Dec. 15, 1920 Optus fletcheri, Parasite of the Melon Fly in Hawaii 425 



This is the active stage of the larva, in which it is specially equipped 

 with long, sharp mandibles for its struggle for survival over other larvae 

 of the same species, which it often finds in the same host individual. 

 This struggle takes place immediately after hatching, and usually within 

 four hours all but one of the larvae of Opius fletcheri have been killed. 

 Many cases have been observed where there were only one living and 

 from two to eight dead parasite 

 larvae in the same host individual. 

 Thus, having all the food material 

 of its host available for itself, the 

 surviving larva is able to proceed 

 with its development to the adult 

 stage. 



The duration of this instar 

 varies greatly and depends upon 

 the development of the host. The 

 larva never molts into the second 

 instar until the parasitized host 

 larva has formed its puparium. 

 Several instances have been ob- 

 served where larvae of Opius 

 fletcheri have developed to adults 

 while other individuals, from eggs 

 laid at the same time, still re- 

 mained first-instar larvae. The 

 host larvae of the former formed 

 their puparia soon after they were 

 parasitized, while those of the lat- 

 ter were still in the larval stage 

 when examined. In all the ex- 

 periments to prove this point the 

 host was Ceratitis capitata,\a.rvgd of 

 which were feeding in the fruits of 

 Mimusops elengi. These fruits 

 become rather dry soon after fall- 

 ing from the tree, so that fruit- 

 fly larvae within them find diffi- 

 culty in obtaining sufficient food for rapid development. This results 

 in retarding pupation, sometimes for over three weeks beyond the normal 

 period. On June 1 1 eggs of O. fletcheri were deposited into fruit-fly larvae, 

 which were examined with the following results: On June 18, 10 of these 

 larvae contained living first-instar larvae of O. fletcheri, and 3, that had 

 formed puparia, each contained a fourth-instar larva of O. fletcheri. On 

 June 22, 3 more larvae and 2 of the puparia of this lot were examined. 

 Each of the larvae contained a well-developed living larva of O. fletcheri 



Fig. 3. — Opius fletcheri: Larva, first instar, ventral as- 

 pect, showing head characters and complete tracheal 

 system, and the egg serosal cells. Length 0.88 mm. 



