Nov. 2s. i*9i8 Biology of Fruit-Fly Parasites in Hawaii 



425 



Fig. 7. — Diackasma 

 tryoni: Mandible of 

 second-instar larva, 

 showing mandible of 

 third instar pushing 

 from within. Length 

 0.C2I nun. 



When the lan^a has molted to the second instar, the molted skin can 

 be dissected easily from the fly puparium (fig. 3). In the second instar 

 the larva is greatly changed (fig. 6). The head does not stand out 

 strongly differentiated from the other body segments as in the preceding 

 instar. It is soft, unchitinized, and without pro- 

 nounced visible characters. The articulations of the 

 14 body segments can be clearly seen. The body is 

 glabrous throughout. The mandibles (fig. 7) are soft 

 and translucent and can be seen only with difficulty. 

 The weight of a coverglass may easily crush them be- 

 yond recognition. They are sharply pointed, short, 

 and about as long as broad, averaging 0.021 mm. in 

 length. Mandibles are not needed in this instar, aS 

 the food is composed entirely of fluids, minute glob- 

 ules of fat, and possibly fragments of disintegrated tissue. As the de- 

 velopment of the larva progresses the mandibles of the third instar may 

 be seen distinctly pushing into ultimate position at the bases of the 

 mandibles. The larv^a averages about 1.5 mm. in length in this stage. 



One striking feature 

 in the second instar is 

 the total absence of 

 tracheae, as careful ex- 

 aminations of more 

 than 100 second-instar 

 larvse under the highest 

 magnification and best 

 light failed to reveal 

 In view of the presence of 



Fig. 8. — Diackasma tryoni: Larva of the third instar, dorsal aspect 

 Length 2.9 mm. 



any evidence of tracheal trunks or branches 



a well-marked respiratory system in the preceding instar, the absence, at 



this stage, of tracheae is of distinct interest. As the lar\^a is now immersed 



in a thin liquid there would seem to be no need for tracheae. The 



digestive tube is filled with food, and, as in 



the first instar, takes the form of the simple 



midintestine. The oily fat globules of the 



host which are ingested are conspicuous in 



this portion of the intestine. This intestine 



is closed caudally, although the short hind 



intestine may be seen leading up to it. In 



this instar the larva is very sluggish, and 



there is no need for action, considering the 



accessibility and character of the food. * The duration of this stage has 



not been determined accurately. There is no vvdde variation in its 



length, however, such as occurs in the first and fourth instars. The 



average duration of the second larval instar is about 48 hours. There 



Fig. 9. — Diackasma tryoni: Mandible 

 of third-instar larva, showing man- 

 dible of fourth instar pushing from 

 within. Length 0.035 mm. 



