June, 1922 bionomics of alphelinus semiflavus 229 



Effect of Parasite on Host. 



On Internal Structures. — As soon as the young Aphelinns 

 larva hatches, it begins feeding on the fat body and developing 

 young of the host. Within three or four days after hatching 

 nearly all the young aphids within the mother are destroyed. 

 The feeding continues on the other structures, until nothing is 

 left, when the parasite larva approaches maturity, but the 

 digestive tract. This is in common with the habits of other 

 parasitic Hymenoptera larva which spare the vital organs of the 

 host until the very last. 



The larva of Aphelinus is armed with well developed man- 

 dibles, which, however, do not project out of the mouth as in 

 the larva of the Braconid parasite, Aphidius; but seem situated 

 back in the mouth cavity where they are less effective in tearing 

 up host tissue. There is no gathering of bunches of fat globules 

 in a host parasitized by Aphelinus, as in one harboring Aphidius, 

 which would indicate that the fat cells are not broken down. 

 However, many bits of tissue and separate cells may be observed 

 floating promiscuously around in the body fluids of a host 

 parasitized by Aphelinus. 



On Exter?ial Appearance. — The first external appearance of 

 parasitism is a change in color of the host from green to light 

 cream. Then a honey-colored spot appears in the abdomen, 

 which is the darker mesenteron of the larval parasite within. 

 This light color of the host deepens into a yellow or light amber 

 in another day, and by the following day begins to turn gray. 

 Within a very few hours after graying begins, the aphid is coal- 

 black, except for the head and appendages, which are trans- 

 parent. Death occurs during this last darkening. Just before 

 death the aphid spreads out its legs in symmetrical form and 

 firmly grasps the plant on which it rests. This tends to make 

 them adhere to the plant after death; still, they are further 

 secured by some form of silk secretion through the ventral wall. 

 No detailed observations were made on this act, but it is evi- 

 dent that no hole is cut through, as in Aphidius. Perhaps only a 

 very small puncture is made and the silk forced through it. 

 The amount is very small, at most, and in some cases there is 

 not enough to detect; nevertheless, the black aphid remains 

 adhere quite firmly to the substratum. 



