Stings of Centruroides, a common scorpion in Mexico, Arizona, and New Mexico, have killed many humans, mostly children. Its venom is neurotoxic and causes convulsions, paralysis or respiratory muscles, and heart failure. However, most scorpion stings are not fatal. Spiders are the most familiar arachnids and comprise 30,000 species. They are terrestrial and prey mostly on insects and other small invertebrates.
Spiders are often confused with insects, but spiders have two body regions and eight legs.
Insects have three body regions and six legs. The spinnerets of spiders are independently moving nozzles allowing release of silk from internal silk glands. Web silk is made primarily of polypeptides of the amino acids glycine, alanine, and serine. When this fluid is emitted it hardens not from exposure to air but from polypeptide cross-linkages that form during release. Spiders can produce silk as strong as nylon, highly elastic, and dry or sticky depending on the construction of the web.