Floral sonication by bees: Mesosomal vibration by Bombus and Xylocopa, but not Apis (Hymenoptera: Apidae), ejects pollen from poricidal anthers

Abstract

Vibration accelerations and displacements were measured in Bombus spp., Xylocopa spp., Apis mellifera L., and Eristalis tenax (L.) (a syrphid flower fly) including the first measurements of thoracic displacements during untethered free flight, and floral sonication by Bombus terrestris (L.). For all except A. mellifera the thoracic displacement during sonication is less than during flight but the frequency is greater. Thus, thoracic acceleration during sonication is greater than during flight for this bee species. However, the sonication frequency of A. mellifera, the honey bee, did not differ significantly from flight frequencies. Its accelerations were lower than required to eject pollen from sonication-pollinated flower anthers having poricidal dehiscence (e.g., Solanum spp.). The vibration mode shape of X. californica arizonensis Cresson, a large carpenter bee, was determined, demonstrating that sonication was produced by the same mechanism as flight. However, for Xylocopa, the sonication frequency stimulated resonant frequencies of both head and abdomen, enhancing acceleration forces transmitted during sonication of flowers. Sonication vibration of the thorax occurred at the second harmonic frequency of the fundamental flight frequency, but the natural frequency of the thorax with the wings decoupled could not explain the higher sonication frequency. It is proposed that the higher frequency is produced by the second harmonic of the indirect flight muscle stretch activation response. The power requirements for sonication by bees is presumably less than during flight such that muscular displacements at the second harmonic frequency of the stretch activation response is adequate to drive this highly evolved morphological, physiological and behavioral system of pollen harvesting. Our results explain the observation that Apis mellifera, the common honey bee, does not use floral sonication in its pollen foraging behavioral repertoire.