366 
OESTRUS. 
ceeds from the chrysalis in the space of about two 
months. 
“ The manner, (says Mr. Clark,) in which this 
species deposits its ova has, I believe, not been 
described ; nor is it easy to see, though close to 
the animal at the time, exactly in what way this 
is accomplished, owing to the extreme agitation 
of the Sheep ; but the motions of the Sheep after- 
wards, and the mode of defence it takes to avoid 
it, can leave but little doubt that the egg is de- 
posited on the inner margin of the nostril. 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 sliould be lying in wait for them. 
If tliey observe one, they gallop back, or take 
some other direction. As they cannot, like the 
horses, take refuge in the water, they have re- 
course to a rut, or dry dusty road, or gravel pits, 
where they crowd together during the heat of the 
day, with their noses held close to the ground, 
which renders it diflicult for the fly conveniently 
to get at the nostril. Observations on these flies 
are best made in warm weather, and during the 
heat of the day, when, by driving the sheep from 
their retreats to the grass, the attack of the fly 
and the emotions of the sheep are easily observed.” 
The other British Oestri are the O. lueinorrlioi- 
dalis of Linnjeus, whose larva, like that of the 
