Which bat fingers support the wing membrane during flight?

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bat Livingstone's fruit bat (*Pteropus livingstonii*) in flight.

In bats, the fingers—excluding the thumb—are greatly elongated and serve to support the wing membrane during flight.1

While the thumb remains free of the membrane to assist with climbing, handling food, or roosting, the remaining digits are joined by a double layer of skin that extends from the forearm to the body and leg. Among these supporting fingers, the third digit is particularly notable for its length and its ability to fold over when the bat is not in flight.1