The Arizona brown recluse spider (shown), and the black widow are the most common “harmful” spiders commonly found in Arizona. Most spiders are harmless to humans. In fact, spiders are considered beneficial because they feed on insects. Outdoors, spiders serve an ecological purpose. When they’re found indoors, however, spiders build unsightly webs, often with leftover dead insects in them, which far outweighs any benefits they may offer in keeping the number of pests down.