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BULLETIN 136&, IT. S. DEPARTMENT OF AGEICULTUEE 



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 

 E. bovis do not differ materially from those of H. ImeaUim, but 

 the larvae 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 



FIG. 7. — Hypo- 

 derma Un-ea- 

 tum: Row 

 of eggs at- 

 tached t o 

 hair. Great- 

 ly enlarged 



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 

 first-stage larvae these are provided with a heavily chitinized, 

 large, circular, elevated base. 



The second-stage larva of H. bovis, which was first described by 

 Phibbs (78), is remarkably similar to that of H. lineatum in the 

 same stage excepting the cephalopharyngeal skeleton and mouth 

 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 

 H. lineatum, but the spinous armature shows no material differ- 

 ences. The cephalopharyngeal skeleton and mouth hooks show 

 the same specific characteristics as in the earlier stages (figs. 

 12 and 13). 



FOURTH STAGE 



Fig. 



derma tovis: 

 Egg attached 

 to hair. 

 Greatly en- 

 larged 



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. 



