to the behaviour 

 of micro-organisms in the nutritional process, but, for the most 

 part, we know as yet very little concerning their actual functions. 

 In illustration of the effects of different host-plants upon the 

 metabolism of Aphis rumicis, as expressed in the reproductive 

 capacity of this insect, certain experiments carried out by 

 Davidson (1921, etc.) at the Rothamsted Experimental Station 

 may be taken as an example. The figures given below represent 

 the average number of aphides (average of five plants in each 

 case) produced on one plant of the undermentioned hosts, by 

 single adult apterous viviparous females, during a fourteen-day 

 period. The aphides used were all the progeny of a single parent, 

 and the experiments were carried out simultaneously under 

 identical environmental conditions. 



Table X. 



Similar experiments carried out with different varieties of broad 

 beans ( Vicia faba) showed that if the longpod bean be taken as 

 the standard or control, and the mean reproductive figure for the 

 aphid on that host-plant be represented by 100, the corresponding 

 figures for other varieties of beans may be grouped into classes 

 yielding reproductive figures ranging from 98 per cent, to 3 per 

 cent. The last-mentioned figure was obtained on Vicia narhonensis, 

 which some authorities regard as the prototype of all the cultivated 

 varieties of the broad bean. It appears that these differences, in 

 the reproductive capacity of the aphid, are correlated expressions 

 of the influence of the cell-sap, of different host-plants, upon the 

 metabolism of the insect. Certain further experiments by 

 Davidson lend support to this contention, since they show that 



