What is popularly known as the "grub" is the larva of the _[oe]strus
oris_, or gad-fly of the sheep. It is composed of five rings; is
tiger-colored on the back and belly, sprinkled with spots and patches of
brown; its wings are striped.
The sheep gad-fly is led by instinct to deposit its eggs within the
nostrils of the sheep. Its attempts to do this--most common in July,
August, and September--are always indicated by the sheep, which collect
in close clumps, with their heads inward, and their noses thrust close
to the ground, and into it, if any loose dirt or sand is within reach.
If the fly succeeds in depositing its egg, the latter is immediately
hatched by the warmth and moisture of the part, and the young grubs, or
larvae, crawl up the nose, finding their devious way to the sinuses,
where, by means of their tentaculae, or feelers, they attach themselves
to the mucous membrane lining those cavities. During the ascent of the
larvae, the sheep stamps, tosses its head violently, and often dashes
away from its companions wildly over the field. The larvae remain on the
sinuses, feeding on the mucus secreted by the membrane, and apparently
creating no further annoyance, until ready to assume their _pupa_ form
in the succeeding spring.
Having remained in the sinuses during the fall and winter, they abandon
them as the warm weather approaches in the latter part of spring. They
crawl down the nose, creating even greater irritation and excitement
than when they originally ascended, drop on the ground, and rapidly
burrow into it. In a few hours, the skin of the larvae has contracted,
become of a dark-brown color, and it has assumed the form of chrysalis.
This fly never eats; the male, after impregnating two or three females,
dies; and the latter, having deposited their _ova_ in the nostrils of
the sheep, also soon perish.
The larvae in the heads of sheep may, and probably do, add to the
irritation of those inflammatory diseases, such as catarrh, which attack
the membranous lining of the nasal cavities; and they are a powerful
source of momentary irritation in the first instance, when ascending to,
and descending from, their lodging-place in the head. But in the
interval between these events, extending over a period of several
months, not a movement of the sheep indicates the least annoyance at
their presence. They are, moreover, found in the heads of nearly all
sheep, the healthy as well as the diseased, at the proper season.
_Treatment._ Though the presence of the grub constitutes no disease,
some think it well to diminish their number by all convenient means. One
simple way of effecting this is, by turning up with a plough a furrow of
earth in the sheep-pasture, into which the sheep will thrust their noses
on the approach of the _[oe]strus_, and thus many of them escape its
attacks. Some farmers smear the noses of their sheep occasionally With
tar, the odor of which is believed to repel the fly. Another plan,
deemed efficacious in dislodging the larvae from the sinuses, is as
follows: Take half a pound of good Scotch snuff, and two quarts of
boiling water; stir, and let it stand till cold. Inject about a
table-spoonful of this liquid and sediment up each nostril, with a
syringe; repeat this three or four times, at intervals, from the middle
of October till January. The efficacy of the snuff will be increased by
adding half an ounce of asaf[oe]tida, pounded in a little water. The
effects on the sheep are immediate prostration and apparent death; but
they will soon recover. A decoction of tobacco affords a substitute for
snuff; and some recommend blowing tobacco smoke through the tail of a
pipe into each nostril.