Feb. 4, 1918 Interrelations of Fruit-Fly Parasites in Hawaii 293 



fruit fly. It is occasionally bred out from fruit-fly material secured in 

 the field. Several times during dissections of puparia from the field its 

 larvae have been found in combination with larvae of Opius humilis or of 

 one of the species of Diachasma. Though soft, sluggish, and armed with 

 small, inconspicuous, blunt mandibles, it nevertheless survives the opiine 

 larvae by sheer force of numbers and consequent rapid absorption of 

 food. When ovipositing, this chalcid usually places about 10 eggs in 

 the host at one insertion of the ovipositor. The opiines deposit but one 

 egg, remove the ovipositor, and look for further larvae to attack. A 

 single oviposition, then, by a T. giffardianus into a host larva already 

 parasitized by an opiine, places about 10 individuals of the chalcid with 

 I of the opiine. If the host larva has already received several opiine ovi- 

 positions, usually only one individual is alive, as already shown. The larvae 

 of T. giffardianus exhibit no cannibalistic tendencies, and do not destroy 

 each other. Many of the chalcid larvae are then killed by the opiine 

 larvae, but seldom all of them. The opiines ultimately die, and one or 

 more of the Tetrastichus larvae develop. The death of the Opius or Dia- 

 chasma larvae results usually from starvation or suflfocation or possibly 

 by the absorption of toxic excretions of the Tetrastichus larvae. Certainly 

 the chalcid larvae inflict no visible bodily injury on the opiine larv^. 



In view of the demonstrated ineffectiveness of this chalcid and of its 

 capability for surviving the Opius humilis when occurring in fruit-fly 

 larvae with it, it is here considered a detrimental introduction because of 

 interference with the work of the latter. 



OPIUS HUMILIS, THE MOST PROLIFIC OF THE INTRODUCED 



PARASITES 



From the foregoing summaries of the data it is obvious that the 

 parasite Opius humilis is killed in the larva stage in almost every instance 

 in which its larvae become associated in a host larva with any one of the 

 three other species of introduced fruit-fly parasites and that the percent- 

 age of cases of such duplication is large. Biological studies of all of these 

 parasites, so far as two and three years' adaptation in a new country may 

 show, have indicated quite clearly that under Hawaiian conditions 

 O. humilis is the most prolific of the four species in all seasons of the year 

 and that none of the others show particular abilities for reaching larvae of 

 the fruit fly that are not as easily accessible to the attack of O. humilis. 



The very considerable activity of the parasite Diachasma tryoni and 

 the occasional heavy parasitism by it has made necessary the establish- 

 ment of careful proof that it is less prolific than Opius humilis, before it 

 can be maintained that the introduction of the former is a detrimental 

 one. The unimportance of the other two parasitic species eliminates all 

 present need for discussing them any further. 



From what has already been shown, we know positively that the 

 parasite Diachasma tryoni has a clear field in Hawaii for unchecked 



