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Bees have a system of sex determination that is known as haplodiploidy. Males are haploid and females are diploid. This means that male offspring result from unfertilized eggs and receive genetic material only from their mothers, whereas female offspring result from fertilized eggs and contain genetic material from both parents.
Female bees can choose the sex of their offspring, how much pollen to provide each sex, and ultimately determine the size of the adult that emerges from the nest. Males often emerge from nests before females; therefore, mothers tend to place male eggs close to the entrance of the nest so that young male bees can leave the nest without disturbing the female cells that are still developing and be ready to mate with the females that emerge after them. The number of eggs a female produces in her lifetime varies from fewer than 8 in solitary species to over a million in highly eusocial species.
Bees undergo complete metamorphosis, which includes four distinct life stages: egg, larva, pupa, and adult. Eggs are pearly white, elongate, and have soft outer membranes. They are usually laid directly on pollen reserves in the nest cell (except for in eusocial species which actively feed their larvae instead of providing them with a single pollen mass). Larvae are cream-colored, legless, and grub-like. Most bee species go through five larval instars. Pupae start to resemble the adult form and have three body segments, as well as antennae, eyes, and legs, but lack wings and hair (though wing buds may be present). Young pupae are cream-colored and darken as they mature.
Bee nests are extremely variable and can consist of a single cell attached to a stem or can be a large underground complex of excavated tunnels lined with many cells. Bees nest in diverse substrates, including soil, rock crevices, stems, snail shells, wood, and man-made objects. Unfortunately, the nesting habits of many species are unknown. The location of the nest and the materials used to create the nest cells can give clues to what type of bee may have built it. For example, Andrenidae and most Halictidae nest in the ground so the nest must be dug up to determine what type of bee built the nest. Megachilidae often nest in crevices, such as in rocks, stems, and man-made structures, making them more likely to be transported to new areas in cargo or baggage. This is one possible reason why most of the invasive bees in the U.S. belong to this family. Megachilidae nests can sometimes be distinguished from Apidae nests by the use of scavenged materials, such as cut pieces of leaves, chewed leaf pulp, plant hairs, resin, pebbles, or mud. Apidae are a diverse group of bees with a variety of different nesting habits, including highly social bees that build their nests out of wax like Apis (honey bees) and Bombus (bumble bees), bees that build their nests in burrows in the ground or in sandstone walls, and bees that lack cells for their brood all together. Further, bees in the Apidae subfamily Xylocopinae, except for Xylocopa (Proxylocopa), nest in dead plant material, such as hollow stems and wood used for crates for the transport of goods, both of which can easily be transported outside of its native range.
Nest cells of bees in the Megachilidae family:
Many people think of bees as belonging to colonies that produce honey; however, social colonies are relatively uncommon. Social behaviors of bees vary widely. Highly eusocial bees, such as honey bees, have a division of labor between egg-laying queens and pollen-collecting workers. The queens of highly social species are both morphologically and physiologically different from workers. Queen honey bees are much larger than honey bee workers and have a greatly expanded abdomen. Since queens are only responsible for laying eggs, they lack pollen-collecting hairs and could not survive without the aid of the workers. Honey bee colonies have multiple, overlapping generations of bees working together in nests that can last for years.
Bumble bees and some bees in the tribe Halictini are primitively eusocial. These bees have queens and workers, but they are morphologically similar to each other. Although these queens are often larger than workers, they still possess pollen-collecting hairs and could survive on their own if necessary. There is still a division of labor - the queen is responsible for laying eggs, while the workers collect pollen and maintain the nest. Further, these bees often create several reproductive females each season that leave the nest to start their own colonies, and the old nest is often abandoned for the next season.
Most bees are solitary, yet some solitary bees may still nest communally. Females of solitary bees are responsible for building nest cells for their young and collecting pollen to provision them by themselves. Solitary bees have no queens, there is no division of labor, and each female can lay their own eggs. Solitary bees offer no maternal care after the nest is built and provisioned. Newly emerged adults leave the nest and construct their own nests elsewhere, rather than returning to the nest they were born in.