The Lure of the Glowworms
Every year in late summer I travel to Tasmania from my base in Brisbane to carry out research on the Tasmanian glowworm.
Glowworms are distributed along the eastern coast of the Australian mainland, so why, you may ask, do I travel so far to study the Tasmanian glowworm?
The answer lies in the Tasmanian species’ wonderful adaptations to their preferred cave habitat. It is emerging from our field-work as well as from laboratory studies carried out at the Brisbane campus of the University of Queensland, that Australia’s glowworms are more diverse than we thought.
In particular, the Tasmanian species shows special adaptations to living in caves and shows some distinctively different behaviours to the local Queensland glowworm, Arachnocampa flava.
Let me tell you a bit about glowworms. They are insects, members of the genus Arachnocampa. The immature stages (larvae) produce light to attract prey into their sticky, webbed snares. Light (bioluminescence) is produced in cells located at the tips of internal tubular structures branching from the gut, known as Malpighian tubules.
In most insects the tubules function solely as excretory structures, but in glowworms they have taken on a dual function; excretion and light production. They are the only insects that produce light in this way. The light-producing cells are located internally at the posterior end of the larva. The cuticle is transparent to allow the transmission of light.
In addition, an internal reflector composed of a mass of air-filled respiration tubes is present around the light-producing cells. They function both as a respiratory system and a light reflector. The larval stage lasts for many months, then the larvae form a pupa and from the pupa an adult fly emerges. The mosquito-like adults mate as soon as they emerge and the female goes on to lay her eggs, then dies only a few days after emerging.
The distribution and relationships of glowworms are keys to understanding the behavioural and evolutionary differences between species. Claire Baker carried out a PhD study at the University of Queensland, completed in 2004, examining the morphology and DNA sequence of all known glowworm groups in Australia and New Zealand.
She named a number of new species and provided a phylogenetic tree: a reconstruction of the most likely relationships among the present-day species, with estimated times (x millions of years ago) when the major divergences took place.
Because glowworms react to natural light, they have no obvious need for a circadian clock that controls their bioluminescence, but our tests using the Queensland forest species, Arachnocampa flava showed that bioluminescence does indeed come under the control of a biological clock The finding made us curious about what happens in cave populations. Glowworms in the deeper zones of caves never see daylight so it was possible that they would be glowing at a consistent level all the time because their clock—if they have one—has never been entrained by daylight. Alternatively, individuals might keep their internal rhythm of glowing but individuals in a colony would be out of phase with each other because each has its own personal periodicity.
The way to test this was to find accessible cave populations and set up long-exposure, time-lapse photography that would remain operative in a cave environment. With my colleague Arthur Clarke, a biospeleologist who lives at Dover, south of Hobart, we set up time-lapse digital cameras that operated unattended for several days. We discovered that, within a colony, individuals are synchronised, i.e. they showed the same time of peak and trough in their intensity curves.
Consequently, the intensity of light produced by any single colony oscillates throughout a day peaking at about 5:00 pm and reaching its dimmest at 5:00 am. Some individuals never turn off completely, others dim to the extent that they are no longer detectable by the camera, but all maintain the same periodicity. The phenomenon occurs in other caves as well.
To explain why whole colonies show the same periodicity and phase we suspected that the glowworm larvae might be synchronising to each other. If we could prove this it would be a truly novel discovery. To definitively test for synchronisation we needed to carry out experiments in the laboratory where environmental conditions could be tightly regulated to explore the possibility of behavioural synchronisation.
With permission from Tasmanian Department of Primary Industries, Parks, Water and Environment (DPIPWE), and assistance from Mike Driessen of DPIPWE who has been studying Tasmania’s glowworms for some time, we collected glowworms from a cave and set them up in the laboratory in Brisbane where we maintained them in incubators at a cool 8°C; the mean annual temperature of Mystery Creek Cave (Driessen).
In the lab, postgraduate student Andrew Maynard carried out the definitive laboratory experiments that proved synchronisation does indeed take place. We made up small habitats where individual larvae recreate their snares. The containers are plastic and transparent so larvae could all see each other when placed together. Individuals were pre-set to different periodicities then three larvae on one cycle were exposed to one larva on another other. The single larva changed its periodicity over several nights until its period and phase both matched the other three, providing concrete evidence that they do see each others’ lights and synchronise to them.
We are now exploring how they accomplish this synchronisation, for example, do they slow down or speed up their internal clock to catch up with the others?
What does the future hold? We suspect that synchronisation in cave-adapted species allowed them to become more efficient at attracting prey: a constellation of lights might be more efficient at attracting flying insects than a free-for-all where individuals are competing with each other to glow the brightest. We want to test whether group synchronisation is indeed more efficient at attracting prey. Also, we plan to derive a mathematical model of synchronisation from laboratory studies and test whether it holds up in caves.
We have found that a subset of individuals in a colony can show differently-phased rhythms to the rest of the colony and we suspect that this is related to the fact that they can’t see each other to synchronise: the topography of the limestone walls where they are located could influence their ability to see each other. Three-dimensional time-lapse photography is the next item on the research agenda.
If you are interested in seeing glowworms, you can visit Hastings Caves south of Hobart, or Mole Creek Karst National Park west of Launceston.
David Merritt is an associate professor in the School of Biological Sciences at the University of Queensland. He teaches and carries out research on insect development and physiology. He became interested in glowworms around 2000 when he decided to take some students on a night field trip to view them in rainforest at night. He regularly visits the world’s most famous glowworm site—Waitomo Glowworm Cave in New Zealand—where he advises the environmental advisory group on glowworm population monitoring. David teaches a number of courses, mostly on entomology, the study of insects. Beside bioluminescence and biological rhythms, his research interests include the developmental biology of insects and the structure and function of insect glands and sense organs.
