230 E. A. HARTLEY Vol. XXII, No. 8^ 



On Production of Young. — Several experiments were under- 

 taken to determine the effect of parasitism in the various instars 

 on their subsequent maturity and production of young. An 

 equal number of parasites were confined with separate lots, 

 of different ages and instars of aphids for a day and then 

 removed. The aphids were observed until they showed para- 

 sitism and died. One nymph, parasitized on the day it was 

 born, carried the parasite through to maturity, but did not 

 mature itself, or produce any young. Three nymphs, para- 

 sitized in the second instar, produced but one young, which 

 was doubtfully from one of these nymphs, since another unpar- 

 asitized one occurred in the same cage. Nine second instar 

 nymphs matured and produced but two young at the end of 

 eleven days from birth. Seven nymphs, parasitized in the third 

 instar, matured and produced sixteen young before succumbing 

 to the parasite. With the larger stages, parasitism was so rare 

 that no data were obtained. 



It appears from these experiments that those aphids par- 

 asitized in the second instar seldom mature and produce young; 

 while those parasitized in the third instar may mature and 

 produce several young before they are killed. 



In the few cases where adult aphids were parasitized after 

 they had begun to produce young, the production continued for 

 six days from the time the parasite's egg was deposited. During 

 this time two females produced twenty-five young. Allowing 

 three days for the egg of Aphelinus to hatch, it will be seen 

 (Fig. 2, curve "d") that the decline in production of young 

 began shortly after the egg hatched. Production ceased alto- 

 gether two days before the death of the host. 



Aphelinus may also disturb the production of young in 

 aphids by worrying or exciting the viviparous females. This 

 is shown in Figure 2, curves a, b, and d, between the points 

 x and xi where the parasites were introduced and removed. In 

 every case a marked drop occurs in the production. However, 

 as soon as a few young are born, the parasite's attention is 

 directed to them so that the adults may go on producing others 

 unmolested. Here is where the preference of Aphelinus for the 

 younger stages of the host is of marked significance as a factor 

 in their control, since it does not attack at the source of pro- 

 duction, the adult female. This may account for the fact that 

 Aphelinus is much slower than Aphidius in exterminating a 

 colony of aphids with which they are confined. 



