Arachnids and Biology
Arachnids first appeared on Earth about 400 million years ago.
Fossil of a Carbonifeorus Harvestman - Ameticos scolos - reconstructed with the aid of computed tomography from palaeontologyonline.com
There are 11 living orders of arachnids, 3 extinct orders and about 100,000 different species in total.
- Acari - mites and ticks
- Amblypygi - tailess whipscorpions/cave spiders
- Araneae - spiders
- Haptopoda - extinct
- Opiliones - daddylonglegs/harvestman
- Palpigradi - microwhip scorpions
- Phalangiotarbida - extinct
- Pseudoscorpions
- Ricinulei
- Schizomida
- Scorpions
- Solifugae- camel spiders/sun spiders/wind scorpions, solpugids
- Thelyphonida (formerly Uropygidae) - vinegarroons/whip scorpions
- Trigonotarbida -extinct
All arachnids share commonalities. They have hard exoskeletons made of chitin, are invertebrates, meaning no backbone, have jointed appendages, have 4 pairs of walking legs attached to the cephalothorax, have bilateral symmetry, meaning both sides of their bodies are alike, have chelicerae, the first pair of appendages used for feeding and defense, and they all have pedipalps, the second pair of appendages used for feeding, locomotion and reproduction. The chelicerae can be shaped as claws, pincers or fangs, like in spiders, and the pedipalps can be shaped as claws or pincers, like in scorpions. They also have respiratory systems that work using either book lungs or a tube like trachea.
Of course there are always exceptions to any rule and arachnids are no different. For example some mites do not have 8 legs and the division between body parts may not be visible with some ticks, mites and harvestmen, so they appear to only have one body part.
Of course there are always exceptions to any rule and arachnids are no different. For example some mites do not have 8 legs and the division between body parts may not be visible with some ticks, mites and harvestmen, so they appear to only have one body part.
Water mite from palaeontologyonlie.com
Most arachnids are terrestrial, living on land, but there are a couple of aquatic mites and spiders. Aquatic spiders still breathe air, not water, and they do this by use of an air bubble. Water mites breathe by absorbing oxygen directly from their own body surface.
Most arachnids are predators and are carnivorous, preying on insects, other animals and even other arachnids. There are some arachnids though that are scavengers or are parasitic, like the mites that live on humans or ticks that feed on animal blood. Most arachnids are only able to intake liquids and so they secrete a digestive enzyme to break down their prey, liquefying it, prior to ingesting it. Many arachnids are venomous and the venom is used for defense and to immobilize prey.
Most arachnids mate by the males passing a sperm package to the females. There are mating rituals performed to assure the passage of the sperm packet. Most young are hatched from eggs, that the females lay, however some scorpions bear live young. Most female arachnids will watch over the eggs or the young for a period of time to ensure survival.
Most arachnids are predators and are carnivorous, preying on insects, other animals and even other arachnids. There are some arachnids though that are scavengers or are parasitic, like the mites that live on humans or ticks that feed on animal blood. Most arachnids are only able to intake liquids and so they secrete a digestive enzyme to break down their prey, liquefying it, prior to ingesting it. Many arachnids are venomous and the venom is used for defense and to immobilize prey.
Most arachnids mate by the males passing a sperm package to the females. There are mating rituals performed to assure the passage of the sperm packet. Most young are hatched from eggs, that the females lay, however some scorpions bear live young. Most female arachnids will watch over the eggs or the young for a period of time to ensure survival.
Scorpion mother carrying her young
Wolf Spider mother carrying her young
