Study of the Aerial Righting reflex in hoverflies

Thesis defence: Friday 12 March 2021

Abstract

Hoverflies (Order: Diptera, family: Syrphidae) feature stunning aerial capabilities allowing them to orient themselves in various positions and orientations. Flies can land on surfaces no matter how tilted itmay be, they can even settle upside down on the ceiling. When taking off from the tilted surface, flies must reorient dorsoventrally and stabilize body roll via active control of their flapping wings. Righting reflex has been shown to exist in mammalian and wingless insects but have never been studied so far in winged insects. After being released upside-down and dropped in free fall, hoverflies systematically rotate their body in roll once the wingbeats triggered. The aerial righting reflex is achieved by Episyrphus balteatus in 48.8ms (median value) within 6 wingbeats. As expected, a wing asymmetric stroke amplitude is at the origin of the body righting. We show that body rotates first at maximumroll speed as fast as 10 000°/s and then that head rotates after a time lag of 16ms (median value) at similar angular speed.

A dynamic model of the righting reflex accounts for that head-body response by implementing a closed-loop control of both head and body combined with a feedforward control of the head body angle. The feedforward control of the head orientation from the body angular speed, provided by the halteres, introduced a time lag between head and body, which was coherent with the fly’s response. Our model suggests that a closed-loop control of both body angle and body speed, merged with a fast head stabilization reflex, are at work at an early stage during the righting process. The model also suggests that halteres are involved both in body and head roll rate and roll angle control. These results highlight the strong coupling existing between the activation of the halteres and gaze (head) stabilization reflex. Tests made tend to confirmit.

During additional experiments, we have modified the stimuli perceived by the syrphe (modification of the environment as well as of its sensory organs). Associated with a preliminary study of Episyrphus balteatus in free fall with flies rightside-up, changes of the environment (presence or absence of proprioception and light source) has allowed us to show the importance of proprioception when hoverflies right themselves: this would indicate the initial orientation and would trigger the rollover via rapid wing flapping during recovery. We also have studied the impact of the dorsal light response (DLR) on the righting reflexe with and without proprioception.

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See the thesis manuscript (in french)