498 Journal of AgrictUtural Research voi.v. no. n 



tightly inclosed in the layer of thin, flat cells. The serosal cells are scat- 

 tered through the body of the host. The chorion shrinks and probably 

 finally dissolves. The young larvae are now 0.7 mm. in length and start 

 feeding, after cutting through the amnion in the mouth region. At this 

 time the mass of cells which forms the caudal vesicle has grown out 

 through the anal opening. 



THE LARVA 



First instar (PI. L, fig. 5). — The first larval instar averages 3>^ 

 days, the first molt taking place, on an average, 8^ days after oviposition. 



The larva grows rapidly, increasing in length approximately from 

 0.7 mm. at hatching to 3.5 mm. at the first molt. 



The head is made up of i segment and the body appears to have 10, 

 but in subsequent growth the tenth segment divides into 2, making 11 

 in all. There are no spines or hairs on the segm.ents, except a few in the 

 oral region. Owing to the rapid growth of the larva, the embryonic 

 envelope in which it is inclosed becomes ruptured and gradually falls off, 

 although portions of it may remain until the first molt takes place. The 

 mandibles are constantly in motion, attacking the fat body of the host. 

 This, together with the blood, is the food of the parasites during this 

 stage and is drawn in by means of a sucking pharynx. The alimentary 

 tract does not change, except to increase in size, it being still further 

 lengthened as the caudal vesicle expands. 



Immediately following hatching, the slender cells of the mass which 

 protrudes from the distended anal opening are compressed lengthwise, so 

 that they become broad, flat cells, thus immensely increasing their 

 exterior and interior surfaces, and there is formed at the end of the larva 

 a large sac, the caudal vesicle, the walls of which are made up of a layer 

 of broad, thin cells (PI. L, fig. 5). The two Malpighian vessels are drawn 

 out into the caudal vesicle, their relative positions being the same as in 

 the embryo. 



The origin of this caudal vesicle and its functions in the two endopara- 

 sitic stages will be considered later. 



The nervous system appears as in the embryo, its growth keeping pace 

 with the growth of the larva. 



No tracheal system is visible during this instar. 



The heart can be seen forming in the early part of this instar. It lies 

 dorsally and has nine pairs of valves, its lateral controlling muscles being 

 readily seen. Anteriorly it narrows to an aorta which opens into the poste- 

 rior region of the head. Instead of ending normally in the posterior 

 end of the body, a rudimentary tube lying dorsally in the caudal vesicle 

 connects with the heart (Pi. L, fig. 4). This tube extends posteriorly, 

 opening in the dorsal posterior region of the caudal vesicle, and forms a 

 channel through which the blood is sucked into the heart. When the 

 heart commences to function, which it does during this stage, the blood. 



