Shell-Nesting Mason Bees: these bees build their nests in empty snail shells, using crushed leaves and soil to form the inner brood chambers and then sealing the entrance with debris
Bees of the family Megachilidae typically build their nests in the gaps and crevices in tree stumps, rocks, plant stems, and wooden structures, but there are a few species that prefer to nest in discarded snail shells. These shell-nesting bees are primarily found in Europe, North Africa, and the Near East; a few can also be found in North America, South Africa, and Japan.
Above: just some cozy little mason bees
These are solitary bees, meaning that they don't form colonies or live together in hives, and each female builds her own individual nest. The nest is constructed as a series of brood cells, and each cell contains a single egg with enough pollen and nectar to sustain the larva until it reaches adulthood.
Shell-nesting mason bees are known to exist in at least four different genera, including Osmia, Rhodanthidium, Wainia, and Protosmia.
Above: Osmia spinulosa and Rhodanthidium septemdentatum, both of which build their nests in snail shells
When the female is ready to nest, she carefully selects a shell and then drags it into a shaded or well-hidden spot. Moving the shell is no easy feat, but she clings to it with her hind legs and pulls herself along by grabbing objects with her mandibles. A single bee may travel like that for several meters before finally settling on the right spot to prepare her nest.
Above: a mason bee dragging her shell into position
This article describes how the nest is then constructed:
The bee begins to build its nest, mainly within the “whorl” or spire of the shell. A typical nest consists of a few chambers (about two or four in number, depending on the size of the shell) known as cells, the walls of which consist of masticated leaf pulp known as leaf mastic. When fresh, the colour of this material is bright green, but with time, it assumes a brownish or black colour. Each cell is provisioned with a mixture of pollen and nectar, an egg is laid on this, and the cell sealed with further leaf mastic.
The female must make dozens of trips just to gather the provisions for a single brood cell, and completing the entire nest can take days.
Above: the the internal structure of this nest is partially exposed, revealing a central brood cell, a single larva, a supply of pollen/nectar, and a layer of debris
Once the brood chambers have been constructed and provisioned, the entrance to the shell is "bricked up" with several layers of plant pulp, soil, pebbles, and shell fragments. In some cases, the female will also apply patches of plant pulp to the outer surface of the shell as a way to provide camouflage.
Above: mason bees sealing their nests with plant pulp
The completed nest is then carefully maneuvered so that the entrance faces the ground. Some females will conceal the entire nest beneath a pile of twigs, pine needles, and plant stems. Moss and blades of grass are also woven throughout the pile. All of the debris is carefully selected, positioned, and then "glued" together with saliva, forming a tangled, tent-like structure over the nest.
Above: Osmia bicolor, commonly known as the red-tailed mason bee. constructing a protective thatch over her nest
In other cases, the female will conceal the nest by creating a small hole in the sand and then dragging the shell into it, ensuring that the nest is partially buried.
Above: Osmia aurulenta and Osmia rufohirta
This is just one of the many peculiar nesting habits that can be found among solitary bees. Several other examples have been featured in my previous posts, which describe the nest-building strategies of woolcarder bees, resin-pot bees, and a ground-dwelling species known as Osmia avosetta.
Bulletin of the Amateur Entomologists' Society: Shell-Nesting Bees
The Little Book of Bees: Snail-Nesting Mason Bees
University of Hradec Králové: Bees Nesting in Empty Gastropod Shells
The Wildlife Trusts: Red-Tailed Mason Bees
Journal of Hymenoptera Research: Comparative Biology of Four Rhodanthidium Species that Nest in Snail Shells
Journal of Hymenoptera Research: Biology of Palaearctic Wainia Bees of the Subgenus Caposmia
Cambridge University Press: The Native Shell-Nesting Bee Osmia conjuncta
Wired: Adorable Bees that Live Inside Snail Shells
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