LIGHT
Luciferin and luciferase:
Light is produced by a reaction between an enzyme (luciferase) and a substrate (luciferin), related to the chemicals that produce light in fireflies. The molecule, ATP, provides energy for the reaction. There is no evidence that waste materials are used in producing the light.
lights-on when vibration is felt:
Some tour operators report that glow-worms react to sound disturbance by dimming their lights. Others report that sound causes the glow-worms to brighten. At Waitomo Caves in New Zealand tourists are asked to remain quiet as they view the glow-worms so that they do not disturb them and affect the display. In other locales such as Natural Bridge at Springbrook the waterfall produces high noise levels and the glow-worms do not appear to be affected.
In a series of experiments as part of a BSc honours project, Julie-Anne Popple showed that glow-worms are much more sensitive to vibration than to air-borne sound. They respond to medium-intensity signals by rapidly increasing their light output and then gradually return to the baseline level. They are most sensitive to vibrations around 100 Hz frequency; about the same as the wing-beat frequency of a small, flying insect. The light response may be an adaptation that increases the chances of attracting prey when detected in the vicinity. Alternatively, it might simply be a startle response.
lights-off when light is seen:
Glow-worms are sensitive to artificial light when they are glowing. Continuous exposure to bright light will cause them to douse their own lights.
Light output from individual glow-worms tends to fluctuate through the night due to their movements within the snare and activity such as maintaining their silk lines. In the laboratory, the glow-worms switch on about 30 minutes after onset of darkness and rapidly reach maximum brightness.
Glow-worms glow most brightly after dusk and gradually decrease in brightness toward dawn.
Glow-worms respond to artificial light by switching off their own light. Recovery time to full light output depends on:
• the duration of exposure
• the intensity of exposure
• the time of night.
Glow-worms are equally sensitive to light of all colours. Although many insects are insensitive to red, glow-worms can see red light and switch off in response.
Key findings
Glow-worms control their light output intensity through the nervous system
Light output comes under circadian regulation
Light output varies through the night in a regular pattern
Light intensity increases rapidly when larvae feel vibration in their snares
Light output is doused when larvae are exposed to light above a threshold intensity
Dave setting up time-lapse camera in a cave
Apparatus for recording responses to vibration
Left: close-up view of the posterior of a glow-worm where light is produced. The white structure is a reflector that dissipates the light. Right. microscope image of a section through the light-producing cells.
The blue streaks are the accumulated images of individual glow-worms taken over 24 hours
