B O T 



fonie convenient retreat, and cliange to the chryfalis ; and 

 in about fix or feven wtv-k?, the fly appears ; for the de- 

 fcription of which, fee the conclufion. 



" The mode purfucd by the parent fly, to obtain for its 

 young a fituatioii in the Uoinach of the horfe, is truly fin- 

 gular, and is effefted in the following manner. Wlu-n the 

 female has been impregnated, and the ecTgs are fuflicicntly 

 matiiied, rtie feeks among the horfes a fnbjeft for her pur- 

 pofe ; and adviincing on the wing, (lie holds her body nearly 

 upright in the air, and her tail, which is lengthened for the 

 purpofe, curved inwards and upwards. In this way, (he ap- 

 proaches the part wliere (lie defigns to depolit the egg ; and 

 fufpending herfelf for a few feconds before it, fiiddenly darts 

 upon it, and leaves the egg adhering to the hair ; (he hardly 

 appears to fettle, bnt merely tonches the hair with the egg held 

 out on the projefted point of the abdomen. The egg is made 

 to adhere bv means of a ghitinous liquor fecretedwith it. She 

 t'nen leaves the horfe at a f.iiali dillance, and prepares a fe- 

 cond egg ; and poizing herfelf before the part, dcpofits it 

 in the fame way. The liquor dries ; the egg becomes (irmly 

 glued to the hair; and this proeefs is repealed by various 

 flies, till four or live hundred eggs are fonietimes placed on 

 one horfe. 



"The horfes, when they become ufed to this fly, and find 

 it docs them no injury, as the tabani and coiiopes, by lucking 

 their blood, hardly regard it, and do not appear at all aware 

 of its infidious olijeft. 



" The inlide of the knee is the part on v^-hich thefe flies 

 are molt fond of depofitmg their eggs, and, next to this, on 

 the fide and back part of the Hioulder, and Icfs frequently on 

 the extreme ends of the hairs of the mane. But it is a faft 

 worthy of attention, that the fly does not place them pro- 

 mifcuoufly about the body, but conilanlly on thofe parts 

 which are moll liable to be licked with the tongue; and the 

 ova, therefore, are fcrupuloufly placed within its reach. 

 Whether this be an ait of recfor. or inllinft, it is certainly 

 a vtry remarkable one ; and one (hould fufjicft, if it was the 

 latter, it ought to dirett the performance of tlie ait in one 

 way only. 



*' Whichever of thefe it may be, it is, without doubt, one 

 of the ftrongefl examples of pure inlliniT:, or of the moll 

 circuitous reafoning, any infeil is capable of. The eggs 

 thus depolittd, I at (iril fuppofed were loofened from the 

 hairs by the moifture of the tongue, aided by its ronghnefs, 

 and were conveyed in this way to the ftomach, where they 

 were hatched ; but on more minute fearch, I do not find 

 this to be the cafe, or at leall only by accident ; for when 

 they have remained on the hairs four or five days, they be- 

 come ripe,after which time the flighteft application of warmth 

 and moilhirc is fufficient to bring forth in an inllant the la- 

 tent larva. At this time, if the tongue of the horfe touches 

 the egg, its operculum is thrown open, and a fmall aftivc 

 worm is produced, which readily adheres to the moid furface 

 of the tongue, and is from thence conveyed with the food 

 to the (lomach. 



" I have often clipped off with a pair of fciflars fome 

 hairs, with the eggs on them, from the horfe ; and on plac- 

 ing them in the hand moillened with faliva, they have hatched 

 in a few feconds. At other times, when not perfeftly ripe, 

 the larva would not appear, though held in the hand, under 

 the fame circumllances, for feveral hours ; a fufficient proof 

 that the eggs themfelves are not conveyed to the ftomach. 



