368 THE BIOLOGY OF INSECTS 



The fact that aphids do not, fortunately for the human 

 population of the world generally, increase to the extent 

 suggested by Huxley illustrates the meaning of the struggle 

 for existence. The food-supply of these insects, derived 

 from the sap of plants, is enormous in extent but not un- 

 limited, and when aphids are very numerous, the leaves and 

 succulent shoots on which they live become dry and 

 withered. Their piercing jaws are adapted for procuring 

 and sucking the sap ; the wings, developed as already noted 

 (p. 164) in many members of the virgin generations, enable 

 them to fly to fresh feeding grounds. The green colour of 

 many of them renders them inconspicuous to their enemies, 

 as they feed on the leaves ; many kinds have the habit of 

 sheltering beneath the leaves, or inducing the formation of 

 blisters in which they obtain some degree of protection. 

 But they fall a prey to numerous creatures of diverse kinds — 

 insect-eating birds, ladybird-beetles and their larvae, the 

 grubs of lacewing flies and the maggots of hover-flies, while 

 small Hymenoptera lay eggs in the aphids' soft bodies so 

 that parasitic larvae devour them internally. All these pre- 

 daceous insects are adapted for stalking, capturing, and suck- 

 ing the juices of the aphids as these are for feeding on the 

 sap of plants, and their dependence on the aphids is shown 

 by their increasing abundance when these are very numerous. 

 On a single shrub hundreds or thousands of young are 

 eliminated in every generation both among the plant- 

 suckers and the insects of prey. We realise, therefore, that 

 the process may be expected gradually to lead to increasing 

 perfection of adaptation and then to ensure the maintenance 

 of a high standard when the characters necessary to survival 

 have been attained. It is self-evident that no characters 

 can be attained by the race except those capable of hereditary 

 transmission ; hence the action of natural selection depends 

 on the preservation of favourable inherited variations 

 through a long series of generations. The special aspect of 

 the subject known as Sexual Selection, of importance as a 

 factor of ** success in leaving offspring," has already been 

 discussed (pp. 201-8). 



