Patagium

Patagium on a flying squirrel

The patagium is a membranous structure that assists an animal in gliding or flight. The structure is found in bats, birds, some dromaeosaurs, pterosaurs, and gliding mammals.

Bats

In bats, the skin forming the surface of the wing is an extension of the skin of the abdomen that runs to the tip of each digit, uniting the forelimb with the body.

The patagium of a bat has four distinct parts:

1. Propatagium: the patagium present from the neck to the first digit
2. Dactylopatagium: the portion found within the digits
3. Plagiopatagium: the portion found between the last digit and the hindlimbs
4. Uropatagium: the posterior portion of the body between the two hindlimbs

Pterosaurs

In the flying pterosaurs, the patagium is also composed of the skin forming the surface of the wing. In these ornithodirans, the skin was extended to the tip of the elongated fourth finger of each hand.

The patagium of a pterosaur had three distinct parts:

1. Propatagium: the patagium present from the shoulder to the wrist. Pterosaurs developed a unique bone to support this membrane, the pteroid.
2. Brachiopatagium: the portion stretching from the fourth finger to the hindlimbs.
3. Uropatagium or cruropatagium: the anterior portion between the two hindlimbs, depending on whether it did or did not include the tail

Other

In gliding species, such as some lizards, rodents and other mammals, it is the flat parachute-like extension of skin that catches the air, which allows gliding flight.

In some lepidoptera insect species, it is one of a pair of small sensory organs situated at the bases of the anterior wings.

In birds, the propatagium is the elastic fold of skin extending from the shoulder] to the carpal joint, making up the leading edge of the inner wing. Many authors use the term to describe the fold of skin between the body (behind the shoulder) and the elbow that houses the outer segments of the latissimus dorsi caudalis and triceps scapularis muscles. Similarly the fleshy pad that houses the follicles of the remiges (primary and secondary feathers) caudal to the hand and the ulna is also often referred to as a patagium. The interremigial ligament that connects the bases all the primary and secondary feathers as it passes from the tip of the hand to the elbow is thought to represent the caudal edge of the ancestral form of this patagium.