Short answers, pulled from the story.
Arachnids are arthropods in the class Arachnida, part of the subphylum Chelicerata. Over 110,000 named species have been described, and scientists estimate there may be more than one million species in total.
Arachnids and insects belong to different arthropod clades and evolved their limb counts independently. Adult arachnids have eight legs plus two additional pairs of appendages, the chelicerae and pedipalps, which handle feeding, defense, and reproduction rather than locomotion.
The term derives from the Greek word aráchnē, meaning spider. The word refers to the myth of Arachne, a human weaver whose hubris led to her being transformed into a spider.
Scorpions, spiders, and pseudoscorpions pour digestive enzymes over prey using their chelicerae and pedipalps, dissolving the tissues into a liquid broth before sucking it into a pre-buccal cavity. Their venom also contains pre-digestive enzymes that begin breaking down prey before ingestion.
Not all arachnids produce venom. Scorpions, spiders, and pseudoscorpions secrete venom from specialized glands to kill prey or defend themselves. Ticks instead use saliva containing anticoagulants and anticomplements, and several tick species produce a neurotoxin.
The extinct order Trigonotarbida dates from the late Silurian to the early Permian. A notable specimen, Chimerarachne yingi, was preserved in amber estimated at around 100 million years old from Myanmar; it had both spinnerets like modern spiders and a tail like the much older Uraraneida.