between body parts means that workers of different sizes differ in shape6,25. We found negative allometry between body weight and head width in 9 of 24 tested species. In other words, larger workers have relatively smaller heads, which is the opposite of what is found in many ant species6. Importantly, there was no association between negative allometry and having larger guards, which suggests that negative allometry does not seem to be linked to the size differentiation between defence workers and foragers in stingless bees. Guards in stingless bees might not need large heads accommodating strong head muscles as, in contrast to ants, they mainly use their mandibles for clamping, rather than cutting, and their legs for grappling9,33. Why some stingless bees show negative allometry and whether developmental constraints or ecological pressures play a role is unknown.Compared to the impressive size differences between soldiers and minors in some ant genera (e.g., Atta), size differences between guards and foragers in stingless bees are moderate where they exist. This raises the question why more extreme worker differences have not evolved in eusocial bees. Several non-mutually exclusive hypotheses have been proposed to explain differences among social insect species in the degree of worker differentiation5,13. The developmental constraints hypothesis predicts that worker phenotypic diversity is more likely to evolve in species with an early queen-worker caste determination as this provides more time for worker larvae to develop along different developmental pathways13,25. As a result, we expect a positive correlation between the variance in worker size and queen-worker dimorphism13. Indeed, we found that queen-worker dimorphism explained more than 60% of the variation in worker diversity between species. This positive correlation between queen-worker dimorphism and worker phenotypic diversity could also be caused by a third factor, namely the reproductive potential of workers. A decreasing ability of workers to reproduce could favor both the evolution of worker differentiation13 and queen-worker dimorphism34. However, Toth et al.21found no link between worker reproduction and queen-worker dimorphism in stingless bees. Thus, we deem it unlikely that worker reproductive potential explains the strong relationship between queen-worker size difference and worker diversity.The size-complexity hypothesis predicts that species with larger colony sizes have a more specialized division of labor and a phenotypically more diverse workforce3,26,27,28. There is good evidence for a size-complexity relationship within ant species6, but evidence among ant species is mixed13,27. We found no relationship between colony size and worker size variation among our 28 species of stingless bees (Fig. 2b), which suggests that colony size does not explain the presence or absence of worker differentiation in stingless bees.A reconstruction of the evolutionary history of worker differentiation suggests that the common ancestor of our 28 species had similarly sized guards and foragers and that, therefore, increased guard size is a derived trait (Fig. 1a). The analysis further suggests that increased guard size evolved five times independently among the 28 study species (Fig. 1a). Stingless bees began their diversification around 80 million years ago18, but all transitions towards increased guard size have occurred relatively recently, during the last 2025 million years (Fig. 1a). This period coincides