Figure 2.

Symsagittifera roscoffensis worms exhibit milling as a step function of increasing density. (a) A circular mill in a small arena. (b) The relationship between worm density (no. per ml) and the presence or absence of circular mills (1 and 0, respectively); green circles: data, black line: predicted probabilities from the fitted binary logistic regression model log(π/(1 − π)) = −4.126 + 0.053x, where π is the probability of milling and x is worm density. The model predicts that with every additional worm per ml, the probability of milling increases on average by 5% (95% CI: 3–8%) and that at density above 78 worms ml⁻¹, the presence of milling becomes more likely than not (see the electronic supplementary material). (c) Part of a 100 × 100 mm Petri dish where three circular mills have formed.