wt is 
5 feo 
798 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
pharyngeal skeleton is composed of two long and nearly parallel rods slightly 
curved outward at the tip on which two crescent-shaped mouth hooks articulate. 
The hooks are pointed at each end, especially the forward one, which termi- 
nates in a sharp point. A prominent inward-curving tooth is located about 
one-third the length of the entire hook from the anterior tip (figs. 9 and 10). 
A stout, sharp spine directed forward projects slightly between the mouth 
hooks. The anterior spiracles appear as two minute circular | 
elevations above the mouth parts and at the tip of the head. 
The armature and spiracles of the first-stage larva of 
H. bovis do not differ materially from those of H. lineatum, but 
the larve are slightly larger. The outstanding difference is_ 
that the mouth hooks of H. bovis are well forked at the anterior 
end and more truncate at the posterior end. The articulation | 
of the mouth hooks is on a small knob extending laterally nearly 
at right angles to the axis of the pharyngeal skeleton, which is 
not curved at the anterior tip like that of H. lineatum. 
SECOND STAGE 
The second or next known larval stage of H. lineatum 
found in the esophagus of cattle (fig. 11) varies in length from 
less than 3 millimeters up to 13 millimeters or sometimes even 
longer. It is cylindrical in form and tapers slightly at both. 
extremities. The spinous armature is present on all segments. | 
On the body segments the spines are- arranged in transverse | 
rows beginning with the heaviest spines along the anterior | 
border and extending well back toward the posterior border. 
The spines are more numerous per row, thinner, and longer 
than those of the corresponding segments of the first stage. 
The posterior half of the anal segment is covered with stout, 
sharp-pointed, curved ‘spines; unlike the spines in this group on - 
H first-stage larve these are provided with a heavily chitinized, 
PIG. i tigen, large, circular, elevated base. | 
tum: Row The second-stage larva of H. bovis, which was first described by 
of eggs at- Phibbs (78), is remarkably similar to that of H. lineatum in the | 
ee same stage excepting the cephalopharyngeal skeleton and mouth | 
ly enlarged hooks, which exhibit the same differences as in the first stage. 
THIRD STAGE 
The third-stage larva of H. lineatum (fig. 6, c), sometimes found in the; 
gullet just before migration to the back, and later immediately after punc- 
turing the skin of the host, is from 12 to 16 millimeters long and from 2.5 
to 3.5 millimeters wide. It is cylindrical, with the ends tapering and ; 
often with the anal end slightly curved toward the dorsal side. With few 
exceptions segments 2 to 10 inclusive are free from spines, and at the anterior 
end only the group of spines below the mouth parts is present. 
The posterior half of the anal segment is thickly dotted with 
spines having heavy circular bases greater in diameter than the 
length of the spine and nearly three times as great as that of the 
second stage (fig. 6, f). The posterior spiracles measure 14 
microns in diameter, and the triangular spines on the border of 
the spiracles are greatly reduced in size. The form of the 
mouth hooks, although heavier than in the preceding stages, 
remains the same. 
The third-stage larva of H. bovis is slightly larger than that of [',S--Hune 
H. lineatum, but the spinous armature shows no material differ- Egg attached 
ences. The cephalopharyngeal skeleton and mouth hooks show to hae 
the same specific characteristics as in the earlier stages (figs. ae ah ene 
12 and 18). 
FOURTH STAGE 
The fourth-stage larva of Hypoderma lineatum (fig. 6, d, e, g) is from 13 to | 
18 millimeters long and from 3.5 to 6 millimeters wide. It tapers consider-— 
ably from the fourth, fifth, or sixth segments to the posterior extremity. The 
spinous armature varies greatly with different specimens. 
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