448 
Journal of Agricultural Research 
Vol. XXI, No. 7 
but they are always individually clear and distinct in outline. The 
color of the stigmal plates remains almost constant during this instar. 
While the number of disks is usually close to 20 in each plate it runs 
as high as 28 or even higher in exceptional cases and as low as 14 in rare 
instances. The yellowish color and the more distinct separation of the 
disks readily facilitate counting these structures. The surface of the 
plates is usually flat in perfect specimens and measures from 0.201 to 
0.301 mm. at its greatest diameter. 
The characters of the stigmal plate of the fourth-stage larva of II. bovis 
(fig. 8) differ from those of H. lineatum (fig. 7) in that the whole plate pre¬ 
sents a somewhat larger area which is black and usually contains about 
30 disks. The borders of the disks are wider and are all or nearly all 
heavily fused together so as to present a solid black mass. The number 
of disks in each plate is also variable, dropping as low as 26 or going as 
high as 38 in different specimens. 
The surface of the plate of a per¬ 
fect specimen is usually convex 
and projecting slightly above the 
surrounding tissue. The size of 
the plate varies from 0.201 to 0.368 
mm. at its greatest diameter. With 
uncleared plates the^ size of the 
Fig. S.—Hypoderma bovis: Posterior stigmal plates of disks appears to be Smaller in H. 
fourth stage larva. Greatly enlarged. bovis than in II. lineatum and they 
are more indistinct, but after clearing they measure practically the same. 
The color of the plate remains constant throughout this stage. 
Discoloration or dryness of specimens sometimes renders determina¬ 
tion rather difficult, but if the surface of the stigmal plate in such in¬ 
stances is kept covered with a film of water or alcohol the characters 
are usually brought out plainly. Occasionally specimens of H. lineatum 
of the fourth instar are found that show stigmal plates of an almost dark 
brown color which may at first appear rather confusing, but even in such 
instances the clear and unfused outline of the individual disks if com¬ 
pared with the solidly fused mass of disks of H. bovis remains constant, 
and it is very easy to determine the species by this character alone. 
DIFFERENTIATION OF H. BOVIS AND H. LINEATUM IN THE THIRD 
INSTAR 
The third-stage larva, or the first stage found in the back and hereto¬ 
fore known as the second stage, presents no distinguishing characters in 
either the structure or the shape of the posterior spiracles or in the 
scanty, minute armature just below the mouthparts and 011 the end of 
the anal segment. The spiny armature is entirely lacking on all the mid¬ 
dle segments, and that which appears around the posterior spiracles and 
