More than 30,000 spider species have been documented worldwide, the vast majority of which cannot be found in urban or suburban settings. The United States is home to only a little more than 3,000 documented spider species, and while many of these species are occasionally found within homes, most lack the muscular strength and/or fang size to inflict bites on human skin. All spider species that can appear within or around homes can be divided into three groups known as synanthropes, occasional invaders, and non-domiciliary species.

Spider species that are categorized as true synanthropes include American house spiders, southern house spiders, cellar spiders, triangulate cobweb spiders, yellow-sac spiders and brown recluse spiders. A “synanthrope” is any animal that benefits from living alongside humans within manmade environments, mainly homes and buildings. For example, house mice have evolved to live alongside humans in structures where a lack of predators, accessible foods and shelter are just a few survival advantages that the natural environment does not provide. Several intelligent mammals are well known to be synanthropes, such as racoons and rats, but many people find it surprising that a group of animals as ancient and primitive as spiders are sophisticated enough to learn that indoor living confers a greater survival advantage than outdoor living.

Urban spiders categorized as “occasional invaders,” or “seasonal invaders” include all spider species that are abundant in urban areas and may appear within homes during certain times of year. These spiders may find temporary respite within homes during adverse weather conditions, or they may enter homes to prey on insects, but they are unable to establish breeding populations indoors. Occasional invaders include species like western and southern black widows, woodlouse spiders, eastern parson spiders, yellow garden spiders, broad-faced sac spiders and many wolf spider species. Occasional spiders may establish indoor egg sacs, but spiderlings quickly move outdoors upon hatching. Non-domiciliary spiders include species that are rarely found in homes, either because they are not locally abundant or because they have nothing to gain by invading homes under any circumstance. With the exception of the brown recluse and the southern and western black widows, all of the spider species mentioned in this article are commonly found in Massachusetts homes, including the potentially dangerous yellow-sac spider. Luckily, however, the northern black widow is very rarely spotted in Massachusetts homes despite their native presence in the state.

Have you ever found numerous spiders of the same species within your home?