to the long axes of the bones exhibit a lower stiffness than the skin that is oriented parallel to the long axes of the bodies. stiffer skin is necessary for bat wing skin oriented in the direction parallel to the long axes of the bones to prevent too much deformation of bat wing skin during flight (with respect to the bone), resulting in the shearing of the bat wing skin off of the bone. flexible skin is necessary for the direction perpendicular to the long axes of the bones for facilitating the shape changes needed for movement and control during flight. this anisotropy of bat wing skin is also useful as method of storing and releasing elastic energy, particularly during the downstroke. during the downstroke, the bat extends its wing and the wing skin experiences an aerodynamic force. the wing skin expands and counteracts the aerodynamic force. after the wing is fully extended, the orientation of the wing is nearly vertical and the aerodynamic forces on the wing are reduced. as the aerodynamic force is reduced, the wing recoils, drawing the digits of the wings together in preparation for the upstroke. gliding lizards there are two different mechanisms by which lizards glide through the air. both mechanisms involve the patagia. active parachuting mechanism in the active mechanism, skeletal supports and muscles run through the patagia of lizards. the skeletal supports and muscle erect the flight membrane and control the gliding using the patagia. most of the lizards that exhibit this active gliding mechanism are agamine lizards such lizards in the genus draco. for the passive mechanism of gliding in lizards, the patagia is unfurled by air pressure alone. the patagia of the passive mechanism differs from patagia of the active mechanism; there is the lack of skeleton support and musculature in patagia of the gliding lizards with the passive gliding mechanism . the passive mechanism of gliding is seen in smaller lizards such as the geckos of the genus ptychozoon. for the passive mechanism of gliding, body movements are believed to control the descent of the gliding lizard. passive parachuting mechanism the surface area to body ratios of lizards with different gliding mechanisms are similar, but how the surface area is distributed is different. the difference in the distribution of surface area indicates the differences in the role of the patagia and accessory areas for the different flight mechanisms. lizards with passive gliding mechanisms tend to have smaller patagia relative to lizards with active flight mechanisms. however, lizards with passive flight mechanism have, ore surface area located in accessory areas (i.e. webbed toes, tail) than lizards with the active gliding mechanism. the structure of the skin of the patagia and accessory areas for the patagia of a ptychozoon kuhli, flying gecko, exhibiting the passive gliding mechanism consists of five layers; a layer of adipose tissue is surrounded by a layer of dermis on each side (ventral and dorsal) and a layer of epidermis is external to the two dermal layers. the distribution of the adipose tissue is thickest close to the body wall. this thick layer of adipose tissue at close to the body wall is believed to provide a "safety factor" for the structural elements of the skin ( i.e. collagen fibers) near the body wall. the thick layer of adipose tissue is more compliant than the structural elements of the body wall (i.e. ribs, muscles), therefore will more readily deform (absorb force) before the structural elements of the skin experience a force. the layer of adipose tissue also aids in the creation of the domed and cambered shape of the patagia. with regards to the structure of the dermal layer of the patagia, there is a thick layer of collagen fibers oriented parallel to the axis of the patagial ribs. these collagen fibers act as the structural support for the shape of the patagia, and provide the stiffness necessary to resist shape chang . the most prominent features of the epidermal layer of the patagia are the scales. the morphology of the dorsal scales of the patagia change as a result of their functional role. a large portion of the dorsal scales of the patagia are arranged in regular rib-like pattern, which guide the flow of air and allow for the lizard to behave as an airfoil. however at the hinge joints (places where patagia folds and attaches to limbs), the regular rib like structure of scales breakdown into a more random distribution of scales. this breakdown of scales is believed to aid in the mechanical loading of the patagia during the unfurling process and also determining the extent the patagia unfurling during flight. birds the wing of a woodpecker avian skin is a bit more complicated than the skin of gliding lizards or bats because the presence of feathers. in addition to the counteracting stresses and stains associated with flight, avian skin must provide a means to monitor and anchor a "coat of feathers", thus the structure of avian skin is different from skin of other flying and gliding animals. to better understand the structure of avian skin, avian skin has been broken down into three different functional components: hydraulic skeleto-muscular apparatus of the feathers this functional component consists of the only of the structural features of the feather tracts, namely the cutis, and connective tissue layer fascia superficialis. this functional component was named "hydraulic skeletal" due to the fact that the fat bodies embedded within cutis and fascia act similar to the hydrostatic bodies within a hydrostatic skeleton. however the functional role of the fat bodies within the hydraulic skeleto-muscular apparatus of the feathers is to counteract forces generated by the erector and depressor muscle of the feathers tracts. rather than to facilitate movement within of a body. dermo-subcutaneous muscular system of integument this functional component of avian skin consists of the smooth muscle of the apertia and striated subcutaneous muscles. the smooth muscles of the apertia counteract the horizontal forces experienced by the feather follicles. the striated subcutaneous muscles also adjust the position of the feather follicles in the directions the smooth muscle cannot. together this system acts as an integrated muscular system that properly positions the feather tracts on the body of the bird. subcutaneous hydraulic skeletal system this functional component of avian skin consists of the fat bodies of the fascia superficialis and fascia subcutanea. the majority of the fat bodies are located either between fascia superficialis and the fascia subcutanea. these fat bodies are stratically located at depression within the body of the bird and function to even out depressions so that feather tracts of the skeleto-muscular apparatus function properly.