Brood comb construction by the stingless bees 'Tetragonula hockingsi' and 'Tetragonula carbonaria'

Abstract

'Tetragonula hockingsi' and 'T. carbonaria' are two closely related species of Australian stingless bees. The primary species-specific character is the architecture of the brood comb. The brood comb of 'T. hockingsi' is an open lattice comprising clumps of about ten cells that are connected by vertical pillars. In contrast, in 'T. carbonaria' the brood comb is a compact spiral in which all brood cells (except on the margins) are connected by their walls to adjacent cells at the same height. We made detailed observations of the cell construction process in two colonies of each species. From these observations we formed a species-specific hypothesis about the algorithm followed by the bees during cell construction. The two algorithms allowed us to make predictions about the locations of new cells. Both 'T. hockingsi' and 'T. carbonaria' share a preference for constructing new brood cells in the clefts formed by two or three adjacent existing brood cells, but there are differences in detail for other components of the building process. The fundamental difference in the cell construction process of the two species is that for 'T. hockingsi', when a cluster of cells contains ten cells, the next cell added to the cluster is offset upwards by half a cell length, or, less often, a vertical pillar rather than a new cell is constructed. In T. carbonaria, cell construction is continuous at the comb margin so that there are no gaps between cells. Furthermore, it seems that 'T. hockingsi' only makes use of local knowledge of the brood comb when deciding to place new brood cells, whereas 'T. carbonaria' could make some building decisions based on knowledge of the total structure. We translated the species-specific algorithms into agent-based lattice swarm computer simulations of the cell construction process for the two species. These simulations produced representations of brood combs that are similar to those seen in vivo, suggesting that our biological rules are realistic.