'Cousins' of the spiders

Araneae, the true spiders: Characteristics of a typical true spider include 8 eyes on the 'head' region of the prosoma, two chelicerae that carry the fangs, and between them and the first pair of walking legs a pair of leg-like pedipalps (often just called palps) which on the males are modified for mating. Spiders cannot ingest solid food but use their mouth parts and fangs to immobilize their prey and predigest it to a liquid form outside the body. Defence is provided by a pair of venom glands in the chelicerae or prosoma. On the underside of the abdomen are one or two pairs of book lungs then openings for mating and egg laying (if female) and for voiding of waste materials, and at or near the rear end of the abdomen are up to 3 pairs of spinnerets that extrude silk for web-making or other purposes.

Acari, the ticks and mites: These are characterised by the presence of a very small prosoma tightly and broadly joined to a much larger and usually oval abdomen. The very small 'head' region of the acarine body may have one pair of eyes and the chelicerae and palps are short appendages that are closely pressed together to form a beak-like structure that is well suited for piercing and sucking. At least on ticks the fused chelicerae have rows of backwards-pointing barbs that make the tick difficult to dislodge while it is feeding. There are four pairs of conventional legs but no spinnerets or other abdominal appendages, though some mites are claimed to make a form of silk from their mouthparts. Ticks are parasitic, generally feeding on the blood of vertebrates. On the other hand, while some mites attack humans and large animals, the majority feed on smaller creatures or plant materials and mites therefore can be found in a wide range of environments, including ponds and buildings. In Australia, ticks such as the paralysis tick, Ixodes holocyclus, are significant hazards to humans and many other vertebrates while mites, though mostly harmless to us, can damage crops and cause skin irritation, sometimes by inducing a secondary allergic reaction. It is not uncommon for mygalomorph spiders such as the funnel-web and trapdoor species to have ectoparasitic mites on their body surfaces.

Scorpiones, the true scorpions: These are the size of large spiders and have 4 pairs of legs plus a very large pair of palps with strong terminal pincers. There are two large eyes in the middle of the head and up to 5 sets of small lateral eyes. The prosoma and opisthosoma are fused together and the latter has a number of segments and a long tail (called a telson) which curves forward and has a venom gland and a sharp stinging barb. Like spiders, scorpions only ingest liquid food and use 4 pairs of abdominal book lungs to obtain oxygen. They produce living young instead of laying eggs and also lack silk-secreting spinnerets. In Australia most are found under logs or rocks.

Pseudoscorpiones, the false scorpions: In many respects these arachnids resemble true scorpions but are somewhat smaller in overall size, most species being only a few millimetres long. They also have a combined prosoma and opisthosoma, the latter being visibly segmented, as well as an enlarged pair of palps with strong pincers. Typically, 1-2 pairs of primitive eyes are present. They lack the curved tail of a scorpion and therefore the capacity to sting like a scorpion. They do not use silken snares or webs to catch their food but they do possess silk glands associated with their chelicerae and these are used to build small retreats. They are most often found in leaf litter and under rocks and loose bark but are also known to survive in a wide range of other habitats, even including seashore ones. There are many Australian species.

Opiliones, the harvestmen: These typically have quite small bodies but very long, slender legs. A prosoma with just a single pair of eyes (sometimes reduced in size) is closely attached to a somewhat larger, visibly segmented opisthosoma and there are also short cheliceae and palps of varying length and diameter. Each palp has a small claw and the chelicerae have pincers rather than fangs with attached venom glands. Harvestmen do not have book lungs but instead use a network of fine air tubes that are scattered around the body like our blood vessels. Like the other arachnids harvestmen ingest only liquid food, mostly squeezed from other small animals that are already dead. Their preferred habitat is long grass or damp vegetation. They are said to have acquired the harvestmen name because they are most common in fields in late summer and autumn. A number of Australian species have been described but few people ever see them in the wild.

Solifugae, sun spiders or wind scorpions: Though not found in Australia these becaue well known during the 1990s because of the image shown here. This was taken by an unknown photographer (probably in Iraq by a US serviceman) and was part of a widely circulated hoax email that implied this creature is much larger and more dangerous than is actually the case. Solifugids have a large segmented abdomen without a telson and a smaller prosoma with a single pair of eyes and very strong, toothed, beak-like chelicerae. The palps and 4 pairs of walking legs are also rather robust and allow these creatures to run fast. The overall body length can be up to 50 mm. Some species live in burrows and most are nocturnal (their name means "flee from the sun").

Amblypygi, tailless whip scorpions: The creatures in this arachnid Order are quite similar in general appearance to the uropygids but an obvious difference is the lack of the whip-like telson at the end of the abdomen. Once again, the first pair of legs overlie the second pair and are much longer and thinner than the three pairs of 'conventional' legs. They are said to serve a sensory role similar to that for a uropygid. Also notably different are the palps, which extend to the sides of the prosoma and have the appearance of folded combs. Amblypygids can be found in many of the same high-humidity places as the uropygids although they probably have a greater overall distribution. Most are nocturnal and hide in leaf litter or under stones and fallen logs, but some are subterranean. There is at least one Australian species and since this is also a creature that survives best in tropical habitats it is normally found in the northern parts of Australia.