218 



E. A. HARTLEY 



Vol. XXII, No. 8 



the first and second stage nymphs after being parasitized; and 

 the difficulty of handling large numbers of the various stages of 

 aphids necessary to obtain accurate results. 



At first a large number of parasites were placed with a 

 greater number of aphids, in the different stages, for a short 

 time and then removed. An equal number of aphids, represent- 

 ing the different stages, were then isolated and the parasites 

 allowed to develop in these until they showed externally. These 

 parasitized individuals were then counted and the percentage 

 taken to indicate the preference of the parasite for a given stage. 

 (See Table I.). 



Table I. 



Showing the Percent of Parasitism in the Various Instars of the Aphid Host, Reared 

 Through Until It Appeared. 



This method was unsatisfactory, as it did not account for the 

 death of a large number of the smaller stages, which would lower 

 the percentage considerably for that group. It also did not 

 necessarily provide an equal number of the various stages, 

 either at the beginning or the end, to eliminate chance. Then 

 finally the separating and keeping the aphids in dift'erent cages 

 for any length of time always made possible the death and 

 escape of many specimens that could not be accounted for. 



To get around these difficulties, the following modifications 

 were introduced: As nearly equal and smaller numbers of the 

 different stages were placed together with a number of par- 

 asites for a day or so and then all were dissected. The percent 

 parasitized in the various groups, as indicated by the presence 

 of the parasite's egg, showed fairly accurately the preference for 

 that particular stage. Three separate lots were thus treated 

 with the results shown in Table II. 



The extreme rarity of parasitism in the adult aphids was 

 clearly demonstrated in the experiments to determine the effect 

 of parasitism on the production of young, in which it was neces- 



