Pergamm Press. Printed in Great Britain
THE PHOTOPERIODIC CLOCK IN THE FLESH-FLY,
D. S. SAUNDERS
Department of Biological Sciences, Stanford University, Stanford, California, U.S.A.;
and Department of Zoology, University of Edinburgh, Scotland
of the flesh-fly,
raised in experimental light cycles with periods (T) of 21 to 72 hr, each cycle
containing a photoperiod of 4 to 20 hr of white light. This ‘resonance’ technique
revealed periodic maxima (N 24 hr apart) of pupal diapause, thereby demon-
in the photoperiodic
The positions of these maxima of pupal diapause suggested that the oscillation,
like that controlling the pupal eclosion rhythm in
is ‘damped out’ by photoperiods longer than about 11 to 12 hr, but restarta at
dusk whereupon it runs with circadian periodicity in a protracted dark period.
With photoperiods shorter than 12 hr, however, the two diapause maxima were
less than 24 hr apart, suggesting that an additional component, possibly a ‘dawn
hour-glass’, was modifying the position of the first peak.
Both photoperiod and the period of the driving light cycle (T) were shown
to affect the length of larval development (the sensitive period) and the number
of calendar days needed to raise the incidence of pupal diapause to 50 per cent
(the required day number, RDN).
Peaks of diapause induction were shown to
be the result of an interaction between a long sensitive period (slow develop-
ment) and a low RDN, whereas troughs in diapause induction were the result of
an interaction between a short sensitive period (fast development) and a higher
Larvae of S.
are unable to distinguish (in a photoperiodic
sense) between 12 and 18 hr of red light ( > 600 nm).
of the flesh-fly,
at short daylength
enter diapause as pupae whereas those kept at long daylength
(1972) has since demonstrated
that the intra-uterine
spp. are particularly
Working with a northern
strain of S.
showed that the ‘sensitive period’ effectively came to an end at puparium formation.
‘add up’ successive
cycles and enter
diapause (as pupae) if a sufficient number
of such signals have been seen before
the end of the sensitive period.
the length of larval development
sensitive period) was temperature
(Q1,, = 2.7) the number of short-day
cycles needed to programme
the insects for subsequent
diapause (the required day