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    Arachnida – Spiders & scorpions

    Web weavers and fang bearers

    Arachnida, one of the first terrestrial animals, with four pairs of legs, basic eyes, and fang-tastic mouthparts. No antennae or wings – they keep it simple and stylish!

    Arachnids typically have a body divided into two parts: a front part (cephalothorax) with mouthparts, sensory organs, and legs, and a back part (abdomen) for digestion and reproduction. The arachnid skeleton is hydraulic. As an alternative to tracheae, the basic arachnid converts carbon dioxide into oxygen using two (or four) pairs of book lungs.

    The majority of arachnids are terrestrial, yet their evolutionary history is rooted in marine environments. The Early Devonian (400 Ma) and Late Silurian (414 Ma) are the earliest known periods for terrestrial arachnids.

    Today’s taxonomy of Arachnids includes about 640 families, 9000 genera, and around 100,000 identified species. The diversity of arachnids is astounding, yet it’s believed to be a small fraction of the class’s actual diversity.

    Did you know?

    • Although this interpretation is controversial, some fossilized scorpions from the Devonian period are supposed to have been marine because they contain what appear to be external gills!
    • The first set of walking legs is elongated and serves a similar purpose as insect antennae in several arachnids (such as amblypygids, uropygids, schizomids, solifuges, palpigrades, and many mites).
    • Arachnids lack extensor muscles at important joints and extend the limb by pumping blood into it, with the exception of pseudoscorpions and scorpions.
    • All arachnids have a rather incomplete fossil record. Since the Silurian Period, less than 50 significant time periods have yielded fossil information of about just 13 extinct and surviving orders.