Fig. 2: Secondarily synchronous hawkmoth flight muscle exhibits delayed stretch activation, a hallmark of asynchronous flight.
From: Bridging two insect flight modes in evolution, physiology and robophysics
a, Intact, downstroke flight muscle (DLMs) from M. sexta (n = 9 independent moths from the same source colonies, each sampled a single time) was mounted on an ergometer and electrically stimulated at 150 Hz to establish tetanus. Muscle viability was maintained with a saline drip at a constant 35 °C. b, We applied stretch–hold–release–hold strains, matching in vivo strain amplitudes55 of 4.5% while measuring stress normalized to tetanus. Positive strain (ε) and force are defined in the shortening direction (opposite stretch). The black line denotes mean muscle stress normalized to tetanic stress, grey lines show individual trials. c, Magnification of the region outlined in b shows the delayed stretch activation response, characteristic of asynchronous muscle physiology. A sum-of-exponentials mathematical formulation of delayed stretch activation (equation (5); red line) accurately fits the mean normalized stress (black line; shaded region is ±s.d.). The initial transient is the viscoelastic response of the muscle and the subsequent rise and fall is the stretch activation. d, Despite being synchronous, the delayed stretch activation rising rate constant (r3) of M. sexta lies near the prior empirical finding of a linear relationship between r3 and wingbeat frequency18 (123.4 ± 52.6 s−1 at 25 Hz; the black star shows the mean, error bars (obscured) show s.d.). Non-lepidopteran data and the black regression line are replotted from Molloy et al.18, with error bars representing the full range of data. We scaled r3 values to ambient temperature using published relationships (equations (2) and (3) from Molloy et al.18). e, Peak stress for M. sexta delayed stretch activation (Fa), tetanic force (Tet) and twitch. Delayed stretch activation (dSA) stress is shown with (IIR) and without (Emp) infinite impulse response correction (Methods). Box plots denote mean and quartiles, and whiskers are 1.5 × the interquartile range.
