OESTRUS. 
time the slightest application of ■warmth 
and moisture is sufficient to bring fortli, in 
an instant, the latent larva. At this time, 
if the tongue of the horse touches the egg, 
its operculum is thrown open, and a small 
active worm is produced, which readily 
adheres to the moist surface of the tongue, 
and is from thence conveyed with the food 
to the stomach. If the. egg itself be taken 
lip by accident, it may pass on to the intes- 
tinal canal before it hatches ; in which case 
its existence to the full growth is more pre- 
carious, and certainly not so agreeable, as 
it is exposed to the bitterness of the bile. 
“ I have often, with a pair of scissars, 
clipped off some hairs with eggs on them 
from the horse, and on placing them in the 
hand, moistened with saliva, they have 
hatched in a few seconds. At other times, 
when not perfectly ripe, the larva would 
not appear, though held in the hand under 
the same circumstances for several hours ; 
a sufficient proof that the eggs themselves 
are not conveyed to the stomach. It is 
fortunate for the animal infested by these 
insects that their numbers are limited by 
the hazards they are exposed to. I should 
suspect near a hundred are lost for one that 
arrives at the perfect state of a fly. The 
eggs, in the first place, when ripe, often 
hatch of themselves, and the larva, without 
a nidus, crawls about till it dies ; others are 
washed off by w'atcr, or are hatched by the 
sun and moisture thus su{)plied together. 
When in the mouth of the animal they 
have the dreadful ordeal of the teeth and 
inasticatiou to j)ass through. On tlieir 
arrival at the stomach, they may pass 
mixed with the mass of food into the in- 
testines ; and when full grown, in drop- 
ping from the animal to the ground, a dirty 
road or water may receive them. If on tlie 
commons, they are in danger of being crush- 
ed to death, or of being picked up by the 
birds who so constantly attend the footsteps 
of the cattle for food. Such are the con- 
tingencies by which nature has wisely pre- 
vented the too great increase of their num- 
bers, and the total destruction of the ani- 
mals they feed on. 
“ I have once seen the larva of this oestrus 
in the stomach of an ass ; indeed there is 
little reason to doubt their existence in ihe 
stomachs of all this tribe of animals. These 
larva attach themselves to every part of the 
stomach, but are generally most numerous 
about the pylorus, and are sometimes, 
though much less fieqiiently, found in the 
intestines. Their numbers in the stomacli 
are very various, often not more than half a 
dozen, at other times more than a hundred ; 
and, if some accounts might be relied on, 
even a much greater number than this. 
They hang most commonly in clusters, be- 
ing fixed by the small end to the inner 
membrane of the stomach, which they ad- 
here to by means of two small hooks, or 
tentacula. When they are removed from 
the stomach they will attach themselves to 
any loose membrane, and even to the skin 
of the hand. The body of the larva is com- 
posed of eleven segments, all of which, ex- 
cept the two last, are surrounded with a 
double row of horny bristles, directed to- 
wards the truncated end, and are of a red- 
dish colour, except the points, which are 
black. The larva evidently receive their 
food at the small end, by a longitudinal aper- 
ture, which is situated between two hooks, 
or tentacula. Their food is probably the 
chyle, which, being nearly pure aliment, 
may go wholly to the composition of their 
bodies without any excrementitious resi- 
due, though on dissection the intestine is 
fbimd to contain a yellow or greenish mat- 
ter, which is derived from the colour of the 
food, and shows that the chyle, as they 
receive it, is not perfectly pure. They 
attain their full growth about the latter end 
of May, and are coming from the horse 
from this time to the latter end of June, or 
sometimes later. On dropping to the ground 
they find out some conveuient retreat, and 
change to the chrysalis ; and in about six or 
seven weeks the fly appears. 
“ The perfect fly but ill sustains the 
changes of weather; and cold and mois- 
ture, in any considerable degree, would 
probably be fatal to it. These flies never 
pursue the horse into the water. This 
aversion I imagine arises from the chillness 
of that element, which is probably felt 
more exquisitely by them, from the high 
temperature they had been exposed to dur- 
ing their larva state. The heat of the sto- 
mach of the horse is much greater than that 
of the warmest climate, being about lOit 
degrees of Fahrenheit, and in their fly state 
they are only exposed to 60, and from that 
to about 80 degrees. This change, if sud- 
denly applied, would in all probability be 
fatal to them ; but they are prepared for it 
by suffering its first eft'ects in the quiescent 
and less sensible state of a chrysalis. I have 
often seen this fly, during the night time and 
in cold weather, fold itself up with the 
head and tail nearly in contact, and lying 
apparently in a torpid state through the 
middle of the summer.” 
O, ovis; wings pellucid, punctured at ths 
