46 INSECTS 



attacks both, or between the plum and the hop, where 

 the insect cannot exist unless both its alternate hosts 

 are present. At other times, as in the case of the melon- 

 louse, the insect has a variety of food plants and the 

 migration is not essential to the continuance of the 

 species. In fact it is not even necessary for all plant 

 lice to go into the egg stage, for some species winter as 

 parthenogenetic females on plant remnants or on stools 

 like the rosettes of cruciferous plants. In tropical coun- 

 tries the resting stage is often during the dry season, 

 when vegetation has little spare moisture. 



Some species attack only the roots of plants, and 

 these are usually wingless, and often without honey- 

 tubes or cornicles. Some inhabit the roots at one 

 season and the leaves at another, the winter being 

 usually spent underground, while a large part of the 

 growing period may be passed on the foliage, the spread 

 of the species being provided for there. Such species 

 may be very little modified, like the black species that 

 occurs on peach, or very greatly specialized like the 

 Phylloxera that occurs on grape, and is so great a 

 factor in all countries where the vine forms an impor- 

 tant part of the agricultural product. This Phylloxera 

 vastatrix winters on the roots as a partly grown, wing- 

 less form. It grows rapidly in spring and lays eggs, 

 the young from which are, like their mothers, wingless 

 and parthenogenetic, laying eggs and producing others 

 like them in turn throughout the season, always from 

 unfertilized eggs. In most localities, about midsummer, 

 some specimens acquire wings, work their way to the 

 surface and fly to other vines, thus providing for the 

 rapid spread of the species. These winged forms lay 

 from three to eight eggs on the leaves, some of them 

 large, producing females, the others small, producing 

 males. These sexed forms seem to be produced only 



