150 DISEASES OF CROPS. 



(winter), the little glossy, black, ovoid eggs of the specieS" 

 are fotuid attached to the terminal twigs, and especially 

 in more or less protected crevices around the buds of 

 different varieties and species of Prunus, both wild and 

 cultivated. From this winter-egg there hatches a stem- 

 mother, which is characterized by somewhat stouter and 

 shorter legs and honey tubes than in the individuals of 

 any other generation. Three parthenogenetic ^ genera- 

 tions are produced upon Prunus, the third becoming 

 winged. This last is what' Lichtenstein called the 

 ' pseudogyna ' or migrant ; and it instinctively flies to the 

 hop-plant, which is entirely free from the attack during 

 the development of three generations upon Plum. A num- 

 ber of parthenogenetic generations are produced upon the 

 hop-plant until in the autumn, and particularly during 

 the month of September, winged females are again pro- 

 duced. This is the ' pupifera ' of Lichtenstein, or return 

 migrant ; and she instinctively returns to the plum. Here 

 she at once settles, and in the course of a few days, 

 according as the weather permits, produces some three or 

 more young. These are destined never to become winged, 

 and are true sexual females. Somewhat later, on the hop- 

 plant, the true winged male, and the only male of the 

 whole series, is developed ; and these males also congre- 

 gate upon the plum, on the leaves of which, toward the 

 end of the season, they may be found pairing with the 

 wingless females which stock the twigs with winter-eggs. 

 . . . Each parthenogenetic female is capable of pro- 

 ducing on an average one hundred young, at the rate of 

 one to six per day. Each generation begins to breed 



' Parthenogenesis is a term applied to a fanction of those females 

 who produce fertile eggs without previous impregnation. 



