it causes the aphid to remain in the apterous form, perhaps because feeding 

 is stimulated as suggested by Herzig. 



By the use of a radio-isotope technique, Herzig's hypothesis that ant- 

 attendance significantly increases the excretion and feeding rates of A . fabae 

 was confirmed (Banks and Nixon, 1958); it was also shown that the aphid 

 controls its rates of feeding and excretion, which are not determined solely 

 by pressures within the plant. 



I have also confirmed that ant-attended A. fabae Scop, on bean plants 

 multiply more than ant-free aphids in the absence of insect enemies, but, on 

 average, ant-attendance increased aj)hid numbers by about one third and 

 did not double or treble numbers as reported by Herzig, in whose experiments 

 predators were not excluded (Banks, 1958). I showed that when aphids are 

 ant-attended they change their behaviour: the excretion behaviour alters, 

 and the normal dispersal of adult apterae from the young apical growth of 

 bean plants is delayed, because the usual tendency of the adult apterae to 

 wander over the plants is inhibited. No significant differences were found 

 between the reproduction rates of individual ant-attended and ant-free aphids 

 living on leaves of similar age, but the reproduction rates were significantly 

 affected by the age of the plant tissue on which the aphids fed. The increased 

 multiplication of ant-attended colonies was attributed to the delay in dispersal 

 of adult apterae from the young growth, where they reproduce more, to the 

 old growth, where they reproduce less. The effect of the aphids' multiplication 

 is therefore indirect. 



Although El-Ziady (quoted by Kennedy and Stroyan, 1959) reported 

 that ^. fabae gains weight faster луЬеп attended by ants than when unattended, 

 B. Johnson (1959) found no differences in weight or in rates of growth 

 of attended and unattended Aphis craccivora Koch (see below); and in recent 

 experiments I could find цр differences in weight of ant-attended and ant- 

 free aphids which луеге collected in the field, or of ncAvly-moulted ant-attended 

 and ant-free adult apterae feeding on young potted bean plants in cages. In these 

 experiments, the average weight of newly-moulted ant-attended and ant-free 

 apterae was doubled within one generation when the insects were transferred 

 from a winter laboratory culture to fresh young plants kept in the cages 

 out-of-doors in the spring. The average weight was maintained through three 

 more generations of the aphids, without significant differences between the 

 two treatments, and there were no differences in the rates of development 

 or of the reproduction rates of ant -attended and unattended aphids in each 

 generation. The increase in size of the aphids followed from the improved 

 nutrition and not from ant-attendance. 



The stimulation of feeding caused directly by attendant ants has little, 

 if any, effect on the aphid's reproduction rate, which is, however, significantly 

 affected by the age of the plant tissue on which it feeds. It seems that the growth 



330 



