BOTS are the Larve or Caterpillars of the Gad-Fly, belonging to the order Diptera and the genus (Estrus, and distinguished by this peculiarity, that they pass the larval state of their existence within some living animal, and feed on the juices or substance of that animal. There are numerous species of them. Every quadruped on which they prey has its peculiar fly. The notice of a few of those most commonly known will suffice.
The (Estrus Equi, or Gad-Fly of the Horse, belongs to the genus Gasterophilus of some entomologists, so called from its larym inhabiting the stomach of that animal. It is distinguished from the other (Egli by the smoothness of the thorax, and by the eyes in both sexes being equidistant from each other, not quite half an inch in length, with gauze-like yellow and brown wings, its chest of a rusty colour approaching to a brown hue on the sides and with a yellow tinge posteriorly, its belly of a reddish-brown superiorly and a dirty gray beneath, with its extremity almost black. The whole insect is thickly covered with down. The Gad-Fly is seen in the latter part of the summer very busy about horses : this is the impregnated female depositing her eggs. She approaches the horse, selects some part which be can reach with his tongue, and which he is in the frequent habit of licking ; she balances herself for a moment, and then suddenly darting down, deposits an egg on one of the hairs, which adheres by a glutinous substance that surrounds it. She continues her labour with wonderful perseverance until she has parted with fifty or a hundred eggs, and then having exhausted herself, she slowly flies away, or drops at once and dies.
If a horse at grass is carefully examined in August, some hundreds of these minute eggs will be found about its legs and the back part of the shoulder, and few or none out of the reach of his tongue. In two or three days these eggs are sufficiently matured to be hatched. Possibly the horse feels a little inconvenience from all this glutinous matter sticking about and stiffening the hair, and he licks the part, and by the pressure of the tongue, and the mingled influence of the warmth and moisture of it, the ova are burst, and a small worm escapes from each. It clings to the tongue, and is thus conveyed into
the mouth ; thence it is either carried with the food into the stomach, or, impelled by instinct, it travels down the gullet, being too small to inconvenience or annoy the horse. Thus it reaches the stomach, and by means of a hook on each aide of its mouth affixes itself to the cuticular or insensible coat of that viscus. It scoops out a little hole, into which its muzzle is plunged, and there it remains until the early part of the summer of the following year, feeding on the mucous or other matter which the coats of the stomach afford. It has now become an inch in length and of corresponding bulk, and ready to undergo its change of form. It detaches itself• from the cuticular coat to which it had adhered, and plunges into the food which the other and digestive portion of the stomach contains ; it paasess with the food through the whole length of the intestines, and is discharged with the dung. Sometimes it is not perfectly enveloped in the fecal mass; it then clings to the sides of the anus, and hangs there firmly until there is a soft place beneath on which it may drop ; it then hastens to burrow into the earth, and, if it has escaped the birds that are eagerly watching for it, it has no sooner hollowed for itself a convenient habitation than a Shelly covering is formed around it, and it appears in the state of a pupa or chrysalis.
It here lies torpid for a few weeks, preparing to undergo its last change. It assumes the form of a perfect fly ; it then bursts from its prison, rises in the air, and seeks its mate. The work of fecundation being accomplished, the male immediately dies : the female lingers a day or two in order to find the proper deposit for her eggs, and her short life also terminates.