Among the winged animals, the only one that is viviparous is the bat; it is the only one, too, that has wings formed of a membrane. This is, also, the only winged creature that feeds its young with milk from the breast. The mother clasps her two young ones as she flies, and so carries them along with her. This animal, too, is said to have but one joint in the haunch, and to be particularly fond of gnats.