A study of circadian rhythmicity of the blow fly, Calliphora vicina, has identified the role of locomotor activity as the 'hands' of the clock system, therefore providing a reliable gauge for the 'black box' approaches applied in the present investigation. Observations in constant conditions confirmed that the circadian rhythmicity of C. vicina is under the control of a self-sustained oscillatory system which is temperature-compensated and free-runs in DD with a mean period length 22.68 h. This, however, lengthened to a value greater than 24 h in continuous light below about 0.03 Wm^-2, above this intensity, locomotor activity became arrhythmic. Phase response curves (PRCs) for 1 h light pulses and temperature steps further described the periodic changes of sensitivity of such oscillatory systems to these two main environmental Zeitgebers. The phase-shifts required for entrainment to a 24 h light/dark cycle were shown to be predictable by the light pulse PRC. The splitting of the activity rhythm into two or more components, spontaneously or induced by light stimuli, provided strong evidence that the circadian rhythm of C. vicina is under the control of a coupled multioscillatory system. The attempt to interpret the observation of multi-components led to the development of a heuristic model which provided support for the multioscillatory hypothesis. One of the main aims - a search for the anatomical locations of the photoreceptor(s) and the circadian pacemaker - was pursued via surgical treatments, including optic tract severance and complete removal of both optic lobes (lobectomy). Both treatments failed to destroy the activity rhythm or prevent entrainment to a light/dark cycle.