F. R. Pethbebridgp] and M. A. Husain 197 



yet the injury caused by their sucking the juices was of no importance. 

 An insect that causes mechanical injury would probably do so to all 

 plants that it feeds on but it is possible that the injection of an insect 

 that causes physiological injury might affect different plants in different 

 ways and even be harmless to some plants and also much less harmful 

 to some varieties of plants of the same species. 



It is interesting to find that certain pests are specific in their attacks 

 whereas others infect a large number of plants. Lygus pratensis is known 

 to attack fifty diff'erent species of plants (7). Plesiocoris rugicollis was 

 formerly known to attack Salix and Alnus but it now attacks apple, 

 black and red currants and under experimental conditions has been made 

 to attack plums. This interesting change in the diet of a species is 

 possibly comparable with mutations in the morphological characters and 

 may be due to some physiological mutation in the organism. It is possible 

 that P. rugicollis may in the future extend its host plants and so become 

 a still more serious pest. 



The change in its diet is difi&cult to explain but it may be due to a 

 very simple cause. Larvae cannot fly and when just hatched do not 

 appear capable of travelling far, and their only chance of living seems to 

 be to suck the juice of the plant on which the eggs are laid. They get 

 used to this diet and so do not change it readily. Suppose a fertilised 

 female to be blown on to a new host and not capable of reaching her 

 former host, she may lay eggs there and if she can live on the juices 

 of this host the larvae which hatch from these eggs will probably be 

 able to live on the tissues of the host on which their mother could live, 

 and in any case are unable to reach the original host of their mother. 

 Actual experimental evidence is wanting, but we know that nymphs can 

 be made to change their hosts, e.g. apple to plum, and black currant to 

 apple, and it would be interesting to see if P. rugicollis could be made to 

 lay eggs on a species other than that on which it was reared. 



The facts that apples and willows are found interlacing and only the 

 willows attacked by P. rugicollis and also apples and black currants 

 interlaced with only the latter attacked show that it normally lays its 

 eggs on the host on which it has fed and does not readily change its host 

 when that host is capable of providing it with food. 



In the Wisbech district P. rugicollis lives on willows, apples, black 

 currants and red currants, whereas in certain districts near Cambridge 

 it does not live on apples but does on the other three. Larvae from black 

 currants near Cambridge were sleeved on apple trees and although they 

 did not feed readily at first eventually became used to their new host 



