or Horse-fly, in zoology; a genus of insects, belonging to the order of diptera. The beak consists of two valves, is cylindrical, obtuse, and hanging; and the feet have several claws. There are four species, distinguished by their wings, &c. The most remarkable is the equina, the pest of horses and cows. This insect is broad, flat, shining, and as it were scaly. Its head, thorax, and abdomen, are yellow, undulated with brown; and the legs are interlocked with yellow and brown. The wings, crossed one over the other, exceed the length of the body by above one half; they are transparent, tinged with a little yellow towards their outward edge, and have a spot near that edge of a brown colour. These insects are very difficult to be killed on account of the hard cutaneous shell which covers them; and they fix so close and fast to the poor animals with their claws, that they cannot rub or bite them off without wounding themselves.