The basitarsal brushes of bees are used to clean the pollen off their faces while they forage.

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The basitarsal brush is a collection of hair on the last segment of the foreleg. This brush is used during pollination. As bees forage, they can rub the anthers of the flower on their faces and collect pollen all along their heads. The bees then use the brush on their leg to remove the pollen and place it on structures designed for pollen transport. The brush may also be modified to collect oil from flowers, with the ability to do so arising independently in multiple groups.

A brush is defined as a group of hairs with little organization, in comparison to a pollen comb where the hairs make up a single row.

Similar brushes may be found on the tarsus and femur of the hind and mid legs. Variation is seen in the size, density and shape of the hairs that make up the brush, based on the type of pollen collected and any specializations for other substances like oil or nectar.

This information is also available from the University of Calgary Invertebrate collection, where it was curated as part of a study on design inspired by bees. 

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References

“Oil-collecting bees have a striking array of pads, brushes, or combs of flattened setae (Figs. 6-3, 110-3a) with which to absorb or scoop the oil and to transport it back to the nest, sometimes mixed with pollen. The morphological details are discussed and illustrated by Vogel (1966 to 1990), Neff and Simpson (1981), and Cocucci, Sérsic, and Roig-Alsina (2000).” Michener 2007:18

“Pollen is commonly removed from anthers by the front tarsi or is dusted onto the body of the bee by its movement among floral parts. The forelegs may be pulled through the mouthparts if the bee eats the pollen, or they are pulled through the flexed middle legs whose opposable midfemoral and midtibial brushes remove the pollen.” Michener 2007:18

Book
The Bees of the World: Second Edition John Hopkins University PressMichener CD.

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