Fig. 8. Local and context-dependent differences in leg coupling strength. Coupling strength (numbers) and efficacy (percentages) of the three main coordination rules (sensu Cruse et al., 1995) known to be present in the walking stick insect. (A–C) Rules 1–3, respectively, for straight walking. L, left; R, right leg. (D–F) Rules 1–3, respectively, for curve walking. O, outer; I, inner leg. Arrows point from sender to receiver legs. Arrow size is scaled to the efficacy of the rule. Coupling strength is the difference between the likelihood maxima (rules 2 and 3) or minima (rule 1) and the baseline. Efficacy indicates the percentage of the maximum coupling strength possible, i.e. the situation if a rule held in each single step cycle. This is equivalent to the fraction of step cycles by which a given coordination rule increased (rules 2 and 3) or decreased (rule 1) the likelihood of protraction. Coloured arrows mark coordination rules for which the coupling strength changed significantly (red, increase in strength; blue, decrease in strength). Coordination strength and efficacy vary strongly between rules and between leg pairs. During curve walking, coordination rules undergo a context-dependent change in strength and efficacy. Rule 1 becomes more effective between ipsilateral front/middle leg pairs. Efficacy of rules 2 and 3 increases between outer leg pairs and decreases between inner hind and middle leg.