The hoverflies’ ability to control their head orientation with respect to their body certainly contributes importantly to their agility and their autonomous navigation skills. Many tasks performed by this insect during flight, especially while hovering, involve a head stabilization reflex. This reflex prevents the visual processing from being disturbed by motion blur and maintains a consistent perception of visual environment. The so-called Dorsal Light Response (DLR) is an example of such a head control mechanisms, making insects sensitive to the horizon contrast, the orientation of which could provide a good approximation of the pitch and roll orientation required to control the wingbeat aerodynamics finely and swiftly. In addition, how this attitude control depends on an internal estimation of gravity orientation is still an open question. It has not yet been established whether hoverflies use gravity perception cues to detect a nearly weightless state at an early stage. To address this question in the case of dipterans, we investigated the following: (i) how do tethered insects react when a free-fall situation is suddenly triggered?; (ii) do hoverflies use gravity perception cues to detect a near-weightless state and stabilize their flight?
Methods. Here we modelled the dynamics of the visual closed-loop system controlling the insects’ head locking to an artificial horizon (consisting in a simple half black-half white contrast) and we examined the head roll response to a reversed artificial horizon (the upside-down configuration) while applying a roll step to the body. We also designed a gravity-defying setup where the unsuspecting flies were dropped instantaneously from an electromagnet, causing them to fall, in order to find out how swiftly the insects responded to the loss of the sensation of gravity and began attempting to fly. Results. Our results allow us toThen with the upside-down configuration, we revealed that a pure visual feedback loop was not sufficient to account for the measured insects’ head orientation with respect to the reversed artificial horizon. We therefore suggested the possible existence of a parallel pathway transmitting the head in body orientation measurement, provided by proprioceptive hair in the neck (prosternal organs for example). In addition, the flies' responses to a free-fall state reveal that the crash-avoidance performance of these insects in various visual environments suggests the existence of a multisensory control system based mainly on vision rather than gravity perception
1. R. Goulard, J-L. Vercher and S. Viollet (2016), To crash or not to crash: how do hoverflies cope with free-fall situations and weightlessness?, J. of Experimental Biology, vol. 219, 2497-2503
2. R. Goulard, A. Julien-Laferriere, J. Fleuriet, J-L. Vercher and S. Viollet (2015), Behavioural Evidence for a Visual and Proprioceptive Control of Head Roll in Hoverflies (Episyrphus balteatus, Dipteran), J. of Experimental Biology, vol. 218, 3777-3787.