Other Arachnids and myriapoda
What are arachnids?
Mention arachnids and you’re likely to think of spiders, but they’re just one of eleven major groups of Arachnida. The best known of these are scorpions and perhaps the mites and ticks. Other common but rarely noticed arachnids are the pseudoscorpions and harvestmen, common in leaf litter worldwide. The other three groups of Arachnida found in Queensland are rarely encountered. These are all types of whip-scorpions: the micro, short-tailed and tailless.
Arachnids have a body divided into two major parts: a front-body and rear-body. The front-body has eight legs, two biting chelicerae and two palps (the appendages between the legs and chelicerae).
In spiders the chelicerae are the fangs, like on this Grey Huntsman.
This form is recognised by most people as an arachnid, but they’re not the only animals with this body plan. The Arachnida belong to a larger group of animals called the Chelicerata. All chelicerates share this basic body plan.
Other groups of Chelicerata are the Horseshoe Crabs, Sea Scorpions and Sea Spiders. Horseshoe crabs are an ancient group of marine animals that come to land to spawn. Sea scorpions are extinct but, as their name suggests, some looked incredibly scorpion-like. In contrast, sea spiders are so strange that some scientists don’t think they should be included in the Chelicerata.
So what distinguishes an arachnid from these other chelicerates? Basically, an arachnid is any terrestrial chelicerate. Long ago, a chelicerate animal - perhaps a scorpion or mite - started to live its life entirely on land. Along with this change came adaptations to living on land, such as book lungs or tracheae to take oxygen from air. Scientists assume this happened only once but it's possible, even likely, that it happened at least twice. However, without any good evidence for this we assume a single aquatic-terrestrial event.
One last point: some arachnids have returned to the water! Most of these spend some time out of water, like water spiders or water mites. However, one diverse group of mites spends its entire life in oceans.
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