Scorpions

Scorpions are common arachnids found in gardens and forests throughout Australia. They are found under logs, rocks and in shallow burrows in earth banks. There are also desert species that construct deep spiral burrows in desert sand. Scorpions are mostly nocturnal but they can be active during the day, especially during prolonged wet weather. Scorpions tend to be larger and more venomous in the northern parts of Australia. The largest Australian scorpions can grow to 12 cm long, but many forest dwellers are only small.

Identification

Scorpions are easily distinguished by their long sting-bearing tail and a pair of pincers on long arms, known as pedipalps, at the front of the body. Despite having six to twelve eyes - an obvious pair at the centre of the carapace and two to five smaller eyes on each side - scorpions do not have good eyesight. However, they can readily distinguish light from dark and appear to have excellent low light sensitivity, which helps them to both avoid harsh sunlight and to navigate by starlight or moonlight. They sense their way around using sensory hairs and slit organs on the legs, pedipalps and body that pick up vibrations and scents (mechanoreceptors and chemoreceptors). They also have special organs on the underside of the body called pectines, which pick up ground textures and scents. Scorpions breathe through four pairs of book lungs on the underside of the abdomen. Female scorpions are more heavily built than males, with shorter tails. Colour ranges from dark grey to light brown or gold, with lighter coloured legs. Scorpions also fluoresce under ultraviolet light, which is a good way for scientists to find them in the field. The fluorescence is thought to serve as an ultraviolet sensitivity mechanism, perhaps allowing the scorpion to avoid damaging light levels.

Four species of scorpion you may commonly come across include;

• Brown Scorpion, Urodacus manicatus:
• Desert Scorpion, Urodacus yaschenkoi:
• Marbled Scorpion, Lychas marmoreus:
• Wood or Forest Scorpion, Cercophonius squama:

Distribution

Scorpions are common arachnids found in a variety habitats throughout Australia. They are found under logs, rocks and in shallow burrows in earth banks.

Feeding and diet

Scorpions are nocturnal hunters, feeding mainly on arthropods such as beetles, cockroaches, spiders, slaters, centipedes and millipedes. One Australian species, Isometroides vescus, is specialised to feed solely on burrowing spiders, especially trapdoor spiders, invading and often occupying their burrows. Many scorpions are lie-in-wait ambushers that forage at or in the vicinity of the burrow entrance (eg Urodacus spp.), but some, notably bark and litter dwellers (eg Lychas spp.), are more active foragers.

Ground vibrations caused by moving prey are sensed both by slit-like tarsal sensory organs on the scorpion's legs and vibration sensitive tarsal hairs. The clawed grasping pedipalps are used to hold the prey while the scorpion stings or crushes it. The scorpion digests its prey by pouring digestive juices onto the prey and breaking it up with its jaws. The hard outer body casings are discarded. The main predators of scorpions are carnivorous marsupials, rodents, lizards, nocturnal birds, centipedes and other scorpions.

Other behaviours and adaptations

Scorpions are arachnids, which means that they are related to animals such as spiders, ticks, mites and harvestmen. Arachnids are characterised by possessing four pairs of legs and a body divided into two parts - the cephalothorax (containing the mouthparts, eyes, pedipalps and legs) and the abdomen (containing the reproductive and digestive organs). One particular group of arachnids that can closely resemble scorpions is known as Pseudoscorpions. These possess large grasping pedipalps, but lack the characteristic tail and sting of true scorpions.

Breeding behaviours

Males and females find each other by vibration, scent and touch. During mating, the sensory pectines under the body are used to find a suitable place for the male to deposit his sperm parcel - the spermatophore. The male and female then perform a mating dance above the spermatophore, with the female being wrestled into position over it in order to draw it up into her genital pore. The fertilised eggs develop inside her body, and she then gives birth to live young. She carries the pale young scorpions on her back for the first few days or weeks, until they are strong enough to become independent. The young then disperse to find food and shelter. Scorpions take a long time to reach maturity, moulting frequently (up to five or six times over two to six years) in order to grow, and may live for two to ten years. Some have been recorded as living up to 25 years.

Danger to humans

Australian scorpions can give a painful sting which can cause inflammation and pain for several hours. First aid for a sting is to apply a cold pack and to seek medical aid if pain persists. It is also a good idea to try to catch the scorpion and have it identified. The scorpion usually seen in houses is the small Marbled Scorpion.

To avoid being stung by a scorpion, wear good gloves and shoes in the garden and don't leave things lying around on the floor in the house or garage. Reduce invertebrate habitat by covering compost and garbage, and cleaning up building materials around the house. Scorpions are great pest controllers in the garden, so if one is found in the house, collect it carefully in a jar and remove it to a safe distance, rather than killing it.

References

• M. S. Harvey & Yen, A.L. 1989. Worms to Wasps: an illustrated guide to Australia's terrestrial invertebrates. Oxford University Press: Melbourne.
• Koch, L.E. 1977. The Taxonomy, Geographic Distribution and Evolutionary Radiation of Australo-Papuan Scorpions, Rec West Aust Mus: 5(2).
• Locket, A.1994. Night Stalkers. Australian Natural History 24(9): 54-9.
• Scorpions. WA Museum Leaflet.
• Scorpions, Centipedes and Millipedes. Queensland Museum Leaflet.
• Lawless, P. 1998. Lo what light…Wildlife Australia Magazine, Winter Edition. [article on scorpion fluorescence]