180 



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 apairof 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 tlie 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 larvse 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 cuntaiua 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 flyappears. 

 " 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 Farenhcit, 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 

 nighttime, 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. 



