Wing Anatomy
A bat's wing is built on the basic pattern of a mammalian limb. It is an analogous structure to the human arm and hand (in fact, the name "chiroptera" is Greek for "handwing"), but the relative sizes of most bones and muscles are very different. The bat also has unique muscles in the patagium, chest and back, to power the wing during flight. The wing consists of the upper arm, forearm, wrist and hand. The bones of the hand and the four fingers are greatly elongated, light and slender to provide support and manipulate the wing membrane, called the patagium.
The patagium stretches between the fingers, connects the fore and rear limbs, and even extends between the legs (incorporating the tail) of the bat. This interfemoral membrane (between the legs) can also serve as a pouch to catch and hold insects. Although the membrane is free of fur, fur may be specialized to aid airflow. The membrane, which can only withstand tensile loads, is basically two thin layers of skin with a high density of nerves, tendons and blood vessels. Elastin fibers within the membrane increase flexibility and may store energy.
The second digit, the proximal parts of the third digit, and the dactylopatagium medius compose the leading edge of the wing, while the 3rd finger forms the wing tip. The 2nd and 3rd digits are either very close together or fused, to make for a stiff leading edge. The trailing edge of the wing is unsupported. In this fashion, the wing serves as a sort of thin aerofoil with very high camber, allowing it to fly well under low-speed, high-lift conditions.
The bones of the arm and the four fingers primarily support and manipulate the patagium. The very light bones of the bat wing are often oval shaped, with two major consequences: 1) the stresses are lateral, reducing breakage, and 2) the wing is more streamlined. The bat uses the thumb, which is short and has a sharp claw, to cling to various surfaces. The hindlimbs are attached at the hip of the bat at a 90 degree rotation, pointing the legs sideways and the knees backward. This is designed both to facilitate roosting upside down, and to support the patagium during flight. The overall wing structure, which is very light, makes it ideal for quick maneuvering and hovering during flight.
