60 
afterwards, and the mode of defence it takes to avoid it, there is 
little doubt that the egg is deposited on the margin of the nos¬ 
tril. 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 running away, and holding their noses close 
to the ground, and looking about them on every side to see if 
the fly pursues, and as they go along they often smell also to the 
grass, and look anxiously into it, lest one should be lying in wait 
for them ; if they observe one, they gallop back again, or take 
some other direction.' As they cannot,, like the horses, take 
refuge in the water, they have recourse to a rut, dry dusty road, 
or gravel-pit, as a defence, as we have before remarked. PI. II. 
fig. 16. 
Observations on these flies are best made in dry weather, and 
during the heat of the day, when by driving the sheep from their 
retreats, the attack of the fly and the emotions of the sheep are 
easily observed. 
I imagine the nostril becomes highly inflamed and sore, from 
their repeated attacks, and the consequent rubbing of the nose 
against the ground, which, together with' certain instinctive 
apprehensions of these flies, occasion their touch to be so much 
dreaded. 
From the difficult and precarious mode these Oestri pursue in 
depositing their eggs, they cannot succeed in depositing but a 
few in each sheep ; whereas if on the contrary they actually 
entered those cavities of the face to effect it, they must deposit 
them all, and in one subject, the improbability of which in re¬ 
spect to the other species is already stated. 
Of the Larva. From one to seven or eight are generally found 
in the cavities of the face, what are called by anatomists of the 
human, the maxillary and frontal sinuses, but which in quadru¬ 
peds are cavities of considerable extent and magnitude, and the 
thin-flexible bones which constitute them are covered with a 
