Invertebrates - Other Arachnids

The arachnids are often thought of as insects but they are completely different. Insects have winged adults, 3 pairs of legs, three main body parts and a pair of antennae. Arachnids don't have wings, usually have four pairs of legs, only two main body parts and no antennae. That makes scorpions, amblypygids, mites and ticks the other members of the arachnid group. (There are other more obscure members which we won't describe here and they are the schizomids, palpigrades, Daddy Longlegs, whip-scorpions, solifuges and pseudoscorpions!)

 
Mites and Ticks

Tiny but sometimes largely annoying, some mites and ticks are responsible for the transmission of serious diseases including Lyme's Disease and Rocky Mountain Spotted Fever (ticks) and Scrub Typhus (mites). Ticks feed specifically on the blood of vertebrates such as mammals, reptiles and birds. Mites, on the other hand, can make life miserable for allergy sufferers (for example, dust mites), cause skin irritations from their saliva, or affect companion animals (i.e., mange in dogs and cats). Mites are predatory to small invertebrates but some are plant eaters and can damage crops.

Not all ticks and mites are so undesirable. Some mites are used to rid crops of pest mites and others assist with the breakdown of decomposing matter in the soil, thereby freeing up nutrients. In the Wet Tropics, one mite is actually attractive. The Velvet Mite is visible to the naked eye and can be found on the forest floor or even in your backyard compost bin.

 
Amblypygids

Hiding under rocks or sheets of bark, the one species of amblypygid we have in Australia (Charinus pescotti) is found in the Wet Tropics rainforests. Its body is only 1 cm (just under ½ inch) but the amblypygid has very long legs which enable it to move quickly. Although resembling the body of spiders, these arachnids do not have the web-creating spinnerets. They lie in wait for their food to come along, using their long, whip-like legs to herd it closer until it is grabbed with their claw-like front legs (called pedipalps).

 
Scorpions

Well known the world over are the scorpions, particularly those of the desert, but there are tropical species as well. Scorpions have been on the planet longer than any other arthropod having first appeared in the Silurian period, over 400 million years ago. They are the largest of the arachnids reaching up to 9 cm (3 ½ inches) long and are distinctive for their elongated, segmented abdomen which ends in a barbed tail. As with many other animals which are highly feared by humans, most species of scorpions pose no threat to us. Their usual food is insects and other small invertebrates, they are nocturnal, very sensitive to vibrations and prefer not to venture too far from their burrows. If you encounter one, it is strongly recommended to leave it alone.

Their reproductive method is interesting and involves a little dance. The male and female grab each other's front legs (pedipalps) and rotate in a circle. The male deposits a spermatophore on the ground and, still holding the female, pulls her forward until her genital opening is over the spermataphore where she can absorb it. After what can be a long gestation period (for some species), live young are born and climb onto their mother's back where they remain until after the first moult or longer.