Arachnocampa is a genus of four fungus gnat species which have a luminescent larval stage, akin to the larval stage of glowworm beetles. The species of Arachnocampa are endemic to New Zealand and Australia, dwelling in caves and grottos, or sheltered places in forests.
A previous synonym was "Bolitiphila," meaning "mushroom lover," in the past. The name was changed in 1924 to Arachnocampa, meaning "spider-worm," for the way the larvae hang sticky silk threads to ensnare prey. The genus Arachnocampa belongs in the family Keroplatidae.
The larval stage lasts about 6 to 12 months, depending on food. The larva emerges from the egg only about 3 to 5 millimetres long, and through its life grows to about 3 centimeters.
The larva spins a nest out of silk on the ceiling of the cave and then hangs down as many as 70 threads of silk (called snares) from around the nest, each up to 30 or 40 cm long and holding droplets of mucus. The larvae can only live in a place out of the wind, to stop their lines being tangled, hence caves, overhangs or deep rainforest. In some species, the droplets of mucus on the silk threads are poisonous enhancing the trap's ability to subdue prey quickly.
The larva glows to attract prey into its threads, perhaps luring them into believing they are outdoors, for the roof of a cave covered with larva can look remarkably like a starry sky at night. A hungry larva glows brighter than one which has just eaten. Prey include midges, mayflies, caddis flies, mosquitos, moths, or even small snails or millipedes. When prey is caught by a line the larva pulls it up (at up to about 2 millimetres a second) and feeds. If prey is scarce the larvae will turn to cannibalism, eating other larvae, pupae or adult flies.
The glow is the result of a chemical reaction that involves luciferin, a waste product; luciferase, the enzyme that acts upon luciferin; adenosine triphosphate, the energy molecule; and oxygen. It occurs in modified excretory organs known as Malpighian tubules in the abdomen.
The body of the larva is soft while the head capsule is hard. When it outgrows the head capsule it moults, shedding its skin. This happens four times through its life.
At the end of the larva stage it becomes a pupa, hanging down from the roof of the cave. The pupa stage lasts about 1 or 2 weeks and it glows intermittently. The male stops glowing a few days before emerging, the female's glow increases. The glow from the female is believed to be to attract a mate, and males may be waiting there when she emerges.
The adults (of both sexes) cannot feed and live only a short time. They glow, but only intermittently. Their sole purpose is to mate, and for the female to lay eggs. Adult insects are poor fliers and so will often remain in the same area, building a colony of glowworms. The female lays a total of about 130 eggs, in clumps of 40 or 50, and dies soon after laying. The eggs hatch after about 20 days and the cycle repeats.
The larvae are sensitive to light and disturbance and will retreat into their nests and stop glowing if they or their snares are touched. Generally they have few predators. Their greatest danger is from human interference.
- Arachnocampa luminosa is found in New Zealand, in both the North and South islands. Its Māori name is titiwai, meaning "projected over water". The Waitomo Caves in the North Island near Pirongia is one well-known habitat, the caves having become a popular tourist attraction. It was first known to science in 1871 when collected from a gold mine in the Thames region. At first it was thought to be related to the European glowworm beetle, but in 1886 a Christchurch teacher showed it was a larva of a gnat, not a beetle. The species was called Bolitiphila luminosa in 1891, before being renamed Arachnocampa in 1924. A species of harvestman preys on the luminosa eggs, larvae and pupae, and even the adult flies. A fungus also affects A. luminosa; it gradually kills the larva. Fungus spores are spread by air movement, but since the larvae live out of the wind the spread of spores is limited. Arachnocampa luminosa is found only in New Zealand.
- Arachnocampa richardsae is found in New South Wales. The Newnes glow worm tunnel in the Blue Mountains is one well-known habitat.
- Arachnocampa tasmaniensis is found in Tasmania (as the name suggests). One habitat is the Marakoopa Cave, Mole Creek near Cradle Mountain.
- Arachnocampa flava is found in Queensland. The Natural Bridge in the Gold Coast hinterland is one well-known habitat.
- Arachnocampa buffaloensis. A colony of Arachnocampa has been found in an alpine cave on Mount Buffalo in Victoria. Early research suggests it is a new species, but related to the tasmaniensis and the New Zealand luminosa. Its presence suggests rainforest may have extended up the mountain in the past. The Victorian Government presently has it listed (called the Mount Buffalo Glow-Worm) as a threatened species.
Glow worm in a cave on the west coast of the south island of New Zealand
- Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
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