THE CABINET OF NATURAL HISTORY 
they have remained on the hairs four or five days they be- 
come ripe, after which time the slighest application of 
warmth and moisture is sufficient to bring forth, in an in- 
stant, 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 up 
by accident, it may pass on to the intestinal canal before it 
hatches ; in which case its existence to the full is more 
precarious, and certainly not so agreeable, as it is ex- 
posed 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 for- 
tunate 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 
water, or are hatched by the sun and moisture thus supplied 
together. When in the mouth of the animal they have the 
dreadful ordeal of the teeth and mastication to pass through. 
On their arrival at the stomach, they may pass mixed with 
the mass of food into the intestines ; and when full grown, 
in dropping from the animal to the ground, a dirty road or 
water may receive them. If on the commons, they are in 
danger of being crushed to death, or of being picked up by 
the birds who constantly attend the footsteps of the cattle 
for food. Such are the contingencies by which nature has 
wisely prevented the too great increase of their numbers, 
and the total destruction of the animals 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 exis- 
tence in the stomachs of all this tribe of animals. These 
larva attach themselves to every part of the stomach, but are 
generally more numerous about the pylorus, and are some- 
times, though much less frequently, found in the intestines. 
Their numbers in the stomach 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, being fixed by the small end to the inner mem- 
brane of the stomach, which they adhere to by means of two 
small hooks, or tentacula. When they are removed from 
the stomach they will attach themselves to any loose mem- 
brane, and even to the skin of the hand. The body of the 
larva is composed of eleven segments, all of which, except 
the two last, are surrounded by a double row of horny bristles, 
directed towards the truncated end, and are of a reddish 
colour, except the points, which are black. The larvae evi- 
dently receive their food at the small end, by a longitudi- 
nal aperture, which is situated between two hooks, or tenta- 
cula. Their food is probably the chyle, which being near- 
ly pure aliment, may go wholly to the composition of their 
bodies, without any excrementitious residue, though on dis- 
section the intestine is found to contain a yellow or greenish 
matter, which is derived from the colour of 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 they 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 convenient 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 moisture, in any considerable degree, would 
probably be fatal to it. The flies never pursue the horse 
into the water. This aversion I imagine arises from the chilli- 
ness of that element, which is probably felt more exquisite- 
ly by them, from the high temperature they had been ex- 
posed to during their larva state. The heat of the stomach 
of the horse is much greater than that of the warmest climate 
being about 102 degrees of Farenheit, and in their fly state 
they are only exposed to 60, and from that to about 80 de- 
grees. This change, if suddenly applied, would in all pro- 
bability be fatal to them ; but they are prepared for it by 
suffering its first effects 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 summer.” Nicholson. 
FRESH AND SALT LAKE OF MEXICO. 
There is no lake in the world, we know of, like this : 
a part of its water is fresh, and the other salt ; which gives 
room to think that there are two sources, though but one 
lake appears. 
The fresh water seems stagnant and motionless, and the 
salt water ebbs and flows as the sea, with this difference, 
that it does not follow the rule of tides, being only produced 
by the blowing of winds, which sometimes makes this lake 
as tempestuous as the sea. 
The fresh water of this lake is good and wholesome, and 
affords plenty of small fish ; it is higher than the salt water, 
and falls into it ; the part of the lake that ebbs and flows is 
brackish, and has no sort offish. 
