ABSTRACT.The behaviour of the circadian locomotor rhythm of the New Zealand weta,Hemideina thoracica(White), supports the model that the underlying pacemaker consists of a population of weakly coupled oscillators. Certain patterns of locomotor activity, previously demonstrated almost exclusively in vertebrates, are presented here as evidence for the above hypothesis. They include after‐effects of various pre‐treatments, rhythm‐splitting and spontaneous changes in the rhythm. After‐effects, which describe the unstable behaviour of free‐running circadian rhythms following particular experimental perturbations, have been observed inHemideinafollowing single light pulses, constant dim light, and laboratory and natural entrainment. Period changes occurred in the activity rhythm after single light pulses of 8‐h and 12‐h duration (25 lx). Constant dim light (0.1 lx) increased the free‐running period (τ) of the activity rhythm, but the after‐effect of constant dim light was either an increase or a decrease in τ. After‐effects upon both τ and the active phase length of the activity rhythm were found following non‐24‐h light entrainment cycles with 8‐h and 12‐h light phases of 25 lx. Qualitative measurements of these after‐effects upon τ are presented which reveal a relationship between both the direction and amount of change in τ, and the difference between entrainment cycle length (T) and pre‐entrainment free‐running period. The after‐effect of natural entrainment was an initial short‐period free‐run (τ<24h) lasting 5–10 days, generally followed by a rapid period lengthening to τ= 25–26 h. Support for the population model was provided by spontaneous dampening, recovery, and period changes of the rhythm, together with the disruption of the active phase following critical light perturbations, and rhythm‐splitting. TheseHemideinaresults are compared with the simulation