Journal of Agricultural Research voi. xii, no. s 



the first instar, and particularly during the early period of this stage, 

 before the body becomes engorged and swollen with food. Plate ii, B, 

 which shows a mature first-instar larva of the parasite Diachasma tryoni, 

 is interesting in this connection when compared with the other illustra- 

 tions of newly hatched Diachasma larvse of this instar. The head is 

 unchanged in size, but the body is greatly distended after two days of 

 feeding preparatory to molting to the second instar. In all of the illus- 

 trations the enlargement is the same. It is obvious that the enlarged 

 and somewhat rigid body of the well-fed larva permits much less free- 

 dom of movement than is possible shortly after hatching. The body is 

 first elastic, flexible, and capable of quick and effective action. This 

 change in the size of the body may occur in from two to two and one-half 

 days. 



The molt to the second instar still further incapacitates the opiine 

 larva for carrying on further cannibalistic action. The mandibles are 

 then small, soft, and are almost imperceptible, even under high magnifi- 

 cation, because of their transparency, being wholly unfitted for active 

 use except in the separation of the semiliquid media in which the larva 

 lies after the host has formed the puparium. The strong and heavily 

 chitinized head of the opiine larva in the first instar is thus entirely 

 discarded upon the molt to the second instar, and all further cannibalism 

 ceases. The helpless condition after the first molt is well suggested in 

 Plate 1 1 , A, which shows a larva of Diachasma tryoni in the second instar. 

 During 191 6 a microscopical examination of the contents of a total 

 of 2,925 parasitized fruit-fly larvse and pupae definitely showed that the 

 parasites readily oviposit in the same host larva more than once and 

 exhibit no discernible instinct of selection of parasitized or unparasitized 

 larvse. All eggs so deposited hatch. Thus, fly larvse commonly opened 

 were found to contain several eggs or larvae of the two species of para- 

 sites and sometimes a third species (Diachasma jullawayi) , a later intro- 

 duction. 



Here w^as certain evidence of a complicated overlapping or duplication 

 of parasitism. In no single instance were two parasite larvse ever 

 observed to develop to maturity in the same host larva, except in the 

 case of the chalcid Tetrastichus gifjardianus (p. 292). All but one were 

 killed while still in the first instar, or occasionally before hatching. At 

 times from 8 to 10 opiine larvae were found within the host larva. Only 

 one would mature and, as a rule, if a larva of Diachasma were one of the 

 number, it survived all others. 



If several eggs of Diachasmu tryoni or D. jullawayi alone are deposited 

 in an individual host larva over a period of two or three days, the last 

 parasite larva to hatch stands the best chance for destroying the others 

 and maturing. The case is identical with the Opius humilis However, 

 a well-grown and fully-fed larva of D. tryoni or D. jullawayi, if still in the 



