OESTRUS. 
359 
in diameter, with an opening at the top of each, 
through which may he observed the larva, im- 
bedded in a purulent fluid: its appearance is that 
of an oval maggot, of a yellowish white colour 
while young, but growing gradually darker as it 
advances in age, till at the time of its full growth 
it is entirely brown. It is chiefly in the months 
of August and September that the eggs are laid, 
and the larvae remain through the ensuing winter 
and till the latter part of the next June before 
they are ready to undergo their change into chry- 
salis. At this period they force themselves out 
from their respective cells, and falling to the 
ground, each creeps beneath the first convenient 
shelter, and lying in an inert state becomes con- 
tracted into an oval form, but without casting 
the larva skin, which dries and hardens round it. 
When the included insect is ready for exclusion, 
it forces open the top of the pupa or chrysalis 
coat, and emerges in its perfect form, having re- 
mained within the chrysalis somewhat more than 
a month. 
Though the history of this insect in its larva 
state has long ago been detailed with suflicient 
accuracy by Vallisneri, Reaumur and others, yet 
the Fly itself appears to have been very generally 
confounded, and that even by Linnasus himself, 
with a very different species, resembling it in size, 
but which is bred in the stomach and intestines 
of horses, the larvco being no other than the whitish, 
rough maggots which Farriers call by the title 
of Cots. This insect is the Oestrus Equl^ ex- 
