Soldier production in a stingless bee depends on rearing location and nurse behaviour

The spectacular morphological variation among workers of certain ant and termite species has fascinated evolutionary biologists since Darwin. In some species, environmental triggers induce larvae to develop into different phenotypes, e.g. minor or major workers (soldiers). Recently, the first soldier subcaste was discovered in a bee, the stingless bee Tetragonisca angustula. In contrast to ants, which raise their offspring by progressively feeding larvae until the pupal stage, T. angustula nurses mass provision individual brood cells after which the bees develop from egg to young workers in sealed cells on a seemingly uniform brood comb. This prompts the question of how this bee creates a morphologically variable workforce without larvae having direct contact with nursing workers. We investigated where T. angustula raises a larger soldier subcaste on its compact brood comb. Additionally, we examined whether size differences among workers could be generated by differential distribution of food by nursing workers. We found that colonies produce c. 1-6 % of soldier-sized workers, which mainly emerge from a small central area of the comb. In this area, cells are wider and a larger number of nursing bees unload larval food here before oviposition. Cell attendance levels prior to oviposition were similar across the comb and, thus, did not explain the larger food volumes found in the centre. Our results suggest that workers determine soldier production via larval food discharges and cell-building behaviour. Nutritional differences among larvae might then induce larvae into one or the other caste developmental pathway.