June, 1922 bionomics of alphelinus semiflavus 217 



In the majority of instances, the aphid appears undisturbed 

 while oviposition is going on. This is especially true of the 

 younger stages. However, there are times when the slightest 

 touch with the antennas or a prick with the ovipositor will cause 

 them to kick up and make off. If this happens when the ovi- 

 positor is fastened in one of the larger stages, it drags the par- 

 asite with it. If the host is smaller, however, the parasite is 

 able to master it and keep it in place in spite of its struggles. 

 This is done by the parasite standing well up on its legs and hold- 

 ing up the victim slightly so that it cannot get a firm foothold on 

 the substratum. The ovipositor is held in place during the 

 struggle by the presence of three retractile barbs near the tip. 

 These function so efficiently that the parasite itself often has 

 difficulty in getting free, often having to brace the hind legs 

 against the aphid and give several vigorous pulls before it is 

 withdrawn. At times, the ovipositor may be fixed so firmly in 

 place that it permits a larger host to drag the parasite about 

 so violently that injury results. Several times Aphelinus has 

 been observed in a crippled condition after getting free from 

 one of these encounters. In this condition, the ovipositor remains 

 extruded, the parasite not having the power to retract the 

 abdominal segments that bring it back in position, and the 

 individual goes stumbling off with head down and abdomen 

 elevated until it topples over apparently dead. 



The ovipositor is usually inserted on the dorsal surface of 

 the host's abdomen, but almost any other part of the body may 

 be chosen, depending on the point of approach. Many times it 

 has been observed on the head between the antennas, and 

 among Macrosiphum pisi on clover, a large species that stands 

 at a considerable angle from the plant, well up on its long legs, 

 the parasite was observed to attack it from beneath, elevating 

 the ovipositor well up and making contact with the ventral 

 surface of the abdomen. 



Stage of Host Preferred. — A distinct preference was early 

 shown for the younger stages of aphids. In order to determine 

 this more exactly, several experiments were planned and carried 

 out. Some difficulty, however, was encountered in eliminating 

 a number of factors that might alter a true expression of this 

 preference. These factors were the variation in numbers of the 

 different stages available for oviposition ; the high mortality of 



