| Summary: | A. dorsata builds its large exposed comb high in trees or under ledges of high rocks. The "open" nest of A. dorsata, shielded (only!) by multiple layers of bees, is highly vulnerable to any kind of direct contact or close range attacks from predators. Therefore, guard bees of the outer layer of A. dorsata's nest monitor the vicinity for possible hazards and an effective risk assessment is required. Guard bees, however, are frequently exposed to different objects like leaves, twigs and other tree litter passing the nest from above and falling to the ground. Thus, downward movement of objects past the nest might be used by A. dorsata to classify these visual stimuli near the nest as "harmless". To test the effect of movement direction on defensive responses, we used circular black discs that were moved down or up in front of colonies and recorded the number of guard bees flying towards the disc. The size of the disc (diameter from 8 cm to 50 cm) had an effect on the number of guard bees responding, the bigger the plate the more bees started from the nest. The direction of a disc's movement had a dramatic effect on the attraction. We found a significantly higher number of attacks, when discs were moved upwards compared to downward movements (GLMM (estimate ± s.e.) 1.872 ± 0.149, P < 0.001). Our results demonstrate for the first time that the vertical direction of movement of an object can be important for releasing defensive behaviour. Upward movement of dark objects near the colony might be an innate releaser of attack flights. At the same time, downward movement is perceived as a "harmless" stimulus.
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