Fig. 1: Phylogenetic comparative analysis of insect wingbeat actuation reveals a probable single origin of asynchronous flight muscle.
From: Bridging two insect flight modes in evolution, physiology and robophysics
a, Synchronous muscle has a 1:1 relationship between neural activation (blue dots) and muscle contraction. Asynchronous muscle contraction is independent of the precise timing of neural activation (red dots), arising from delayed stretch activation2. b, The physiological signature of an asynchronous muscle is that when impulsively stretched it produces a delayed force of magnitude Fa that peaks after a characteristic time t0, determined by the rising and falling rate constants r3 and r4 (Methods). c, Ancestral state reconstruction14 based on muscle ultrastructure (not physiology) reveals that a single evolutionary origin of asynchronous fibre types is more probable using an insect-wide phylogeny resolved to the ordinal level15. Tip states were identified from the literature (Methods). Pie charts represent the posterior probabilities of the ancestral state reconstruction at these particular nodes given an equal rates model of evolution (full posterior probabilities in Extended Data Fig. 3 and Supplementary Table 5). d, By iteratively constraining ancestral nodes (Methods), we find an 87% posterior probability that some node ancestral to Lepidoptera and Trichoptera (including M. sexta) was asynchronous (making this clade secondarily synchronous) as opposed to all nodes ancestral to Lepidoptera being synchronous (ancestral synchronous). Myr, million years.
