294 Journal of Agricultural Research voi. xii, No. s 



reproduction. Opius humilis, though present everywhere, does not 

 hinder its activities. Extensive records kept in Hawaii during a period 

 of three years on emergences from more than 100,000 fruit-fly puparia 

 from many sections of the islands and from all available types of fruits 

 have clearly shown that the average maximum degree of parasitism by 

 D. tryoni does not exceed or equal that of O. humilis. This alone is suffi- 

 cient proof of superior prolificness on the part of the latter. 



In no instance have fruits of all varieties been found harboring larvae 

 of the fruit fly that are less frequented by Opius humilis than by Dia- 

 chasma tryoni. The ability of the former to find the host is, then, equal, 

 if not superior, to that of the other parasite. In no case has evidence 

 been found to show that the longer ovipositor of the D. tryoni is really 

 an advantage over that of the O. humilis, whose ovipositor is less than a 

 third as long. The comparative difference is considerable, but the dif- 

 ference when considered in fractions of an inch is really small. 



Preliminary records of individual females of both species to determine 

 the total number of eggs deposited by an individual in a lifetime show no 

 superiority of the Diachasma tryoni over Opius humilis in this respect. 



The life cycle of Diachasma tryoni is consistently longer than that 

 of Opius humilis. From 28,410 records on the length of the life cycle of 

 the former, secured during 191 6 and 191 7, it is almost invariably found 

 to be from two to four days longer than that of the latter during most of 

 the year, and in the winter months a great number of the Diachasma 

 individuals, hibernating in the larva stage, extended the cycle to from 

 one to six months longer. From 22,889 cases under observation in 1916 

 and 1 91 7 on the life cycle of O. humilis, in no single case has an individual 

 ever been known to so hibernate or extend the length of the life cycle 

 beyond the average for more than a few days. Cool weather and drouth 

 seem most favorable for inducing this hibernating tendency in the 

 larvae of the D. tryoni. Of 3,077 cases under observation by the junior 

 author in January, February, and March, 191 7, in which fruit-fly larvae 

 had been parasitized by a Diachasma, a total of i ,404 cases occurred in 

 which the parasite went into hibernation as a mature larva. This seems 

 to explain the great reduction in abundance of D. tryoni in the field in 

 winter. Its capacity as a parasite in the winter months is thus strik- 

 ingly less than that of O. humilis.^ 



From the standpoint of longevity, all experiments so far show no great 

 superiority of one species over the other, except as noted above in regard 

 to hibernation. Individual adults of both Opius humilis and Diachasma 

 tryoni have been kept alive for about four months. 



' Credit is here due Mr. J. C. Bridwell for valuable suggestions offered in connection with the study 

 of the hibernation of these parasites. 



