13 14 A number of insects fly over long distances below the natural canopy where the physical 15 environment is highly cluttered consisting of obstacles of varying shape, size and 16 texture. While navigating within such environments animals need to perceive and 17 disambiguate environmental features that might obstruct their flight. The most 18 elemental aspect of aerial navigation through such environments is gap identification 19 and passability evaluation. We used bumblebees to seek insights into the mechanisms 20 used for gap identification when confronted with an obstacle in their flight path and 21 behavioral compensations employed to assess gap properties. Initially, bumblebee 22 33 the gap, in our case, indicated by the optic flow contrast between the region within the 34 gap and on the obstacle, which increases with decreasing distance between the gap and 35 the background wall. As the optic flow contrast decreased the bees spent increasing 36 time moving laterally across the obstacles. During these repeated lateral maneuvers the 37 bees are likely assessing gap geometry and passability. 38 39 40 65 and path planning. For long distance navigation, flying insects might use, apart from 66 vision, other sensory modalities such as odor and geomagnetic fields (Knaden and 67 Graham, 2016) In order for a flying animal to arrive at its intended destination or ensure 68 safe locomotion, at a basic level, the animal needs to process the obstacles that lie in its 69 path and identify gaps. Obstacle and gap detection may thus be considered the most 70