POTATO PLANT LOUSE. 243 



August 5. None of the transferred specimens have left the new plants. 

 They are feeding and producing young actively and not a single indi- 

 vidual is to be seen upon the cloth of these new cages, all signs of the 

 restlessness evinced in the old cage having left them. It seems reason- 

 able to suppose that their desertion of the plants upon which they had 

 been reared was caused by the unhealthy condition of these plants due to 

 the two weeks presence of the plant lice. Provided with fresh plants 

 they were content. It is probably due to this migrating instinct that 

 makes possible the even infestation of a whole potato field, — the first 

 winged forms developing upon stalks over crowded and consequently 

 sickly, seeking uninfested tips for their own feeding places and for their 

 progeny. 



August 13. The progeny of the foregoing winged viviparous forms are 

 partly wingless viviparous forms and partly, as indicated by wing pads, 

 to be winged viviparous forms. The colonies do not seem particularly 

 vigorous. The plant tips are badly speckled with beak wounds and the 

 leaves a little discolored with honey dew fungus. 



September 20. — The material recorded August 3 and August 13 was in 

 the case of 2 cages left unmolested until today when both the true sexes 

 are found to be present. The males are winged. The females are wing- 

 less. Further description of these forms is reserved for another place. 



September 21. — About 8 males and some 20 oviparous females were 

 removed from the potato and placed upon a young shepherd's purse 

 plant in a cloth cage. Females were added from time to time and a 

 very few males. From September 21 to October 11 from one to 3 pairs 

 of these were noticed in copulation each day. On October 11 examination 

 of the sheperd's purse showed Nectarophora eggs variously placed on the 

 upper and under sides of the leaves and along the stem. One was depos- 

 ited on a cheese cloth thread in the cage. 



It should be emphasized here that although the true sexes 

 developed upon the potato in the insectary and thereby showed 

 that another plant was not a necessity for these forms, the 

 situation was practically forced. A single caged potato plant 

 had been stocked August 3 with 20 winged viviparous forms 

 reared on potato from the wingless viviparous forms collected 

 July 18 and left to them and to their progeny until September 

 20, there being no choice for the prisoners except the potato 

 or death, for a period of 2 months. This period extended con- 

 siderably past the season of migration for N. solanifolii in the 

 field which had been observed to have occurred for 3 years 

 early in September or late in August. The fact that the 

 imprisoned insects then developed the true sexes upon potato 

 is no indication that such would be the case in the open field. 

 Indeed the fact that the true sexes did not appear until long 

 after the season of out-of-door migration would rather indicate 



