422 



Journal of Agricultural Research 



Vol. XV, No. 8 



organized body, wherein its food, which seems largely* the fat body of the 

 host, is in a semisolid state, in part isolated into definitely compacted 

 masses. The larva is lodged in an area which is well organized in mus- 

 cular, digestive, nervous, and respiratory structures, all of which com- 

 bine to interfere with its freedom of action. Special characters, not 



appearing in the succeeding in- 

 stars, are peculiarly adapted to 

 this stage. The head is large, 

 heavily chitinized and brownish, 

 and bears a pair of sickle-like 

 mandibles, with bases broadly 

 separated and capable of wide 

 movement and quick action. 

 Above the mandibles and seem- 

 ingly on the labrum is a pair of 

 small, short antennal structures, 

 which are frequently extended 

 and withdrawn in a rapid, vibra- 

 tory manner as the larva feeds 

 and moves about. On the ce- 

 phalic edge of the chitinized ven- 

 tral portion of the head is borne 

 a pair of pointed teeth, well sepa- 

 rated and together forming a dis- 

 tinct letter U with the basal con- 

 necting line more or less straight. 

 This lies below and directly be- 

 tween the bases of the mandibles 

 (fig. 3). Its shape affords the 

 best character for differentiating 

 the larva of this species from the 

 newly hatched larva of Diachas- 

 ma fullawayi or that of Opius 

 humilis. A clearly defined, sim- 

 ple tracheal system is present 

 (fig. 4) and becomes filled with air 

 shortly after the lar\^a has 

 hatched. No spiracles occur, but 

 eight minute, oval swellings can 

 be seen along each main tracheal trunk in body segments i to 8. The 

 larva lies strictly within the host, and the .air which quickly fills the 

 tracheae must .be obtained by osmosis from the aerated liquid media 

 surrounding it. The tracheae are filled with air before food has been 

 taken, which shows that the air is not extracted internally from the 

 ingested food. The digestive canal (fig. 5) is a simple, straight tube 



Fig. s.—Diachasma iryoni: Cast skin of first-instar larva, 

 showing head characters of first instar and serosal cel- 

 lular mass still clinging to ventral surface. Length 

 I mm. 



