Megachilidae

The Megachilidae are a cosmopolitan family of (mostly) solitary bees whose pollen-carrying structure (called scopa) is restricted to the ventral surface of the abdomen (rather than on the hind legs like in all other bee families). Megachilid genera are most commonly known as mason bees and leafcutter bees, reflecting the materials they build their nest cells from (clay or leaves, respectively); a few collect plant or animal hairs and fibers, and are called carder bees. Most species feed on nectar and pollen, but a few are cleptoparasites (informally called "cuckoo bees"). Parasitic species do not possess a scopa. The brightly colored scopa leads to a colloquial name used occasionally in North America - "Jelly-belly bees." Megachilid bees are among the world's most efficient pollinators because of their energetic swimming-like motion in the reproductive structures of flowers, which moves pollen, as needed for pollination. Ironically, one of the reasons they are efficient pollinators is their frequency of visits to plants, but this is because they are extremely inefficient at gathering pollen; compared to all other bee families, megachilids require on average nearly ten times as many trips to flowers to gather sufficient resources to provision a single brood cell.

North America has many native Megachilid species, but Alfalfa leafcutter bees (Megachile rotundata) are an imported species used for pollination. The most significant native species is Osmia lignaria (the "Orchard Mason Bee" or "Blue Orchard Bee"), which is sold commercially for use in orchard crop pollination, and which can be attracted to nest in wooden blocks with holes drilled in them (which are also sold commercially for this purpose).

Non-parasitic species
The general life cycle of non-parasitic Megachilidae is as follows (but see below for variations): Nests are typically divided into cells, each cell receives a supply of food (pollen or a pollen/nectar mix) and an egg; after finding a suitable spot (often near where she emerged), a female starts building a first cell, stocks it, and oviposits. Then she builds a wall that separates the completed cell from the next one. The larva hatches from the egg and consumes the food supply. After moulting a few times, it spins a cocoon and pupates. Then it emerges from the nest as an adult. Males die shortly after mating, but females survive for another few weeks, during which they build new nests.

Variations:
 * Nests are often (but not always) built in natural or artificial cavities. Some embed individual cells in a mass of clay or resin attached to a wall, rock surface, or plant stem.
 * Nest cavities are often linear, for example in hollow plant stems, but not always (snail shells are used by some Osmia, and some species will readily use irregular cavities).

Parasitic species
Some genera of Megachilids are brood parasites and, therefore, have no ventral scopa (e.g. Stelis, Coelioxys). They often parasitize related taxa. They typically enter the nest before it is sealed and lay their eggs in a cell. After hatching, the parasite larva kills the host larva, unless the female parasite has already done so, and then consumes the provisions. Parasitic species are of equal size or smaller than their victims.

Genera of Megachilidae

 * Anthidium are leaf-cutting bees who use conifer resin, plant hairs, earth, or a combination of these as material for the nest walls. Their abdominal bands are usually interrupted in the middle. There is no lobe (arolium) between their claws. Anthidium manicatum (pictured) is commonly known as the Wool Carder Bee which uses comblike mandibles to "comb" plant fibers into its brood cell walls. It has spread from Europe to North and South America. The males are much larger (ca. 18 mm) than the females (ca.12 mm) which is not uncommon among Megachilidae, but very rare among other bee families (e.g., the true honey bees, genus Apis). The males also have three "thorns" at their abdominal apex which they use as weapons when defending their territory.
 * Paranthidium
 * Dianthidium
 * Anthidiellum
 * Trachusa
 * Coelioxys are brood parasites of Megachile. Females have a pointed conic abdominal apex (tip), males have several spikes on their apex.
 * Dioxys are brood parasites of Megachile, Anthidium and Osmia.
 * Heriades are mason bees with narrow abdominal bands. They resemble small Osmia, but they are oligolectic (specialized on a few subfamilies of Asteraceae) and use resin from conifers, as well as plant fibers and sand, as cell wall material.
 * Megachile are mostly leaf-cutters, but some are masons. In the former species the whole cell (not just the wall between cells) is made from leaf pieces. There is no lobe (arolium) between their claws.
 * Osmia are mason bees, they build their nests in natural or artificial cavities such as hollow plant stems, abandoned nests of wood-boring beetles or carpenter bees, or even snail shells. They do not excavate their own nests. The material used for the cell can be clay or chewed plant tissue. A few species are referred to as orchard mason bees.
 * Ashmeadiella
 * Hoplitis
 * Stelis Panzer (and other related genera), or the stelidine bees, are cleptoparasites on other Megachilidae. They belong to the tribe Anthidiini.
 * Heterostelis are parasitic on Trachusa.

Gallery
See also http://www.bugguide.net (Search for Megachilidae, North America only)