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 viaidis (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, Lasitis alienns (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 



