The Class Arachnida

Arachnids

These are almost all land animals which normally breathe through gill-like structures called book-lungs. Their bodies are divided into two main parts. The first part is the prosoma, bearing two pairs of appendages (the chelicerae and the pedipalps) which are often pincer-like, and four pairs of legs; the second part is an abdomen which lacks limbs. Arachnids are generally aggresive, predatory creatures, preying on small arthropods. There are ten orders:

Order Scorpionida
Scorpions: 600 species
These secretive, nocturnal animals are abundant in tropical regions. Their pedipalps are large pincers, but their chelicerae are small. The scorpions body, which may be 1 - 16 cm long, has a sting on its last segment.
Order Pseudoscorpionida
Pseudoscorpions: 1100 species
Found throughout the world, these small arachnids live in leaf litter and damp nooks and crannies. They lack stings but have two pairs of pincers: large pedipalps bearing poison glands and small chelicerae containing silk glands.
Order Solifugae
Camel spiders or sun spiders: 570 species
These large, swift-moving, tropical and sub-tropical animals prefer arid environments. They have large, pincer-like chelicerae and leg-like pedipalps with sensitive adhesive organs.
Order Palpigradi
Micro-whip scorpions: 21 species
These tiny, soil-dwelling arachnids are found in the warmer parts of the world. They have a pair of well-developed pincer-like chelicerae and leg-like pedipalps. The trunk is segmented and ends in a long flagellum, or tail.
No picture available
Order Uropygi
Whip scorpions: 105 species
These small to medium-sized arachnids are named after the very long,narrow, bristle-like structure borne by the last segment of the abdomen. The chelicerae are large and hooked, the pedipalps are short, stout and often pincer-like, and the first pair if legs is very long.
Order Amblypygi
Amblypygids: 50 species
These medium-sized arachnids, ranging in length from 0.4 cm to 4.5 cm, are found in warm countries, usually in damp, dark habitats. The chelicerae and pedipalps are similar to those of whip scorpions, and the first pair of legs, which are long and thin, is held out in front like antennae.
Order Araneae
Spiders: 20 000 species
After mites and ticks, spiders are the most widespread and abundant arachnids. Spiders which range in length from less than 0.075 cm to 25 cm, have chelicerae bearing poisonous fangs and have silk glands in the abdomen. The pedipalps, leg-like in the female, are modified into copulatory organs in the male. Many species have excellent vision.
Order Ricinulei
Ricinuleids: about 15 species
These small, compact, little-known arachnids, from Africa and the warmer parts of America, live in leaf mould. Both the chelicerae and the pedipalps are small and pincer-like.
No picture available
Order Opiliones
Harvestmen or daddy long-legs: 2400 species
Long legs and short compact bodies distinguish these arachnids, which are normally found in most environments. Their chelicerae are small with pincers, and their pedipalps are leg-like. They are called harvestmen in Britain and daddy long-legs in North America.
Order Acarina
Mites and ticks: 10 000 species
Most of this abundant and widespread group of animals are tiny. Many are parasites, and most of the free-living species inhabit leaf-litter. They are arachnids with a false head, or capitulum, set apart from the rest of the body, and carrying mouth-parts. External segmentation is reduced or absent. Larval stages normally have three pairs of legs;nymphal and adult stages four pairs. The chelicerae and pedipalps are pincer-like, leg-like or needle like, depending on their function.

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