They are beneficial animals that feed on all sorts of arthropods, including insects. About 3,000 species of spiders are found in the United States. Spiders rarely bite people, and most species found in the world are harmless. However, some people may be allergic to a spider's bite, and a few species of spiders are known to produce bites that may have serious medical implications for humans. Two of the more medically important types are the black widow spiders, and the brown recluse spider and its relatives. These two types of spiders are rarely encountered in Pennsylvania. Although black widows can be found native in Pennsylvania, the brown recluse spiders must be introduced and cannot survive in Pennsylvania's climate unless they remain inside heated structures. Many spiders indigenous to Pennsylvania will come into homes, mostly during the autumn, and a few of the more common and important of these will be discussed here.
To facilitate an accurate description of each of these species, and to aid the reader, a glossary of terms is included at the end of this publication. Sizes indicated in the "Description" subsections under each species refer to the length of the spider from the front of the cephalothorax to the rear of the abdomen.
A Note on Venom: "Toxic Venoms" and "Poisonous Venoms"
For the most part, all spiders have jaws and fangs to deliver venom. However, most spider bites have little or no effect on humans. The exceptions include people with compromised immune systems or other medical conditions that leave them vulnerable to spider venoms. Most spiders are too small to be of concern to humans. The fangs of these tiny creatures cannot penetrate the human epidermis unless the spider is held in place and allowed to bite the very thin skin between the fingers or toes. Larger spiders can bite, but they are shy and will attempt to escape unless trapped between the skin and some other object (e.g., clothing, sheets, and shoes).
Venoms from any spider are poisonous and/or toxic to their prey, but most are not poisonous or toxic to humans. The terms "poisonous venoms" and "toxic venoms" are somewhat misleading and should be replaced with "potentially harmful venoms." Very few spiders produce venoms that are potentially harmful to humans.