FIELD BOOK OF INSECTS. 



plants, where another series of wingless agamic or asexual 

 generations are brought forth. This may continue for 

 some time and after a certain number of generations the 

 plants again become crowded, winged females are produced 

 and there may be a return migration to the original food 

 plant, where one or more generations may be produced 

 and ultimately perfect males and females, \vhich latter 

 pair and deposit eggs in crevices of the bark or other 

 shelters, as stated above, and remain unhatched over 

 winter." 



This changing from one mode of reproduction to another 

 and from one food plant to another, together with still other 

 complications, is very confusing. Lichtenstein has noted 

 twenty-one different forms assumed by Phylloxera quercus 

 in its life-cycle. It is probable that the four hundred or 

 so forms which have been described from the United 

 States as distinct species include phases of a smaller num- 

 ber of real species, but it is certain that many species are 

 still undescribed. 



A common species on apple is Aphis mail. Professor 

 Webster said concerning it: "It would appear almost 

 visionary to advocate spraying apple orchards in mid- 

 winter to protect the wheat crop, but nevertheless one of 

 the most serious enemies of young fall wheat passes its egg 

 stage on the twig of the apple during the winter season." 

 The Woolly Apple-aphis, Schizoneura lanigera (Plate 

 XXIV), secretes a waxy substance, which accounts for 

 its name. It is often seen on twigs and around wounds, 

 clustered in bluish-white masses that look like mold, but 

 the individuals which are probably doing the most damage 

 are feeding upon the roots where they cause gall-like 

 swellings. Phylloxera vastatrix is one of the few Ameri- 

 can insects which have become injurious in Europe. 

 With us it forms galls on grape leaves (see p. 470) but 

 is not usually found on the roots; in Europe it rarely 

 attacks the leaves but forms galls on the roots, causing 

 them to decay. 



Aphids excrete a sweetish substance, called honey-dew, 

 which is much sought after by ants. In fact, aphids are 

 called "ants' cows" and many species of ants go to con- 

 siderable trouble to care for them. A variety of Lasius 



88 



