26 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE 
than is H. lineatum. Frequently cattle will run for a knoll if shade 
is not at hand, and when the breeze is strong the grouping of the 
stock on high ground seems to give some: immunity from attack. 
When these natural protections are not accessible the flies, especially 
H. bovis, will keep up their attack intermittently, causing the cattle 
to run from one part of the pasture to another, in small pastures 
causing them to run round and round until they froth at the mouth, 
breathe heavily, and even drop from exhaustion. Under range eon- 
ditions the stock usually have better opportunity of escaping, but the © 
tendency to a general stampede is increased, especially if cattle are 
being worked in large herds. 
When grazing, cattle usually detect the presence of a fly very 
quickly, largely by sight and ‘hearing, but also by touch. Cattle 
readily differentiate between the presence of a heel fly and other i in- 
sects. The listening attitude assumed when a fly is in the vicinity is 
characteristic, as 1s “also the action of shaking the foot (fig. 5, @) to 
dislodge a fly, the rolling action of the tail, “and the look of fright 
when a fly attacks in earnest. 
~% 
FT 
DESCRIPTION OF STAGES 
THE EGG 
The egg of Hypoderma lineatum (fig. 6, a: fig. 7) is dull yellowish white 
and the surface is smooth and shining. It is narrowly ovoid, slightly larger 
at the base than at the tip, and its greatest diameter is at the middle. The 
average length of the egg proper is 0.76 millimeter and the average diameter at 
its greatest thickness is 0.21 millimeter. The unattached end of the egg hasa 
slight ridge across it from side to side along which the egg splits when hatching 
takes place. (Fig. 6, 0.) This ridge crosses the end slightly on the side toward 
the hair and the micropyle is located centrally at the apex. The clasp with 
which the egg is attached to the hair is oval in outline. The average length of 
the base attached to the hair is 0.381 millimeter. The petiole averages 0.1 
millimeter in length. It is flat and curved and very narrow when viewed from 
the side. The position of the eggs when attached in a row is at an angle of 
about 45° to the axis of the hair, except the last one toward the tip of the 
hair, which is usually more nearly parallel with it. : 
The eggs of H. bovis are sim:-lar to those of H. lineatum except in size, meas- 
uring 0.81 millimeter in length and 0.29 millimeter in width. The clasping 
base is slightly more truncate at the end toward the tip of the hair and the 
petiole arises from the clasp more nearly at its middle than in H. lineatum. 
The most striking difference between the eggs of these two species is the attach- 
ment of the petiole to the base of the egg. With H. lineatum the attachment is — 
on the side of the base of the egg away from the hair whereas with H. bovis the 
attachment is more nearly in the middle of the base and the petiole is more — 
elbowed. (Fig. 8.) Furthermore, the eggs of H. lineatum are normally at- 
tached to the hair in rows, whereas with H. bovis the eggs are always laid 
singly. 3 
ay THE LARVA 
FIRST STAGE 
The larva of Hypoderma lineatum when hatched from the egg measures 
from 0.55 to 0.65 millimeter in length and from 0.15 to 0.18 millimeter in width ~ 
at its greatest diameter. The width is greatest at the posterior end and the © 
larva tapers to the head. It is creamy or dull white in color, and densely 
covered with spines on all segments, the anterior borders bearing the 
heaviest spines in transverse rows, followed usually by six rows of spines, — 
more or less regularly placed, and slightly decreasing in size toward the pos-— 
terior border of the segment. The anal segment differs from all those pre- 
ceding in that it bears spines of three distinct types. The posterior spiracles, — 
which are represented by two dark circular spots, are protected by two or ~ 
three rather large, triangular spines located near their borders. The cephalo- 
