Wireless Stimulation of Antennal Muscles in Freely Flying Hawkmoths Leads to Flight Path Changes

Insect antennae are sensory organs involved in a variety of behaviors, sensing many different stimulus modalities. As mechanosensors, they are crucial for flight control in the hawkmoth Manduca sexta. One of their roles is to mediate compensatory reflexes of the abdomen in response to rotations of the body in the pitch axis. Abdominal motions, in turn, are a component of the steering mechanism for flying insects. Using a radio controlled, programmable, miniature stimulator, we show that ultra-low-current electrical stimulation of antennal muscles in freely-flying hawkmoths leads to repeatable, transient changes in the animals' pitch angle, as well as less predictable changes in flight speed and flight altitude. We postulate that by deflecting the antennae we indirectly stimulate mechanoreceptors at the base, which drive compensatory reflexes leading to changes in pitch attitude.United States. Defense Advanced Research Projects Agenc

Analysis of changes in flight trajectory elicited by in-flight stimulation of antennal muscles.

<p>(<b>A</b>) Ground speed (v_ground), altitude, as well as pitch and yaw heading of a moth's body vector calculated from a 3D reconstruction of a free-flight trial during which extrinsic muscles of the left antenna were stimulated electrically. Stimulus timing is indicated by the gray bar. The change in pitch angle is the only parameter change that could be elicited repeatedly and in a similar fashion in multiple animals. Changes in ground speed, altitude and yaw heading are unique to a specific trial. (<b>B</b>) Still images of a moth outfitted with an on-board stimulator shortly before (<i>1</i>), and during the stimulus (<i>2</i>). (<b>C</b>) Average change in pitch angle (red line) for 4 successive trials (underlying grey lines) in one animal shows a consistent response to stimulation. The arrows indicate time points corresponding to the still images in B.</p

Summary of responses to free-flight antennal stimulation of extrinsic antennal muscles in six animals.

<p>Responses in body pitch angle are quantified by calculating the difference ΔΘ between the mean pitch angle (Θ) of a 200 ms period before stimulus onset and the maximum Θ during 400 ms after stimulus onset. All pitch responses were nose-up, with pitch magnitude indicated as follows: <b>+</b>: ΔΘ>10°; <b>+ +</b>: ΔΘ>15°; <b>+ + +</b>: ΔΘ>20° (maximum was 29°). Yaw axis responses are defined as changes in yaw heading that happened within 400 ms of stimulus onset. Yaw responses varied from one stimulation to the next. Animals either turned clockwise (+), counter-clockwise (−), or showed no change (n.r.) in yaw heading. Symbols in the Δ_altitude column indicate whether a change in the z-coordinate of the moth occurred after stimulation. A change in altitude of ±10 cm between a 200 ms interval before, and a 200 ms interval after stimulus onset is symbolized with a − or +, depending on whether the moth lost or gained altitude, respectively. Similarly, a change greater than ±15 cm is shown as either − − or ++ (maximum was 22 cm/s ). Similarly, the Δ_speed column indicates changes in the mean body speed before and after stimulus onset. (+ +: >1 m/s²; −: <−0.5 m/s²; − −: <−1 m/s²; − − −: <−1.5 m/s²(max. was −2.8 m/s²). In most cases, stimulation is associated with a decrease in ground speed. In cells labeled “n.a.”, parameters could not be computed because a full 3D path was impossible to reconstruct.</p

Antennal motion evoked by electrical stimulation of extrinisic antennal muscles in tethered moths.

<p>(<b>A</b>) Changing the stimulus frequency of a 3 V, 50% duty-cycle pulse train of 200 ms duration leads to changes in antennal motion: Increasing stimulus frequency leads to an increase in antennal deflection amplitude. (<b>B</b>) Antennal deflection amplitudes of 8 animals to 12 consecutive electrical stimulation events of extrinsic muscles. For each animal, the deflection amplitudes are normalized to the mean deflection for all 12 stimulus repeats. There is no significant difference between deflection amplitudes elicited by the first compared to the last stimulus trains (gray boxes; t-test p = 0.78). Likewise, linear fits to each animal's responses show a negligible trend in any direction. The overall mean slope for all fits is 0.003/stimulus event (S.D. = 0.043).</p

Overview of how antennal stimulation in free-flying animals was achieved.

<p>(<b>A</b>) Top view of a moth's head, with one electrode pair placed (indicated by red arrow), but not yet waxed down, to target extrinsic antennal muscles. The other electrode pair has not yet been placed. (<b>B</b>) Photograph of a typical pair of tungsten electrodes used for electrical stimulation of extrinsic muscles. (<b>C</b>) Photograph of the “RadioFlyer” microcircuit that is mounted ventrally on a moth to provide telemetrically triggered electric muscle stimulation. (<b>D</b>) Simplified schematic (redrawn and modified from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052725#pone.0052725-Kloppenburg1" target="_blank">[10]</a>) showing the two muscle groups involved in positioning a moth's antennae. Extrinsic muscles, which move the whole antenna with respect to the head, were targeted for the experiments presented here.</p