Glow-worms and its distribution in Australia

Glow-worms (Arachnocampa tasmaniensis) are the spectacular underground sight. Thousands of them all cluster on ceilings and walls—a myriad of bright blue lights resembling stars in the night sky. They are only found in Australia and New Zealand. The moist, sheltered surface habitats are their ideal choice, such as rainforest gullies and wet caves. (Baker, et al., 2008) The glow-worm caves are regarded as the natural heritage and the fantastic choice for travelling.

The geographic distribution of the glow-worms in Australia encompasses the montane regions of the eastern Australian coastline from the wet tropics region of northern Queensland to the cool temperate and montane rainforests of southern Australia and Tasmania.

Map of the eastern coast of Australia showing some typical locations of Glow-worms. The lightly shaded regions roughly correspond with the distribution of rainforest. Source: (Baker, et al., 2008)

Why the glow-worms can display light?

• Chemical reaction involving fungus

Glow-worms are the luminous larval stage of a fungus gnat. A chemical reaction in their abdomen produces a cold blue light. They are able to switch on and off at will.

• Attracting mates

The chemical reaction and the resultant unstable by-product are often referred as ‘an excited state’. This may be a clue as to why an animal might be bioluminescent but the ability to glow is different. Fireflies use the ability to glow for attracting mates.

• Attracting insects as food resource

The larva builds a hollow, tubular nest of silk and mucous from which it suspends sticky threads up to 30 cm long. Flying insects, attracted to the lights, become trapped and are then eaten. In stream caves, the main insects caught are stoneflies and mayflies. The aquatic, larval stages of these insects are carried underground by the stream. When they emerge from the water and metamorphose into adult flies, they are attracted up to the lights and become entangled in the sticky threads. Glow-worms quickly haul up the appropriate thread and consume their victim.

Hollow, tubular nest of silk and mucous built by glow-worm larva

source: Discover Wildlife (website)

The life of glow-worms

After several months of growth, the glow-worm larva pupates inside a chrysalis, then emerges as an adult gnat. The adults live only a few days during which time they don’t feed. Instead, the female lays her eggs on the wall in the cave. Glow-worm colonies are dependent upon the continued availability of flying insects for their food, especially aquatic insects carried into caves by streams. This is the beginning of another lifecycle of glow-worms.

Human impacts on glow-worms

Intensely visited populations in caves may be at risk of permanent loss of this outstanding underground sight. Specifically, Marakoopa Cave in Tasmania is the only cave with capacity to accommodate viewing by approximately 30,000 visitors per year. (Merritt & Clarke, 2013) Human threats include disturbance through touching of larvae or their webs, harmful activities such as lighting of fires, changes in water quality, and changes in cave. Glow-worms will stop glowing if people shine bright lights on them, or make loud noises. In addition, the cave lighting itself and some creation (paths, stairs and roads) may also cause the larvae to dim.

The conservation of glow-worms

Because of the tourism, commercial and educational values of these sites, the conservation is necessary and it should focus on ensuring the survival of the population.

• Enhancing the humidity and the presence of streams

It is important to maintain the natural conditions of stream flow and native forest within the cave catchment area to preserve the glow-worms. It has been observed becoming torpid in response to dry conditions, ceasing to glow and eventually shrivelling and dying. The native forest should be reserved and reduce the construction of roads and stairs. More streams should be introduced in the glow-worm caves to increase humidity.

• Improving the water quality and food source

Some management can be done on the water quality in the caves to keep the continuous production of the prey for the glow-worms. The abundance of prey plays an important role on the glow-worm population.

• Close the caves in winter and control the number of visitors in summer

In summer, cold air draining out of the lower cave entrances while warm air is drawn in through chimneys. The warm air cools down to cave temperature causing water to condense on the cave walls. But the condition is winter is reverse. The caves would be very dry. Pupae and adults are most common during winter, and larvae are most common during spring and summer. So close the caves in winter is available. The glow-worm population is abundant in summer, so controlling the number of visitors is necessary to avoid some human impacts.

Reference

Baker, C. et al., 2008. Distribution and phylogenetic relationships of Australian glow-worms Arachnocampa (Diptera, Keroplatidae). Molecular Phylogenetics and Evolution, Volume 48, p. 506–514.

Driessen, M. M., 2010. Enhancing conservation of the Tasmanian glow-worm, Arachnocampa tasmaniensis Ferguson (Diptera: Keroplatidae) by monitoring seasonal changes in light displays and life stages. Journal of Insect Conservation, 14(1), p. 65–75.

Merritt, D. J. & Clarke, A. K., 2013. The impact of cave lighting on the bioluminescent display of the Tasmanian glow-worm Arachnocampa tasmaniensis. Journal of Insect Conservation, 02, 17(1), p. 147–153.

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