Larvae of the genus Arachnocampa, known as glowworms, are bioluminescent predatory insects that use light to attract prey. One species, Arachnocampa
flava, is known to possess true circadian regulation of bioluminescence: light:dark
cycles entrain the rhythm of nocturnal glowing. Given the absence of natural
light as a cue in caves, we addressed the question of whether cave populations of
Arachnocampa tasmaniensis, a species known to inhabit caves as well as epigean
environments, are rhythmic. We found that the major dark-zone cave populations of A. tasmaniensis maintain a high-amplitude 24-hour rhythm of bioluminescence, with the acrophase during external daylight hours. Populations of A.
tasmaniensis in caves many kilometers apart show similar, but not exactly the
same, timing of the acrophase. Systematic investigation of colonies in the
dark zone of a single cave showed that some smaller colonies distant to
the main ceiling colony, also in the dark zone, glow in antiphase. Periodic monitoring of a single colony over several years showed that the acrophase shifted
from nocturnal to diurnal some time between October 2008 and January 2009.
Prey availability was investigated as a possible zeitgeber. The acrophase of prey
availability, as measured by light trapping, and the acrophase of bioluminescence
do not precisely match, occurring 3 hours apart. Using in-cave artificial
light exposure, we show that after LD cycles, cave larvae become entrained
to bioluminesce during the foregoing photophase. In contrast, epigean larvae
exposed to artificial LD cycles after a period of DD become entrained to bioluminesce during the foregoing scotophase. One explanation is that individuals
within colonies in the dark zone synchronize their bioluminescence rhythms
through detection and matching of each otherís bioluminescence.