​Physical Characteristics of Mason Bees

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Female mason bees carry pollen on scopal hairs located on the undersides of their abdomens, a trait that helps identify them -- and which they share with other members of the Megachilidae family, such as leafcutter, resin and cactus wood-borer bees.  The scopae of mason bees may be white, yellowish, dark brown, black or even electric orange.  Mason bees sometimes have orange hairs on their lower faces as well, as sported by the Osmia chalybea featured on this page.

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Osmia mason bees are small-to-medium-sized, with robust builds and relatively large heads.  The color and size of mason bees vary significantly by subgenus.  Bees of the subgenera Diceratosmia and Helicosmia, for example, are usually metallic green or blue.  Their bodies are partly covered with pale hairs and their abdomens may be striped with bands of pale hair as well.  Mason bees of the subgenus Melanosmia tend to have more robust builds and very dark green or black coloring, while the subgenus Osmia includes two imported species with a markedly different appearance: the hornfaced bee and the bull mason bee, which are large, covered with dull brown hairs and endowed with brightly-covered scopal hairs.

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Mason bees possess other distinctive characteristics of the tribe Osmiini:  their forewings have only two submarginal cells (as opposed to the more usual three), and their front feet sport an extra part called an arolium. These traits, which aid greatly in identifying mason bees, are illustrated in the photo strip here.