Plant species used as wood substrata for nesting by Megalopta genalis Meade-Waldo, 1916 and M. amoena Spinola, 1853 (Hymenoptera, Halictidae)

Halictine bees are a group that are useful for understanding the evolutionary origins of social behavior and the evolution of nest architecture. Most species nest in the soil, but there have been several evolutionary transitions to nesting in wood. Megalopta is a nocturnal genus and represents one of the larger radiations of wood-nesting bees in the Halictinae. These bees have been observed nesting in branches and stems from trees and lianas, but it is not known to which species they belong. There is only a record of a nest in a dead trunk of Apeiba tibourbou Aubl. (Malvaceae, Tilioideae). This research aims to identify the wood plant species used as substrata by Megalopta genalis and M. amoena bees to build their nests. Fieldwork was conducted on Barro Colorado Island. Fifty-nine occupied nests were grouped in 14 morphospecies and anatomically described using transverse, longitudinal and radial sections cut with a microtome following standard histological techniques. Anatomical features of the samples belong to 11 plant species, represented by eight families of angiosperm plants and 11 genera. Considering there are anatomical features that make possible to distinguish trees and lianas like wide vessels, vessel dimorphism, abundance of axial parenchyma and others, we can determine that Megalopta bees prefer wood substrates with high proportion of soft tissues from trees or shrubs and possibly mostly liana plant species. The species most used as substrata for nesting were Uncaria tomentosa DC. (Rubiaceae), Hylenaea praecelsa (Miers) A.C.Sm. (Celastraceae) and Guettarda L. (Rubiaceae).