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 by some species of ants, who feed upon the honey 
dew secreted from their “ honey-tubes.” 
The Phylloxera vastatrix, which infests vines, affords a 
good example of the complicated life- 
history presented by the Aphididae. The 
wingless root-dwelling forms—vradicola— 
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 parthenogenetic 
egos, 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 
Fic. 206.—Root-inhabit- 
ing form (radicola) of many agamic generations succeed one 
Phylloxera vastatriz : : 
with proboscisinserted another, and the rate of increase is so 
into tissue of root of creat that it has been calculated that 
vine. After Girard. © : ; ; 
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 
