274 
Journal of Agricultural Research 
Vol. XXVIII, No. 3 
some one of the body segments. These spines, although similar in shape and 
size to those in rows on the earlier stages, are not placed in regular order as they 
appear in either the first or second stage. The occurrence of a small group of 
irregularly arranged spines on the otherwise spineless body segments of the third 
stage can be explained as an occasional individual variation, and clearly indi¬ 
cates their presence as the result of a molt, as spines so arranged are never present 
in the first or second instars. 
Although the characters as set forth above should be sufficient to show clearly 
that there are at least three distinct stages of Hypoderma lineatum in the course 
of the migration of the larvae from the point of entrance to the place of encyst- 
ment on the back, where two subsequent larval molts take place, it has been 
the author’s good fortune to obtain two larvae of H . lineatum during the process 
of molting from the second to the third stage, thereby definitely establishing 
this fact for one of these molts, at least. These larvae, both approximately 13 
mm. in length, were taken, one from the gullet and one from the diaphragm 
of two different animals slaughtered in a packing house at Dallas, Tex. 
The specimen taken from the gullet, although almost completely free from 
its cast-off second-stage exuvium, was still possessed of part of it. The newly 
molted specimen, a third-stage larva, is free from spines on the body segments 
while the remaining part of the exuvium extending back nearly as far as the third 
segment of the larva, and plainly showing the torn borders, is covered with the 
usual second-stage armature. In the case of the larva taken from the diaphragm, 
i>he entire and unbroken second-stage exuvium was present and was easily 
removed from the smooth larva. This newly molted third-stage specimen, 
spineless on all the body segments, together with the cast-off second-stage 
-exuvium possessed of the entire spinous armature of that stage, was cleared, 
mounted, and preserved. 
The rarity of finding a larva in the process of molting is no doubt due to the 
very delicate structure of the exuvium and the extreme difficulty in removing 
the larva from the connective tissue of the host without losing the skin. In the 
«ase of the first molt, which probably takes place soon after the larva gains 
-entrance to the host, when it is extremely small, it is almost hopeless to obtain 
& specimen at the instant of molting from the first to the second* stage. In fact, 
no first-instar larvae have ever been found after having left the point of entrance. 
THE SECOND-STAGE LARVA OF HYPODERMA BOVIS 
The second-stage larva of Hypoderma bovis was unknown in 1921, at the time 
the writers preceding paper was published, but its existence was considered 
probable and deemed necessary to fill the gap between the first and third stages 
which are so similar to those stages in H. lineatum. Since then Phibbs 6 of 
Ireland has collected and described this stage. At about the same time Wells 
and Brundrett, of the Bureau of Entomology, who conducted investigations in 
the State of New York, made a number of collections of second-stage larvae of 
H. bovis , all of which, with the exception of a single specimen from the gullet, 
were taken from the neural canal of slaughtered cattle. 
The characters of the second-stage larva of Hypoderma bovis are remarkably 
similar to those of H. lineatum in the same stage, except, of course, the difference, 
as pointed out before, in the mouth hooks and in the anterior end of the cephalo- 
pharyngeal skeleton. The arrangement of the spinous armature of the head 
segments, the transverse rows of spines on the body segments, with the heavier 
spines in the anterior rows of each segment, and the spines, with broa’d bases, on 
the anal segment are all almost identical in appearance with those of H. lineatum 
in the same stage. 
6 Phibbs, Q.— the labval mouth-hooks of hypodebma, Irish Nat. 31: 25-30, illus. 1922. 
