Abstract
The initial stages of diapause induction - as summer gives way to autumn - involve a process of time measurement in which the duration of daylength (or nightlength) is determined by a photoperiodic 'clock' based upon the circadian system. In many insects so far examined, a photophase of sufficient duration and illuminance resets a photoperiodic oscillator to a constant phase equivalent to the beginning of the 'subjective night' (Circadian time, CT 12 h) whereupon it proceeds to measure nightlength in a clock of the external coincidence type. A possible exception may be found in the linden bug, Pyrrhocoris apterus, which - in laboratory studies using relatively low light intensity - suggests that daylength is measured rather than the night. Earlier studies of photoperiodic responses (pupal diapause induction) in the flesh fly Sarcophaga argyrostoma, however, showed that 'weak' or short light pulses giving rise to Type 1 phase response curves could be converted by increasing light intensity to Type 0 responses that phase set the oscillation to the beginning of the subjective night (CT 12 h) whereupon it could begin to measure the night. Based upon these data it is therefore suggested that the photoperiodic clock in P. apterus might also measure nightlength if the bugs were exposed to photophases of higher irradiance simulating daytime exposure to the light intensity experienced by these diurnally active insects in their natural environment.
Highlights
Marcovitch (1923, 1924) – no doubt encouraged by Garner & Allard’s (1920) seminal observations with plants – provided the first experimental evidence for photoperiodism in insects, for the regulation of seasonal morphs in aphids
Models for photoperiodic induction began with Bünning (1936, 1960) who proposed that light had two roles: entraining the endogenous circadian rhythm and activating a ‘long-day’ response by illuminating a particular light-sensitive phase in the night
In laboratory experiments conducted at an irradiance of about 240 μW cm–2 (Saunders, 1987), photoperiodic time measurement in P. apterus appeared to begin at light-on and to measure the duration of daylength
Summary
Marcovitch (1923, 1924) – no doubt encouraged by Garner & Allard’s (1920) seminal observations with plants – provided the first experimental evidence for photoperiodism in insects, for the regulation of seasonal morphs in aphids. Light ‘intensity’ in these examples was recorded as either illuminance (lx) or irradiance (W m–2) and not compared, these data suggested that insects varied in their sensitivity to light, often according to their ecology, some ‘seeing’ given light as ‘dim’, others ‘seeing’ the same light as ‘bright’. These variable responses to light intensity are further examined in this review with particular emphasis on the relationship between light pulse duration and illuminance within the context of diapause induction and the nature of the photoperiodic clock. This review is dedicated to the memory of Ivo Hodek who in his long career championed Andrewartha’s (1952) concept of ‘diapause development’ and contributed much fundamental work on the physiology of diapause (Hodek, 1999, 2002), in that of adult insects
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