in zoology, a species of worms which can be produced and nourished only in the intestines of a horse. It is there alone they can enjoy the proper temperature of heat, and receive the nourishment necessary for them. See Oestrus.

Besides the long worms which have been observed in the bodies of horses, there are also short ones.—By these are to be understood what we call botts.

All authors, both ancient and modern, who have treated of the diseases of horses, have taken notice of these worms; but M. Vallifnieri is the first who has traced them to the last stage of their transformation, and has seen them change into a hairy kind of fly like the drone.

The flies from which these botts are produced inhabit the country, and do not come near houses, at least not near those of great towns; and therefore horses are never liable to have these worms (i.e. botts) in their bodies, if they have been kept in the house, especially in a town, during the summer and autumn.

It is in the former of these seasons, and perhaps too in the beginning of the latter, that the females of these flies apply themselves to the anus of horses, and endeavour to gain admittance, in order there to deposit their eggs, or perhaps their worms.

The precise instant of their entrance will scarce admit of an eye-witness, but by the merit chance; yet M. Vallifnieri says, that Dr Gaspari had attained this very uncommon sight. The Doctor (he tells us) was one day looking at his mares in the field; and from being very quiet, he observed, that on a sudden they became very restless, and ran about in great agitation, prancing, plunging, and kicking, with violent motions of their tails. He concluded, that these extraordinary effects were produced by some fly buzzing about them, and endeavouring to settle upon the anus of one of them; but the fly not being able to succeed, he observed it to go off with less noise than before, towards a mare that was feeding at a distance from the rest; and now the fly taking a more effectual method to obtain its design, passed under the tail of the mare, and so made its way to the anus. Here at first it occasioned only an itching, by which the intestine was protruded with an increasing aperture of the anus; the fly taking the advantage of this, penetrated further, and secured itself in the fold of the intestine;—this effected, it was in a situation proper for laying its eggs. Soon after this, the mare became very violent, running about, prancing, and kicking, and throwing herself on the ground; in short, was not quiet, nor returned to feeding, till after a quarter of an hour.

The fly then, we see, can find means of depositing its eggs, or perhaps its worms (i.e., botts), in the fundament of the horse; which once effected, it has done all that is necessary for them. If these bott worms are not hatched when first deposited in the horse, but are then only eggs, it will not be long before it happens, from the nutritive heat they there receive.

These bott-worms soon make their way into the intestines of the horse: they occupy such parts of this region as are to them most convenient; and sometimes (as we shall see presently) they penetrate even to the stomach. All the hazard they appear to be exposed to, is that of being carried away from the places they have fixed on by the excrement, which may seem likely to drive all before it. But nature has provided for all things; and when we shall have further described these bott worms, it will seem that they are able to maintain their situation, and to remain in the body of the horse, as long as they please.

There is a time when these bott-worms are of themselves desirous to leave this their habitation, it being no longer convenient for them after the purposes of their growth are answered. Their transformation to a fly must be performed out of the horse's body; and accordingly, when the time of their transformation draws near, they approach towards the anus of the horse; and then leave him of their own accord, or with the excrement, with which they then suffer themselves to be carried along.

According to Mr de Reaumur's observations, the bott worms have two unequal claws, by which they are enabled to remain in the intestines of the horse in opposition to all efforts of the excrement to force them out.—These claws are of a sort of anchor, differently disposed from those of common anchors, but contrived to produce the same effect. Besides these two claws, nature has given them a very great number of triangular spines or bristles, very sufficient to arm them against the coats of the intestines, and to resist the force employed to drive them towards the anus, provided the head be directed towards the stomach of the horse.

It will be asked, no doubt, if these bott worms are not dangerous to horses?—The mares which afforded Mr de Reaumur, for several years, those on which he made his observations, did not appear to be less in health than those which had none; but it may sometimes happen, that they are in so great a quantity in the body of the horse as to prove fatal to him. M. Vallinieri supposes these bott-worms to have been the cause of an epidemical disease that destroyed a great many horses about Verona and Mantua in the year 1713.—The observations communicated to him by Dr Gaffari sufficiently confirm his supposition. This gentleman, upon dissecting some horses that died of this distemper, found in their stomachs a surprising quantity of short worms; of which to give us some idea, he compares them to the kernels of a pomegranate opened: each of these, by gnawing on the coat of the stomach, had made for itself a kind of cellule therein, each of which would easily contain a grain of Indian wheat. It is easy to imagine by this means the stomach must be reduced to a wretched condition; the outer membranes were inflamed, and the inner ones ulcerated and corrupted; a very small quantity of these worms were found in the small intestines, and only a few in the larger, to which last they were found affixed, but had not corroded them. It is only perhaps when these bott-worms are in great numbers, and thereby inconvenience each other in the intestines of the horse, that they make their way towards the stomach; and indeed a very few flies must be enough to overstock the inside of a horse, provided they should deposit all their eggs, and such should all be animated, M. Vallinieri having counted 700 and odd in the body of one single fly.

When one of these botts has left the anus of the horse, it falls on the ground; and immediately seeks out for some place of safety, where it may retire, to prepare for the last stage of its transformation, by which it is to become a fly. And now by degrees the skin hardens and thickens; and at length forms a solid shell or cocoon, the form of which scarcely differs from that of the worm. It is first of a pale red colour, which changes into chestnut; and at length, by the addition of gradual and successive shades of brown, the shell is rendered black. The worm, or bott, before it passes into a nymph, is of the form of an oblong ball; it remains in this form much longer than worms of the flesh-fly kind. M. de Reaumur met with worms that retained this figure five or six days; as yet, one can perceive no traces of the legs, wings, and head of the nymph. Hence he first learned, that those bott worms do not become nymphs immediately upon their first change; but that, in order to become flies, they must undergo one change more than caterpillars ordinarily do to become butterflies.—For the cure of horses troubled with botts, see Farriery, § XV.