HEM IP TERA . I 59 



the species is provided for. This winged form also con- 

 sists entirely of females, and is known as the ^vinged agamic 

 form. They produce in turn the wingless agamic form, a 

 single, winged individual, which has flown to a new plant, 

 starting a new colony. Generally on the setting in of cold 

 weather, or in some cases on the failure of nourishment, 

 the weather being still warm, there is produced a generation 

 including individuals of both sexes. These are known as 



o 



the sexual forms. The males may be either winged or 

 wingless, but these true females are always wingless. The 

 sexual forms pair, and the female produces one or more 

 eggb. It is in this state that the species usually pass 

 the winter, and consequently these eggs produced by the 

 sexual form are often called the winter eggs, to distinguish 

 them from eggs produced by the agamic forms of certain 

 species, and which are incorrectly termed pseudova (pseu- 

 do'va). From the winter egg there hatches, usually in the 

 spring-, an agamic female, which, as she is the stock from 

 which the summer generations spring, is often called the 

 stem-mother. The peculiar reproduction of the agamic 

 forms is often termed reproduction by budding. 



Plant-lice are often very destructive to vegetation. The 

 ordinary methods of combating them are either by the use 

 of a strong solution of soap or with kerosene emulsion. 



Plant-lice vary greatly in their habits. Certain species 

 live in the ground on the roots of plants. The Lettuce 

 Earth-louse, RJiizobius lactucce (Rhi-zo'bi-us lac-tu'cae), is a 

 good illustration. This occurs on the roots of lettuce, often in 

 great numbers. Other species are found on the roots of grasses 

 or herbaceous plants and usually accompanied by ants. 



On the other hand, many species, in fact the majority of 

 Aphids, pass their lives on the foliage of plants, infesting 

 especially the tenderer leaves. Familiar examples are the 

 Cabbage Aphis, Aphis brassicce (A'phis bras'si-cae), the Apple- 

 tree Aphis, Aphis mail, the Cherry tree Aphis, Mysus cerasi 

 (My'sus cer'a-si), and the Peach-tree Aphis, Mysus persicce 



