424 



Journal of Agricultural Research 



Vol. XV, No. 8 



Fig. s. — Diachastna tryoni: Larva of first instar about to molt, lateral 

 aspect showing food canal filled with fat globules and illustrating 

 the beginning of the formation of the meconium. Length 1.5 mm. 



The duration of the first-instar larva is dependent upon a curious 

 circumstance. The lar\-a never molts until the fruit-fly larva attempts to 

 pupate. Thus, a small fly larva may be stung by the parasite while the 

 larva is in a fruit of dry texture or hard pulp. Usually the larva will 

 not mature or try to pupate until from 6 to lo days later when in such 

 a fruit. In this case the parasite egg hatches in the usual time (54 to 73 

 hours) and the parasitic lar\^a remains in the first instar during the 

 long 6- or 7-day period remaining until the host prepares to pupate. 

 _ Under ordinary cir- 



cumstances the para- 

 site stings a mature 

 larva, which usually 

 forms a puparium a 

 few days later. Here 

 the parasitic larva 

 hatches about the 

 time the host larva is 

 fully developed and 

 ready to attempt pu- 

 pation. The first instar in such cases lasts from 36 to 48 hours. 

 The following specific cases were observed during 1917: Two small 

 fruit-fly larvae, each stung by a female of Diachasma tryoni on June 

 14, still contained first-instar tryoni larvae on June 21. Thirty-one 

 small fruit-fly larvae, stung by tryoni females on August i , still contained 

 first-instar tryoni larvae when opened on August 8. On August 12 four 

 had formed into puparia by 9 a. m.^ They were opened on the same 

 day at 4 p. m. and each was found to contain a freshly transformed 

 second-instar tryoni 

 larva. The other fly 

 larvae did not show 

 signs of pupating after 

 several days. Four of 

 them were opened on 

 August 16 and each 

 contained a first-instar 

 tryoni lar\^a. The re- 

 maining four, still active, were opened August 1 8 and found to contain a 

 well-developed tryoni larv^a in the first instar. In this series of examina- 

 tions of the 12 parasitized fly larv^ae the first 4 produced tryo7ii \a.ryse 

 whose first instar lasted about 36 hours, the second 4 contained tryo7ii 

 lan^ae whose first instar had already lasted about 6 days and the last 4 

 contained tryoni larvas: whose first stage had already extended about 8 

 days. The extended time in the duration of the instar was in each case 

 entirely controlled by the delay of the host in attempting to pupate. 



Fig. 6. — Diachasma tryoni: Larva in second instar, dorsal aspect, show- 

 ing general shape of body and food canal. Length 1.5 mm. 



'AH references to clock time refer to "standard time." 



