Spiders are abundant (over 1,000,000 individuals per acre in a grassy field) and can be found almost anywhere from the bedroom closet to the 22,000 foot level on Mt. Everest. There are about 2,500 different species in North America. Some live in holes in the ground while others may be found under rocks or logs. Those that prefer dark, shaded areas may also occur in cellars and basements. These species may become established in homes. Most spiders found in homes and buildings are "accidental invaders" that have entered around doors, windows, or other openings. Homes in wooded areas or with naturalized or landscaped foundations may be prone to more frequent invasion because the surroundings are ideal spider habitats. Spiders are beneficial, feeding mostly on small insects and other arthropods. Some trap their prey in webs or snares; others are active hunters that use excellent vision to stalk or ambush their food. Virtually all spiders have poison glands that connect with the fangs. Venom produced by the glands apparently is used to kill or paralyze prey and in defense. Only a few species, such as the black widow and the brown recluse, have venom that is very toxic or harmful to humans. Most species do not attempt to bite; many have fangs that are not capable of piercing the skin. However, some will, with results that are similar to a wasp or bee sting. There is the chance that a person can have an allergic reaction to the venom.