B. Smith ex Wallace, 1860
Females may reach a length of 38 mm (1.5 in), with a wingspan of 63.5 mm (2.5 in). Males only grow to about 23 mm (0.9 in) long. M. pluto is believed to be the largest living bee species.
Wallace's giant bee has only been reported from three islands of the North Moluccas in Indonesia: Bacan, Halmahera and Tidore. Very little is known about its distribution and habitat requirements, although it is thought that it is restricted to primary lowland forests.
The species was originally described by Alfred Russel Wallace in 1858, and given the common name "Wallace's giant bee"; it is also known as the "giant mason bee". It was thought to be extinct until it was rediscovered in 1981 by Adam C. Messer, an American entomologist, who found six nests on the island of Bacan and other nearby islands. The bee is among the 25 “most wanted lost” species that are the focus of Global Wildlife Conservation’s “Search for Lost Species” initiative.
Wallace's giant bees build communal nests inside active nests of the tree-dwelling termite Microcerotermes amboinensis, which may have served to hide their existence even from island residents. The bee uses tree resin to build compartments inside the termite nest. The association of the bee with the termite may be obligate.