Landing behavior is one of the most critical, yet least studied, aspects of insect flight. In order to land safely, an insect must recognize a visual feature, navigate towards it, decelerate, and extend its legs in preparation for touchdown. Although previous studies have focused on the visual stimuli that trigger these different components, the complete sequence has not been systematically studied in a free-flying animal. Using a real-time 3D tracking system in conjunction with high speed digital imaging, we were able to capture the landing sequences of fruit flies (Drosophila melanogaster) from the moment they first steered toward a visual target, to the point of touchdown. This analysis was made possible by a custom-built feedback system that actively maintained the fly in the focus of the high speed camera. The results suggest that landing is composed of three distinct behavioral modules. First, a fly actively turns towards a stationary target via a directed body saccade. Next, it begins to decelerate at a point determined by both the size of the visual target and its rate of expansion on the retina. Finally, the fly extends its legs when the visual target reaches a threshold retinal size of approximately 60 deg. Our data also let us compare landing sequences with flight trajectories that, although initially directed toward a visual target, did not result in landing. In these ‘fly-by’ trajectories, flies steer toward the target but then exhibit a targeted aversive saccade when the target subtends a retinal size of approximately 33 deg. Collectively, the results provide insight into the organization of sensorimotor modules that underlie the landing and search behaviors of insects.
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/215/11/1783/DC1
This research was supported by grants from the Air Force Office of Scientific Research (AFOSR) [grant no. 66-1257 to M.H.D.], a Hertz Fellowship and a National Science Foundation (NSF) Graduate Research Fellowship.
LIST OF SYMBOLS AND ABBREVIATIONS
- distance to the center of the post
- no. trajectories
- no. data points (100t data points for each trajectory, where t is the duration of the trajectory in s)
- post radius
- relative retinal expansion velocity
- retinal size-dependent expansion threshold
- flight speed in the x–y plane
- retinal size (deg), see Eqn 1 and Fig. 1B
- expansion threshold, see Fig. 9
- post angle, see Fig. 1B
- turn angle, see Fig. 1B
- angular velocity
- retinal expansion velocity (deg s–1), see Eqn 3
- © 2012.