Active sensing: different plume structures affect movements of antennae in honey bees (Apis mellifera)

Insects move their antennae to actively sense their environment. Regarding olfaction, it is not clear how these movements might be optimized for sampling the odor environment. Honey bees have movable rod-like antennae, the last of segment of which is covered in several thousand pore plate sensillae that contain dendrites of olfactory sensory neurons. Walking honey bees typically move their antennae in an almost constant manner. These movements can be impacted by odor valence, either innate or learned, suggesting that these movements are under sensory control. However, it is unclear whether these movements are under active control or are simply fixed responses to stimulation. Here, we evaluated antennal movements of stationary bees when placed in odor plumes with different structures. Antennae took up on average two different positions in the plume, in both the absence and presence of odor. One corresponded to upwind and toward the odor source. The other position was across the plume. Bees rapidly switched between these positions regardless of the presence of odor. The frequency of forward and lateral positioning depended on the presence/absence of odor and on the structure of the plume, which suggests that movement is involved in sensing odor filaments. We conclude that these movements represent active sensing, analogous to sniffing in mammals. Future investigations need to focus on the connection between antennal movements and physiological sensing as well as on analyses of odor-driven antennal movements in freely moving bees. Our results also suggest that active sensing may differ across insects with different antennal morphologies.