572 



BOT. 



prepares a second egg, and, poising herself before 

 the part, deposits it in the same way. The liquor 

 dries, and the egg becomes firmly glued to the hair 

 This is repeated by various flies, till 'four or five 

 hundred eggs are sometimes placed on one horse.' 

 Here two circumstances are observable ; first, that, 

 unlike the oestrus of the oxen, this fly instils no 

 terror into the object of its attack ; and, secondly, 

 that as the grub when hatched, does not, like that o! 

 the ox, gnaw its way into the body of the animal, it 

 is essential that the instinct of the parent fly should 

 be exerted to the utmost, in the selection of those 

 precise spots where its offspring will be secure of 

 obtaining the means of conveyance into the stomach 

 of the animal. Now the accomplishment of this 

 exploit, for so we may well term it, presents an exam- 

 ple of that reasoning Vacuity, as we may almost con- 

 sider it, which certain animals amongst, the lowest 

 ranks of the creation exhibit in an extraordinary 

 degree. That this is the case will at once be perceived, 

 when it is stated that these eggs are placed only in 

 those precise spots which the horse can lick with its 

 tongue. After remaining in this situation for four or 

 five days, the eggs are sufficiently ripe to bring forth 

 the larvae on the slightest application of warmth and 

 moisture. " At this time," continues Mr. Clark, " 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 

 growth is more precarious, and certainly not so agree- 

 able, as it is exposed to the bitterness of the bile. I 

 have often with a pair of scissors clipped off some 

 hairs with the 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 larvee would not appear, though 

 held in the hand under the same circumstances for 

 several hours, a sufficient proof that the eggs them- 

 selves are not conveyed to the stomach." We have 

 seen that during the act of oviposition no alarm is 

 created by the fly, nor is this the case subsequently, 

 although the contrary is stated in " Insect Architec- 

 ture," p. 409. Mr. Clark, indeed, expressly states that 

 it is owing to the irritations of " other flies, such as 

 conopes, tabani and muscse, who, by settling on the 

 skin, occasion a horse to lick himself on those parts, 

 and thus receive the larvae on the tongue and lips, and 

 a horse that has had no ova deposited on him, may 

 yet have the bots by performing the friendly office of 

 licking another horse that has." As, however, Mr. 

 Clark has stated, that the eggs, when ripe, often hatch 

 of themselves, and that the larva without a nidus 

 crawls about till it dies, we should be inclined to 

 consider that somewhat of the irritation above alluded 

 to may be occasioned by the motion of the grub 

 amongst the hairs of the body. Of the great num- 

 bers of these insects which we have stated are depo- 

 sited on the body of a single horse in the egg state, 

 few arrive at the perfect state, being subject to nume- 

 rous casualties, 'so that probably near a hundred 

 are lost for one which becomes a fly ; first there is 

 the chance of these eggs being washed off by wet ; 

 others are hatched and not taken up by the mouth ; 

 others, when in the mouth, are crushed to death ; 

 some may pass into the intestines of the horse ; and 



when full grown and fallen to the ground many may 

 be trodden to death, fall into water, or be eaten bv 

 birds. If however they escape the dangers which 

 threaten their early existence, they make their way 

 to the stomach of the horse, where, in a degree of 

 heat far greater than that of the hottest climate, 

 10-2 Fahr., they attach themselves in clusters, espe- 

 cially about the pylorus, varying in number from six 

 to a hundred, fastening themselves by the small end 

 of the body, with the assistance of two small hooks, 

 which are not found in such species as reside beneath 

 the skins of various animals. Here their food con- 

 sists probably of the chyle ; and Mr. Clark is of opinion 

 that they are not so very injurious to the horses as 

 is generally conceived. After arriving at their full 

 growth, these larvae detach themselves and" are passed 

 with the faeces, when falling to the ground they .seek 

 out a convenient spot where they are changed to 

 pupae, and in about six or seven weeks the fly appears, 

 during the month of August. It is to be observed 

 with respect to these insects, that Mr. Clark, who 

 has given a very minute account of the larvap, as well 

 as of what he considered to be the pupae of several 

 of the species, was not acquainted with the real pupaa. 

 It is true, that, previous to becoming flies, the insect 

 appears of a very different form to that which it pos- 

 sessed in the larva state, but this change of form 

 takes place in the covering of the larva, which be- 

 comes shorter and hardens, forming, in fact, a cocoon, 

 in which the real pupa is enclosed, as is the case with 

 many other flies belonging to the same order. The 

 Gasterophilus eqni, whose habits we have thus detailed, 

 is one of the largest in the genus, being about two- 

 thirds of an inch long ; it is of a dirty yellowish colour, 

 with the back of the thorax, and a line of spots on 

 the abdomen (which is of a redder colour) black, the 

 wings are white, with a band and two small apical 

 spots of a dark brown colour. The other species in 

 the genus whose habits has been noticed, G. /iccmor- 



Gasterophilus haemorrhoidalis. 



rhoidalis, resembles the former in the form and habits 

 of the larva ; but the economy of the perfect insect 

 as to the mode of oviposition is very different* Lin- 

 naeus, on the authority of Reaumur (who derived his 

 nformation from Dr. Gaspari), describes it as " mire 

 per anum cntrans" a perfectly fabulous statement, 

 which has nevertheless been repeated by many 

 authors, and which seems to have originated from 

 confusing this insect with the forest fly, Hippobotca 

 quina, which sometimes gets beneath the tail of the 

 lorse. With respect to the mode of depositing its 

 eggs, Mr. Clark observes, " the part chosen by this 

 nsct for this purpose is the lips of the horse, which 

 s very distressing to the animal, from the excessive 

 Atillatiou it occasions, for he immediately after rubs 

 us mouth against the ground, his fore legs, or some- 

 times against a tree. At the sight of this fly the 

 lorse appears much agitated, and moves his head 

 )ackwards and forwards in the air, to baulk its touch 



