The cephalothorax is covered by a shield, or carapace, and bears eight simple eyes. The spider then liquefies the tissues of the prey with a digestive fluid and sucks this broth into its stomach where it may be stored in a digestive gland. Arachnid book lungs are similar to the gill books of horseshoe crabs but are internal and adapted to a terrestrial habitat. Three pairs of spinnerets toward the tip of the abdomen produce protein-containing fluids that harden as they are drawn out to form silk threads.
Man uses spider silk for the cross hairs in certain optical instruments.
Young, growing spiders can regenerate missing legs and parts of legs. Several species of spiders have bites that are exceptionally painful, or even dangerous to man. Species of black widow spiders, which are found in the warmer parts of the world including the United States and S Canada, have a virulent neurotoxic venom. The bite venom of the brown recluse spider of SE and S central United States decomposes tissue, resulting in slow healing and sometimes leaving a sunken scar as large as a quarter. Spiders are classified in the phylum Arthropoda , class Arachnida, order Araneae.