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Definition of arachnoid. 1  1 : of or relating to a thin membrane of the brain and spinal cord that lies between the dura mater and the pia mater. 2  2 : covered with or composed of soft loose hairs or fibers.
Araneae, or spiders, are the most familiar of the arachnids, and the most species-rich with around 40,000 described species. All spiders produce silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen.