XIX INSECTA .- RHYNCOTA 285 



This is often to be noticed on the pavements of a town below 

 sycamore and lime trees which are infested with Green-iiy. 



For the sake of this sweet juice, Aphides are visited and 

 often protected by ants, different species of ants being 

 associated with different species of Aphides. The black garden- 

 ant seems to visit chiefly the Aphis of the rose-tree, or some 

 other shoot-sucking green-fly, whilst the small yellow ant gets 

 its honey-dew from certain subterranean root-sucking Aphides 

 (see p. 411). 



The juices of the plant are sucked up by the Aphis by 

 means of a proboscis similar to that in all Rhyncota (p. 276), 

 and this results in serious injury to the plant, for though 

 individually each Aphis is insignificant, they often occur in 

 such numbers that they seriously menace the life of a plant. 

 The rapidity with which they multiply is 



^tiMi^''^ astounding. It has been calculated that a single 

 Aphis would, if unchecked, in two or three years 

 produce so many millions of descendants that, owing to their 

 ravages, there would be left in the world none of the plants 

 on which they are accustomed to feed. They are, however, 

 kept in check not only by such means as heavy rain, and 

 sudden changes in the temperature of the air, to which they 

 are very sensitive, but also by the fact that many other 

 insects feed on them. 



The method by which their unusually rapid multiplication 

 is brought about is immediately obvious if we study the 

 course of the life-history of one Aphis from the egg stage. 



An egg which is laid on the plant in the 

 autumn hatches out in the spring as a wingless 

 form, which at once pushes its proboscis into the plant tissues 

 and begins to feed, very soon giving forth from the end of 

 its body minute living Green-fly, at the rate of two or more 

 a day. These young ones, in their turn, begin to reproduce 

 in the same way in a very short time, warm dry weather 

 favouring rapid production. All through the summer an 

 enormous number of young is produced thus, partheno- 

 genetically (i.e. without any fertilisation of an egg having 

 taken place), and most of these are wingless like the parent 

 form, but a small and varying proportion of winged partheno- 

 genetic females is also produced. The production of these 

 winged individuals seems to be correlated with the amount of 



