The circadian locomotor (walking) rhythms of forager honeybees (Apis m
ellifera ligustica L.) were entrained to eight different 24 h light-da
rk cycles. The phases of activity onset, peak activity, and offset wer
e correlated with the lights-off transition, suggesting lights-off as
the primary zeitgeber for the rhythm. Further support for this hypothe
sis was provided by LD 1:23 experiments, in which entrainment occurred
when the light pulse was situated at the end, but not at the beginnin
g, of the subjective photophase. Steady-state entrainment of the locom
otor rhythm was achieved with square-wave temperature cycles of 10-deg
rees-C amplitude under constant dark: most of the activity occurred wi
thin the early thermophase. Smaller amplitude temperature cycles yield
ed relative coordination of the rhythm. Interactions of temperature an
d light-dark cycles resulted in entrainment patterns different from th
ose elicited in response to either cycle alone or those formed by a si
mple combination of the two separate responses. Furthermore, temperatu
re cycles having amplitudes insufficient for entrainment of the rhythm
nevertheless modified the pattern of entrainment to light-dark cycles
, suggesting a synergism of light and temperature effects on the under
lying circadian clock system.