366 



ZOOLOGY 



jointed. Antennae three- to seven-jointed. The sixth ab- 

 dominal segment often bears a pair of tubes through which a 

 sweet fluid, the honey dew, is excreted. Some excrete also 

 a powdery bloom. 



Plant-lice are usually brown or greenish in colour ; they 

 live upon cell sap, inserting their proboscis into the tissues of 

 the leaf, stem, or root, and in this way often produce galls. 

 Their life-history is very complex ; as a rule males and females 

 coexist in the autumn. The females lay fertilised ova, the 

 winter eggs, from which in the spring incomplete females, with 

 no receptacula seminis, emerge. These give rise to innumer- 

 able parthenogenetic generations, but after a certain number of 

 these, males again make their appearance. In many species, 

 however, the male is not yet known. 



The genus Aphis is very common on plants, and causes 

 great trouble to gardeners. Certain of them are tended and 

 protected hy some species of ants, who feed upon the honey 

 dew secreted from their " honey-tuhes." 



The Phylloxera vastatrix, which infests vines, affords a 

 good example of the complicated life- 

 history presented by the Aphididae. The 

 wingless root- dwelling forms — radicola — 

 are found with their proboscis firmly 

 fixed in the tissues of the young roots. 

 They do not move about, but lay little 

 clumps of thirty to forty j)arthenogenetic 

 eggs, which give rise in six to twelve days, 

 according to the temperature, to young 

 larvae. These moult once or twice, creep 

 about a little, and then fix themselves by 

 their proboscis and lay parthenogenetic 

 eggs, like their mother. In this way 

 many agamic generations succeed one 

 another, and the rate of increase is so 

 great that it has been calculated that 

 the descendants of a single insect which 

 laid its eggs in March would number twenty-five millions by 

 October. 



As autumn comes on some of the eggs give rise to larvae 



PiQ, 



206. — Root-inhabit- 

 ing form (radicola) of 

 Phylloxera vastafrix, 

 with proboscis inserted 

 into tissue of root of 

 vine. After Girard. 



