426 Journal of Agricultural Research voi. xv, no. s 



is little to distinguish the third (fig. 8) from the second lar^'-al instar, and 

 it is even less pronounced in character The mandibles (fig. 9) are 

 slightly heavier and are about 0.035 "^^i^- i" length. Otherwise they are 

 almost identical with those of the previous instar. The mandibles of 

 the forming fourth instar can be seen pushing from within into the 

 bases of the mandibles. There are no hard, darkened or chitinized parts 

 in any portion of the body. As in the preceding instar, no traces of tra- 

 cheae occur, but late in the development of this stage the strong, heavy 

 tracheal trunks, branches, and stigmata of the succeeding instar may 

 be seen organizing beneath the skin. The stigmata do not, at any 

 period in the development of the third instar, become opened to the 

 surface, as they are not a developed accessory of this stage, but belong 

 strictly to the succeeding instar. The bod)^ is glabrous throughout. 

 Late in the development of this stage the spiny cuticula of the succeeding 



instar may be seen beneath 

 the integument. No change 

 has been noted in the digest- 

 ive tract, other than that of 

 a gradual increase in the 

 volume of food ingested and 

 the increased volume of \vaste 

 matter accumulating in the 

 closed midintestine (fig. 10). 

 This stage averages about 



Fig. 10. — Diachasma tryoni: Third-instar larva, lateral 2.d. mm. in lenSTth. It Still 

 aspect, showing digestive canal. Length 3.1 mm. ,. . , . , , •, 



lies immersed in the body 

 fluids of its host, although shortly before molting to the last instar a 

 large part of the body is usually exposed in the hollow puparium. 

 The average duration of the third larval instar \n\\ approximate 48 

 hours. 



When the fourth instar is reached, the conditions surrounding the 

 larva have undergone a great change. Much of the liquid and semiliquid 

 contents of the host have been consumed, and within a short time little 

 remains in the puparium but the parasitic larva. To meet this condi- 

 tion, the parasitic larva is possessed from the first of a well-defined 

 tracheal system. Nine large, open stigmata are borne on each side of 

 the second to the eleventh body segments (fig. 11). No stigmata occur 

 on the third segment, however, although a branch from the tracheal 

 trunk on each side leads to points on the surface corresponding in posi- 

 tion with the stigmata on the other body segments. A main tracheal 

 trunk extends along each side of the body with a special branch to each 

 stigma and to the dorsal and ventral portion of each body segment. A 

 connecting branch joins the two main trunks posteriorly and anteriorly. 

 With the exception of the head the entire body is closely covered with 



