498 
Journal of Agricultural Research 
Vol.V, No. 13 
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. 
THK IyARVA 
First instar (PI. T, fig. 5).-—The first larval instar averages 3^ 
days, the first molt taking place, on an average, 8K 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 i9 made up of 1 segment and the body appears to have 10, 
but in subsequent growth the tenth segment divides into 2, making n 
in all. There are no spines or hairs on the segments, 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. T, 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, 
