Fig. 1.

Reversals in shape space correspond to reversals in the crawling direction. (A) Tracking video microscopy gives both the x-y trajectory of the worm as it crawls on an agar plate, and the shape of the worm’s body at high resolution. (B) Shape is described by the tangent angle θ vs. arc length s, in intrinsic coordinates such that ∫dsθ(s) = 0. (C) We decompose θ(s) into four dominant modes. (D) The joint probability density of the first two modes. Amplitudes along the first two modes oscillate, with nearly constant amplitude but time varying phase ϕ = tan^-1(a₂/a₁); here the amplitudes are normalized so that . (E) The phase trajectories exhibit abrupt reversals, moments when ω ≡ dϕ/dt change sign. The red cross marks the onset of a body wave reversal and the green and magenta dots mark times prior to and during a reversal. These same times are also marked in A demonstrating that phase reversals correspond to reversals in the crawling direction.