The small (about the size of your thumbnail), venomous yellow sac spider, pictured at the end of this post, is extremely common in Western Pennsylvania and tends to head indoors, this time of year, as the outside air becomes cooler.
They hunt at night by stalking their prey (they don’t use webs) and hide during the day in silken tubes they spin in the corners of walls, or behind pictures, or in other low-activity areas. Cheiracanthium inclusum is not aggressive and most bites occur when they inhabit bedding or clothes and people trap them next to their skin. Take care and shake out stored blankets and sheets before making your winter bed!
These arachnids are responsible for the majority of reported spider bites in North America, because their powerful jaws easily penetrate human skin and their mildly-toxic venom produces symptoms that mimic brown recluse spider bites. Its bite is initially painful with redness and swelling that should abate in several days. If it doesn’t heal promptly, or becomes ulcerated, see a physician for antibiotic therapy.
I like spiders and rarely kill them; that said, when I find a yellow sac spider, and most homes have at least a few, I quickly and cautiously relocate them, well away from my abode. These spiders are not yellow; they are usually a pale white, and extremely good pictures of them abound on the Internet.