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Colorful bee nests

Many people believe that all bees are social, like the well known honey bee or bumble bees, whose colonies consist of (1) larger numbers of worker bees that collect provisions for the young, build the nest, and defend the colony from attack, (2) fewer males, which have just one thing in mind, and (3) a single egg-laying queen. Not so, for most bees are solitary. Indeed, the total world bee fauna numbers about 20,000 known species; nearly 75% are solitary. With these, each female constructs her own nest, provisions the brood chambers with larval food gleaned from flowers, and thereafter deposits an egg in each chamber before closing it. She has no help from workers. Depending upon the species, solitary bee nests can be found on rocks in the open, in beetle larva galleries in wood and hollow plant stems, or in the ground.
Nests may consist of a single chamber or of multiple chambers, again a species-specific matter. It can be a simple hollow in the substrate, but almost never is, because chambers are where eggs—the next generation—are deposited, and where resulting larvae live their entire feeding immature lives (up to 10 months long for a species with a single generation a year). The larval stage terminates with the development of the pupa, the last brief step before metamorphosing into adult bees. For the species to survive, the nesting chambers (also called brood cells) must be well protected wherever they occur. Threats include the danger of being crushed (if the substrate is soil), dried out (if the nesting chamber is in an old dry tree trunk), overheated (if the nest is on a rock exposed to the sun), or food spoilage (if the nest gets too wet), not to mention such biotic threats as mold, viruses, bacteria, parasites, and predators.
Rozen et al. (2010) report on a most unusual nest that they, in two separate field parties unaware of one another, discovered by chance on the same day but in different places in the Middle East. The nests, belonging to the same species of the rare solitary bee Osmia (Ozbekosmia) avosetta, were found by one party near Antalya, Turkey, the other in Fars Province, Iran. At both sites, nests consist of only one or two vertical ovoid brood chambers shallowly buried in the ground, 1.5 to 5 cm below the surface.
By entering the chamber at its open top end, a female shingles the wall of her brood chambers with large pieces of petals or with whole petals, often of many hues, a behavior uncommonly used by bees. She applies the first petals to the lowest part of chamber wall and gradually works her way to the top. She then imports claylike mud through the opening and plasters the entire inner surface except for the upper 3 mm with a layer of mud about 0.5 mm thick. This layer is followed by another layer of plated petals, thus completing the sandwich-like cell wall.
Each female provisions this colorful, wallpapered chamber with a sticky mixture of nectar and pollen placed at the bottom, and deposits her egg on its surface. Then comes cell closure; she folds toward one another the petals at the top of the innermost petal layer, thus closing off the opening. She brings in more mud to cap the closure, and then folds the outer petal layer over the mud top. After a few days the closed chamber hardens to a nutlike consistency.
Open brood cell of Osmia avosetta, showing coloration of outer envelope.
Thus, the developing young bee is sealed in, protected from predators and parasites. Humidity within the chamber remains high despite the shallowness of the nest and hot sun on the surface because of moisture provided by the inner layer of petals and drying mud while the larva quickly eats its lifetime rations of food. After spinning a cocoon, the larva enters its 10-month sleep, to awaken with the next spring bloom. Alas, the larva never enjoys the brilliant colors of its walls: bee larvae have no eyes.