158 
THE STUDY OF INSECTS. 
ratus for producing the honey-dew been developed ? Writers 
long ago showed that ants protect plant-lice by driving away 
from them lady-bugs and other enemies. Recently, how¬ 
ever, Professor Forbes has demonstrated that, in certain 
cases at least, a more important service is rendered. In his 
studies of the Corn Plant-louse, Aphis maidis (A'phis 
mai'dis), he found that this species winters in the wingless, 
agamic form in the earth of previously infested corn-fields, 
and that in the spring the plant-lice are strictly dependent 
upon a species of ant, Lasius alienus (Las'i-us al-i-e'nus), 
which mines along the principal roots of the corn, collects 
the plant-lice, and conveys them into these burrows, and 
there watches and protects them. Without the aid of these 
ants, the plant-lice were unable to reach the roots of the corn. 
In addition to honey-dew, many Aphids excrete a white 
substance. This may be in the form of powder, scattered 
over the surface of the body, or it may be in large floccu- 
lent or downy masses; every gradation between these forms 
exists. 
The plant-lice are remarkable for their peculiar mode of 
development. The various species differ greatly in the de¬ 
tails of their transformation, but the following generaliza¬ 
tions can be made. 
There are several distinct forms in each species, each 
form playing a peculiar part in the history of the species. 
If a colony of plant-lice be examined during the summer 
months it will be found, usually, to consist very largely of 
wingless individuals; these are females, which reproduce 
without the intervention of males. This is the wingless 
agamic form . In many cases this form gives birth to living 
young, instead of laying eggs; and the reproduction of 
this form is so rapid, that it would be disastrous to the 
species, by the destruction of the infested plants and the 
consequent starving of the insects, if another form of the 
species did not arise. But from time to time young are 
produced which become winged, and thus the spread of 
