Gap perception in bumblebees

Abstract

A number of insects fly over long distances below the natural canopy, where the physical environment is highly cluttered consisting of obstacles of varying shape, size and texture. While navigating within such environments, animals need to perceive and disambiguate environmental features that might obstruct their flight. The most elemental aspect of aerial navigation through such environments is gap identification and 'passability' evaluation. We used bumblebees to seek insights into the mechanisms used for gap identification when confronted with an obstacle in their flight path and behavioral compensations employed to assess gap properties. Initially, bumblebee foragers were trained to fly though an unobstructed flight tunnel that led to a foraging chamber. After the bees were familiar with this situation, we placed a wall containing a gap that unexpectedly obstructed the flight path on a return trip to the hive. The flight trajectories of the bees as they approached the obstacle wall and traversed the gap were analyzed in order to evaluate their behavior as a function of the distance between the gap and a background wall that was placed behind the gap. Bumblebees initially decelerated when confronted with an unexpected obstacle. Deceleration was first noticed when the obstacle subtended around 35 deg on the retina but also depended on the properties of the gap. Subsequently, the bees gradually traded off their longitudinal velocity to lateral velocity and approached the gap with increasing lateral displacement and lateral velocity. Bumblebees shaped their flight trajectory depending on the salience of the gap, indicated in our case by the optic flow contrast between the region within the gap and on the obstacle, which decreased with decreasing distance between the gap and the background wall. As the optic flow contrast decreased, the bees spent an increasing amount of time moving laterally across the obstacles. During these repeated lateral maneuvers, the bees are probably assessing gap geometry and passability.

Wind and obstacle motion affect honeybee flight strategies in cluttered environments

“…One potential hypothesis for why bees, on average, flew more quickly when approaching moving obstacles in wind is that the obstacles' lateral movement provides enhanced visual information (e.g. Kirchner and Srinivasan, 1989;Srinivasan et al, 1996) that bees would normally gather by slowing down and performing lateral casting motions (Ravi et al, 2019), and this allowed the bees to traverse moving obstacles without slowing to gather this information. However, if this were true, we would expect to have seen the same result in still air (i.e.…”

“…We analyzed the final 50 mm of each approach (longitudinal distance along the axis of the tunnel) because this is the region in which bees were expected to display strong behavioral responses to the obstacles, based on previous studies (e.g. Ravi et al, 2019). When bees were approaching obstacles in a headwind, the filmed approach region would lie entirely within the wake produced by the obstacles, as vortices shed from obstacles in wind at this Reynolds number (∼240) create a turbulent wake downstream for distances >50 mm (e.g.…”

“…We calculated the median ground speed (speed relative to the ground, not incorporating flow velocity) of each approach, based on the velocity of the bees within the horizontal plane (i.e. incorporating movement along the lateral and longitudinal axes of the tunnel, but ignoring vertical motion; Baird and Dacke, 2012;Mountcastle et al, 2016;Ravi et al, 2019).…”

“…When bees approach an obstacle directly in front of them, they can decelerate smoothly to land on the obstacle by maintaining a constant apparent rate of image expansion (Baird et al, 2013), although this strategy is not always observed in the presence of wind (Chang et al, 2016). Additionally, bees may cast from side to side when approaching obstacles, presumably to gather visual information about the obstacle (Ravi et al, 2019). Bees also perform braking maneuvers, in which they increase their body pitch angle and reverse direction to avoid collisions with obstacles (Crall et al, 2015).…”

From Paths to Routes: A Method for Path Classification

“…The habituated bees were allowed to travel through a foraging tunnel (140 × 30 × 30 cm 3 ) connected to the hive box and a foraging chamber via 2.5 cm diameter tubes and acrylic boxes (see Figure 1). The walls of the tunnel were covered with a red and white 1/f noise pattern (as in (Ravi et al, 2019)). When an individually marked bumblebee returned from the foraging chamber, it was rerouted by using small acrylic gates into an experimental tunnel, parallel to the foraging tunnel.…”