Unique nest entrance structure of Partamona helleri stingless bees leads to remarkable ‘crash-landing’ behaviour

Partamona helleri stingless bees construct a unique, funnel-shaped entrance that resolves an evolutionary conflict between foraging efficiency and defence. The large outer entrance allows many foragers to pass while the narrow inner entrance requires few guards to defend. This structure has given rise to a remarkable behaviour in returning foragers, which appear to approach the nest entrance at high speed and ‘crash’ head first into the entrance. We compared P. helleri landing behaviour with two related species with architecturally different entrances that land conventionally using their legs: Melipona scutellaris, whose narrow entrance allows only a single bee to pass, and Scaptotrigona depilis, which has a wide entrance tube. All three species initially decelerated on their approach to the nest entrance. However, 0.2 m from the entrance P. helleri began accelerating, whereas the other species continued to decelerate. Partamona helleri entered its nest at 1.14 ms−1, double the velocity of the other species. Despite its greater velocity, P. helleri made no fewer errors than the other species when attempting to enter its nest, probably by virtue of the large target provided by the outer entrance. We then used a bioassay that suggests that this behaviour is a defence against ambush predators at the nest entrance. Finally, we use a scaling argument to show that the crash impact should not cause any damage to a small animal such as a bee, such that no morphological adaptation is required.