" It is fortunate for the animals infclled by thefe infefts, 

 that their numbers are limited and kept within bounds, by 

 the hazards they are cxpofed to. I Ihould fnfpedl near a 

 hundred are loll for one that arrives at the perfcil (late of a 

 fly. The eggs, in the firll place, when ripe, often hatch 



B O T 



of themfelves; and the larva, without a nid';:, crawls about 

 till It dies ; others are fubjeiSl to be waflied oft" by the water, or 

 are hatchtd by the fun and moillure thus applitd together ; 

 when in the mouth of the animal, they have the dread- 

 ful ordeal of the teeth and mallication to pafs throuch ; 

 and on their arrival at the llomaeh, they may pafs, mixed 

 with the mafs of food, into the inteftints ; and when full 

 grown, on dropping from the anus to the ground, a high 

 road or water may receive them ; if on the connr.oii, they 

 are in danger of bcin^ crnflied to death, or of being pi'ckcd 

 up by bii-ds, which fo conllantly for food attend the (oot- 

 lUps of the cattle. Siitli are the contii;giiicies, by which 

 nature has prevented tlie too great iucreafc of their num- 

 bers, and the total dellruitionot the aninialj they feed upon."- 

 I have once fcen the larvrc oi tliis oeltrus in the ftomach 

 of an afs. Indeed, there is little reafon to doubt their exift- 

 ence in the llomachs of all this tribe of animals. 



The pcrfedt fly but ill fullains the changes of weather ; 

 and cold and moillure in any conliderablc degree would pro- 

 bably be fjtal to it. Thefe flies never purfue the horfe into 

 the water ; this averfion, I imagine, arifes from the chillnefs 

 of the atniofphere over that element, which is probablv felt 

 more exquilitely by them, than by other flies, froni the 

 high temperature they had been cxpofed to duriii" their 

 larva ftate. The heat of the ftomach of the horfe is" much 

 greater than that of the warmeft climate ; being about 102*' 

 of Fahrenheit ; and in their fly ftate, they are only expofed 

 to 00", or from that to about 8o°, in very warm Wiaiher. 

 Such a change, if fuddeuly applied, would in all probability 

 be fatal to them ; but they are prepared for it, by fufferin/r 

 its lirlt eff'ecls in the quiefcent and lefs fenfiblc ftate of a 

 chryfalis. I have often feen this fly in cloudy we.^.ther, and 

 during the night time, fold itfelf up with the head and tail 

 nearly in contaft, and lying apparently in a torpid ftate, 

 though in the middle of the fummer. 



I nerer obferved tlufe botta hanging to the extremity of 

 the reftum, previous to quitting it, as the ha;morrhoidales, 

 or fmall horfe -bots, are feen to do. 



Whether thefe botts can at all exift in the ftomach of a 

 carnivorous animal, I am not afTurel. I gave upwards of 

 a hundred eggs, proved by trials to be ripe, and containing 

 a living caterpillar, to a cat in milk at various times ; and 

 on deftroying her, at the end of two months after the lirft 

 portion had been given, I could difcover no traces of thera 

 in the llomaeh or intellines. 



1 gave fix of the larva; of the great horfe-bot, recently 

 taken from a dead horfe's ftomach, and perfeilly alive, to 

 another horfe, in a ball made of meal, in the centre of 

 which they were enclofed : their efifefts on the ftomach were 

 not noticeable by any external indications; 27 of them were 

 the next day exhibited to a horfe with the farcy, without 

 their producing any fenfible cffeil. 



Of the oejlrus hiemorrhoidalis, or fmall horfe-bot. Thl^. 

 larva, or grub, producing this infeiSl, is in moll refpedls 

 like the former, occupying the fame fituation alfo in the 

 ftomach of the horfe : it is ealily diftinguilhed by its fmaller 

 iize, and being nearly or quite dcftitute of fpines, on leavinij 

 the rcdlnm ; when full grown, it alTumes a reddifh green 

 colour, and in about two days becomes a chryfalis. 



None of the larva: of this family of infe<Ss appear to 

 change their Ikin, which at length becomes alfo the (hell of 

 the chryfalis : when fqueezed, they forcibly contraA them- 

 felves into a fmaller fpace, and become very hard. It is 

 probable tliey in this way refill the violent prefliire they mull 

 occafionally luftain from the weight of the food, and the 

 adiions of the ftomach, and in pafling through the inteftines 

 and the fphinder ani. 



P 2 After 



