234 E. A. HARTLEY Vol. XXII, No. 8 



Cannibalism. — Aphidius is much better armed with man- 

 dibles and a spiny body, than Aphelinus, and is also more 

 active. It might, therefore, be expected to kill Aphelinus by 

 feeding directly on it or injuring it fatally by striking against it 

 in the host. Yet, in the many cases that were examined, Aphid- 

 ius was never found feeding on Apheli?uis, nor did there ever 

 appear evidence of mutilation in the dead larvae. In each case, 

 Aphelinus simply stopped movement and turned darker in 

 color, in which condition it remained until it finally shriveled up 

 and disintegrated. 



Starvation. — This theory does not carry much weight when 

 we consider the abundant supply of food present at the time 

 of death. 



Toxic Substance. — It is quite possible that something of this 

 nature may be thrown off by Aphidius that has an injurious 

 effect on Aphelinus, but it is difficult to prove. 



Lack of Oxygen. — Our knowledge of respiration in this 

 group of parasites is still very meager, so it is scarcely possible 

 to say just how the above condition might effect the parasites. 

 It appears that both derive their oxygen from the blood of the 

 host, in which they float, and it is also possible that Aphidius, 

 being the more active of the two and better adapted for breath- 

 ing might get around and monopolize it; though it is doubtful 

 if this could be reduced low enough to kill Aphelinus and not 

 work permanent injury on Aphidius. 



Economic Importance. 



While the above theories are of considerable importance 

 from a biological standpoint, it is sufficient for economic pur- 

 poses simply to know which of the two parasites will dominate 

 when the two come together. This the writer has shown. Since 

 Aphidius is the more efficient one in every respect, there need 

 be little fear over the fact that it will kill Aphelinus, since this 

 efficiency will in no way be lessened where they accidentally 

 come together. The aphids that Aphelinus parasitized would 

 not interfere with Aphidius, and would be so many more dead 

 aphids in addition to those killed by Aphidius. 



