An 1863 article on Stingless Bees (Meliponini) in Brazil stated:
"Mr. H.W. Bates often observed these bees collecting clay, which they plaster on their broad flat posterior tibiae. For some time he was at a loss to ascertain the use they made of it, but he subsequently observed that it was the material with which they wall up any large opening in decaying trees, leaving only an outlet necessary for their entrance into and their exit from the hive" (Smith, 1863).
The bee shown here is engaged in this behavior, collecting the substrate on the flat area of the hind tibiae.
Photo location: Sentier de Bagne des Annamites, French Guiana.
American Insects site