The calcar, also known as the calcaneum, is the name given to a spur of cartilage arising from inner side of ankle and running along part of outer interfemoral membrane in bats, as well as to a similar spur on the legs of some arthropods. The calcar serves to help spread the interfemoral membrane, which is part of the wing membrane between the tail and the hind legs. Calcar (femorale) also refers to the dense, vertically oriented bone present in the posteromedial region of the femoral shaft inferior to the lesser trochanter. Usage history It is unclear who first coined the word "calcar" to apply to bat anatomy; records of its usage date to Joel Asaph Allen in 1893. The word calcar is derived from Latin "calx," meaning "heel". Other terms or phrases that refer to the same feature include "supplementary calcaneal bones", "styliform bones", "les éperons" (French), "Fusswurzelstachels" (German), "spurs", and "stylets". Prevalence Not all bats have a calcar, as not all bats have a well-developed uropatagium. Of the bats that have a developed uropatagium, most, but not all, have a calcar. The Kitti's hog-nosed bat is the only species of bat that has an extensive uropatagium while lacking a calcar. Structure The calcar varies widely among bats. It can be as small as , or longer than . In some species of bat, the calcar is very long and bladelike. Examples of this include species in the genera Noctilio and Diclidurus. In other species, the calcar is very small or absent, such as the Kitti's hog-nosed bat or species in the genera Rhinopoma, Diaemus, Mystacina, Syconycteris, Harpyionycteris, and Notopterus. In the hairy-legged vampire bat, the calcar has a unique, finger-like form that extends approximately beyond the edge of the uropatagium. Some insectivorous species of bats have an elaborate, pronounced calcar. This form of calcar is referred to as a "keeled calcar", and it is hypothesized that it may be useful in converting the uropatagium into a basket for catching insects. For species in some genera (for example, Vampyrum and Phyllostomus), the calcar is entirely made of cartilage. In other genera (e.g. Saccopteryx, Pteronotus, Molossus), the calcar is calcified. Intermediate between these two forms is when one end of the calcar is calcified, while the other is cartilaginous (e.g. Noctilio and Trachops). All megabats, however, lack any calcification in their calcar. In microbats, movement of the calcar is controlled