Contactless Acoustic Trapping of Hoverflies for Behavioral Studies

Studying the sensorimotor response of flying insects is usually done in tethered conditions. The righting reflex, for example, is studied with insects that are released either from a rigid or a magnetic tether. The attached tether or magnet may, however, induce proprioceptive biases and behavioral artifacts through physical contact and additional mass or inertia. Here, we propose a contactless tether apparatus based on acoustic levitation. It relies on the emission of ultrasonic waves on both sides of the device, creating standing pressure waves-visualized using the schlieren technique-that enable insect levitation. We demonstrate its effectiveness on living hoverflies Episyrphus balteatus. By quantifying the rate of motion of four natural body markers (head, abdomen, wing, and leg) and the wingbeat response, we show that hoverfly levitation can be stable for a long period of time, with minimal behavioral perturbation under ultrasound stimulation. We further illustrate the application of ultrasound tethering to study the hoverfly righting reflex. Beyond hoverflies, ultrasound tethering was applied successfully to levitate other insect species, such as dead Drosophila and ants. More broadly, ultrasound tethering might be relevant in the context of behavioral studies with flying or walking insects.