22 BOARD OF AGRICULTURE. 
male, and the color is lighter. The thorax is ash-colored, 
with four black lines and small black spots. The abdomen is 
light ash, more or less spotted with blackish. The female has 
a tapering abdomen, with a long ovipositor, which is curved 
forward when about to deposit the eggs. 
The flies appear during the whole summer, and according 
to Mr. Riley, deposit the young larve previously hatched in 
the oviduct, on the margins of the nostrils of 
sheep. “The moment the fly touches this part of 
the sheep, they shake their heads and strike the 
ground violently with their fore-feet ; at the same 
time holding their noses close to the earth, they 
run away, looking about them on every side to 
see if the fly pursues. They also “smell to the 
grass” as they go, lest one should be lying in 
wait for them. If they observe one, they gallop back, or take 
some other direction. As they cannot, like the horses, take 
refuge in the water, they have recourse to a rut or dry, 
dusty road, or gravel pits, where they crowd close together 
during the heat of the day, with their noses close to the 
ground, which renders it difficult for the fly conveniently to 
get at the nostril.” (Clark, 1796). This description, al- 
though written long ago, applies equally well to the habits 
both of the fly and sheep, as observed at the present day.. 
The young larve at once proceed to ascend into the nostrils 
by means of their hooks and writhing motions, thus causing 
great irritation and an increased secretion of mucus, upon 
which they feed. They finally reach the frontal and mazxil- 
lary sinuses and other cavities, connected with the nose, and 
attach themselves securely to the mucus membranes by means 
of the two sharp hooks upon their heads. They thus cause 
great inflammation, if in cousiderable numbers, and severe 
illness, or even at times convulsions and death result. This 
disease is often known as “ grub in the head”? among farm- 
ers. The disease is indicated by loss of appetite, frequent 
coughing or sneezing, unusual discharge of matter from the 
nose, slow and weak gait, dullness, inability to stand, tossing 
Fig. 24. 
Figure 24.—Larva of the Sheep-bot (@strus ovis), natural size. From Clark. 
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