Acarian chemotaxis by a sinusoidal walk: A non-directional mechanism modulating counterturning patterns generates robust chemotaxis in mites

We identified a chemotaxis strategy of mould mites, Tyrophagus putrescentiae, by analyzing trajectories that they drew in a radial odor gradient on a table-type locomotion compensator. All of the mites’ pathways to an odor source consisted of forward walking with sinusoidal angular oscillation, but some engaged in direct guidance and others in indirect guidance. Side-to-side movements of the course while walking apparently described direct guidance as klinotaxis. It was achieved by biasing individual turns to keep the average course direction within ±45 degrees of the odor source. The alignment with up-gradients was primarily achieved with counterturns that followed an outward turn across the transition from up- to down-gradients, thereby inevitably directing the mite back to the up-gradient. The presence of odor gradients promoted the counter-steered reorientations by shortening latencies to initiate the counterturns and by accelerating their angular velocity. For the indirect guidance, which apparently corresponds to classically authorized chemo-klinokinesis in the related species, T. casei, the probability of counter-steered reorientations was only 69.0%; 31.0% of the reorientations needed a series of alternate turns, thus losing individual alignments with the up-gradients. Nonetheless, such prolonged reorientations were involved in late accelerations of turns but without directional relation to the up-gradient. Our findings firstly demonstrate that robust chemotaxis in ambulatory arthropods is produced by a non-directional mechanism in which negative odor gradients activate intrinsic self-steered turning patterns. Robust reorientation maneuvers from composite directional turns indicate that the Tyrophagus strategy is algorithmically intermediate between bacterial klinokinesis and the klinotaxis of larval flies.