Osmia is a large and diverse group of robust, cosmopolitan megachilids. They often have dense, conspicuous body hair and occasionally a cuticle with metallic reflections. Females have large heads with powerful mandibles and a ventral scopa underneath the abdomen. Males often resemble the females, but smaller in size and with longer antennae. They emerge a few days before their sisters and patrol the entrance of the nearby nests. They are solitary bees that typically build their nests in pre-existing cavities such as abandoned beetle burrows, rock cracks or in empty snail shells. Several species also inhabit human made structures and are commonly found inside nest boxes in urban areas. They usually use mud or leaf mastic as lining for their brood cells, but they can employ a diverse range of construction materials, including flower petals and small pebble-stones. The females of some species even have facial horns that they use to manipulate wet mud during the construction process.