Feb. 4. i9i8 Interrelations of Fruit-Fly Parasites in Hawaii 289 



first instar, seems entirely able to destroy larva of O. humilis that are 

 newly hatched and unencumbered with a body engorged with food. 



All opiine larvae within an individual fruit-fly larva or pupa may some- 

 times be fatally wounded and no parasite develop. This is not frequent. 

 Often the deposition of 8 or 10 parasite eggs into a single fly larva causes 

 its death. In such cases the parasite eggs usually hatch and the resulting 

 larvae die within a short time. 



Cases occurred in which as many as 10 dead larvae of Opius humilis were 

 dissected from a fruit-fly larva, together with a single, vigorous, active 

 larva of Diachasma tryoni. This does not necessarily mean that all of 

 the 10 larvae were killed by the latter. No doubt some of the former 

 larvae destroyed each other, but it clearly shows the superior aggressive 

 and defensive power of the larva of D. tryoni. Many cases have been 

 observed in which a larva of O. humilis was badly cut and distorted from 

 attack by a larva of D. tryoni. Some such cases are shown in Plates 

 10, 12, and 13. Occasionally the body may be found entirely severed from 

 the head. These extreme cases are no doubt caused by reattack upon the 

 larva a day or more after it has died and has become somewhat softened. 

 In most cases the death of the larva seems to be caused by the first grasp 

 or pinch of the attacking larva. A single perforation in the body wall 

 should be sufficient to cause the death of the larva in a short time. 



Occasionally an opiine larva will destroy eggs of its own kind when it 

 occurs in the same individual host with the eggs. In this manner mature 

 embryos are sometimes very much distorted and almost unrecognizable. 

 This is not frequently seen. 



Cool weather materially retards the development of the opiine &gg, 

 particularly in the case of Diachasma tryoni and D. fullawayi. At such 

 times many cases have been observed in which larvae of Opius humilis 

 have developed to the second instar before an egg of a species of Dia- 

 chasma, which had been deposited into the same host larva harboring 

 O. humilis, had hatched. Upon hatching the small, active Diachasma 

 larv^a quickly destroyed the large, bulky Opius larva. This unusual con- 

 dition has been observed only in January in the cool, elevated, coffee 

 districts on the Island of Hawaii. 



From over 2,900 cases where parasitized fruit-fly puparia have been 

 opened, in no single instance has a case been observed in which the host 

 pupa was fofmed. A host lar\'a once parasitized quite readily forms 

 into a normal puparium when sufficiently developed, but the presence 

 of a single small opiine egg within its body invariably prevents any 

 further development. The puparium is formed, the histolysis of the 

 tissues is completed, and here all development of the fruit fly ceases. 

 The broken-down and liquid medium thus prepared within the puparium, 

 in which the parasite larvae may move about and feed, enables them to 

 reach all portions within and easily to come in contact with any other 

 parasitic individuals that may occur there with them. 



