INTRODUCTION

The approach commonly assumes there is a single clock, or oscillator, which acts to produce some observable phenomenon once every twenty-four hours. Although there are descriptions of the circadian clock as a population of interacting oscillators, for instance that of Christensen & Lewis (1982), the underlying assumption remains that there is a single master clock, with the population of oscillators comprising the mechanism. The complexity of the hypotheses is necessary because the clock is somewhat inaccurate in relation to the solar day, and under constant conditions of illumination (LL) or, more commonly, lack of illumination (DD) the observed phenomenon recurs with a consistent periodicity which is more or less than 24h - hence the use of the term "circadian". To correct the inaccuracy in the clock that would result from the phenomenon being controlled solely by the circadian period (t), the postulate is that the daily natural cycle of light and dark (LD) in some way exerts a resetting influence. Here there is another assumption which takes only a single-step, upwards (usually taken as light-on) or downwards (light-off), as being the daily clock-correcting action, the "zeitgeber".

A succinct summary of the common approach was given by Crosthwaite et al. (1995). In their list of criteria, they stated that the universally dominant zeitgeber in all circadian systems is the presence or absence of visible light. The clock is photoresponsive, with the magnitude and direction of the response (advances or delays) reflecting the subjective time of day (morning versus evening; night versus day). The most viable model is where light, instead of acting gradually to alter the speed of the clock, acts very rapidly to effect a step change in the level or activity of a state variable and thereby to change the clock itself. The light signal is simple and discrete, and the clock imparts a "time of day-specific" interpretation to the signal.

Although Saunders (personal communication) has suggested that Aschoff has stressed the importance of both light-on and light-off for over 30 years and given his own view that circadian pacemakers entrain to the whole of the light:dark cycle, it has to be noted that even very recent authors clearly take neither point. For instance, Watari & Arai (1997) wrote - "lights-on or lights-off function as a primary zeitgeber (Aschoff, 1981)" for the "circadian oscillator". Similarly Tomioka et al. (1997) wrote only of the circadian clock driving locomotor rhythms.

This Section draws on observations of the timing of flight activity in mosquitoes to develop an alternative and, in some ways, inverse view of how the timing is determined. In the first Section a double-plot format (named as the "photoperiodogram" for convenience) was used to show how the percentage of total daily activity varies per consecutive half-hour of the normal solar 24h-day. By overlaying the activity patterns in various LD regimes, the format showed also how the control of the timing of activity can be revealed. This control appeared clearly to be effected by more than one clock. Here, the evidence, together with results from experiments in which activity was recorded in LL or DD, is used to show how the apparent clear and uncomplicated daily pattern of unimodal or bimodal activity, as observed in LD 12:12, is the end product of complex interactions between endogenous oscillators and the exogenous effects of the natural LD cycle. The key to the concept is an acknowledgement that it is oscillators in plural and steps up and down in light intensity that interact. The study (Taylor, 1969a) which generated the considerable amount of data seems both unusual in its approach of examining activity patterns under a range of LD regimes, and unique in its being a comparative study of taxonomically related species from across the full range of possible geographical latitudes. For instance, Tyschen (1978), who studied the mating rhythm of a fruit fly, Dacus tryoni, in a range of LD regimes and reviewed other similar studies, listed only three insect species - Drosophila pseudoobscura, Blatta and Oryzias; and Hong & Saunders (1994) commented on the paucity of data for locomotor activity in "higher" flies. Several species of tsetse flies have been studied but all of them are Afrotropical and seasonal changes in the natural LD regime are small, LD 11:13 to LD 13:11 (Kyorku & Brady, 1994). Very recently, Watari & Arai (1997) presented their results on the effects of photoperiod on the Onion Fly, Delia antiqua.

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©1998, 2010 - Brian Taylor CBiol FSB FRES 11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K. Comments to dr.b.taylor@ntlworld.com

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