Loose screens and cracks under doors, windows, and other openings are all possible entryways for a spider. These pests may move indoors while searching for food, mates, warmth, or moisture. The presence of insects and other prey in homes is a common reason for spiders to come inside. While most spiders pose little or no danger to people, some species can deliver venomous bites that may cause medical issues. In the U.S., the two most common venomous spiders are the brown recluse, distinguished by the violin-shaped marking on the top of its cephalothorax.