Megachile sp. near Primorsko, Bulgaria. ID by bee expert John Ascher on iNaturalist.

The Megachilidae are a cosmopolitan family of solitary bees whose pollen-carrying structure is restricted to the bottom of the abdomen (rather than on the hind legs as in other bee families). Megachilid genera are most commonly known as mason bees and leafcutter bees, reflecting the materials from which they build their nest cells (soil or leaves, respectively). Megachilid bees are among the world's most efficient pollinators. The motion of megachilidae in the reproductive structures of flowers is highly energetic and swimming-like. This vigorous agitation releases large amounts of pollen. Megachilidae also require an average of ten times as many trips to flowers as other bee species to gather sufficient resources to provision a single brood cell. Lithurgus chrysurus collects pollen only on certain Asteraceae, namely thistles and knapweed. For transportation, the females have a hairbrush on the abdomen bottom. They nests in the old trees, in the wooden structures, dry stalks of plants, galls, in the empty shells of shellfish on the rocks, in crevices in the clayey and sandy soil, cover cells by pieces of leaves or flower petals, grains of sand, bonded with saliva.