Hand-wing
Chiroptera—the scientific name for the order of bats—literally means "hand wing." A bat's wing is actually two layers of skin stretched between the second through the fifth fingers and also attached to the arm. The first finger (thumb) is detached from the membrane and bats use it to move along trees, cave walls and other surfaces. Young bats use the thumb to cling to their mothers. Of course, they use their locking foot claws to hang upside down when roosting and hibernating.