ttle may be treated during the summer with fly repellents (p. 502) to
keep off the warble flies. The efficacy of repellents against these flies
is probably, however, not very great.

In localities where the character of the cattle industry is such as to
render practicable the systematic examination of cattle and the removal of
the grubs--that is, where the herds are comparatively small and subject to
the close supervision of the owners--it is possible, by the exercise of a
little care and with very little effort on the part of the cattle owners,
provided they work together, each doing his share by seeing to the removal
of grubs from his own cattle, so that as few as possible survive to
transform into flies, to reduce the number of grubs within one or two
seasons almost, if not entirely, to the point of extinction.

Investigations not yet completed indicate that grub eradication may
sometimes be accomplished by the use of arsenical dips, which are
extensively used at the present time for destroying cattle ticks. (See p.
497.) It is possible that the destructive action of arsenical dips upon
warbles is more or less dependent upon the fact that arsenic is stored up
in small quantities in and upon the skin of cattle that are repeatedly
dipped in arsenical dips. The arsenical dip appears to act, not upon the
well-developed grub beneath the skin, but upon the eggs or the newly
hatched larvae, probably the latter. Accordingly the dipping of cattle to
destroy grubs should be carried out during the fly season and repeated
treatments should be given every two or three weeks, as in dipping cattle
to eradicate ticks.

[Illustration: FIG. 6.--The warble fly (_Hypoderma lineatum_): _a_,
adult female; _b_, eggs attached to a hair, x 25; _c_, larva as seen
in egg; _d_, larva from esophagus of an ox; _e_, later stage of larva
from beneath the skin of the back; _f_, larva at the stage when it
leaves the back of cattle and falls to the ground--all enlarged
(after Riley).]

LICE.[16]

Cattle in the United States are commonly infested with three species of
lice, two of them sucking lice (_Haematopinus eurysternus_, the short-nosed
cattle louse, and _Linognathus vituli_, the long-nosed cattle louse),
commonly known as blue lice, and one biting louse (_Trichodectes
scalaris_), commonly known as the red louse.

The blue lice (figs. 7 and 8) suck the blood of cattle and are more
injurious than the red lice (fig. 9). Unl