Spiders (Subphylum Chelicerata) are distinguished from other
arachnids by having four pairs of walking legs and an anterior pair of
appendages called chelicerae that bear terminal poison fangs. Although all of the approximately 35,000 species of spiders are venomous to some extent, only about 80 in the world are even capable of injecting
their venom into humans. Of these, only two
forms in the United States present any real danger to human beings,
the black widow (Latrodectus spp.) and the
brown recluse (Loxosceles reclusa).