Parasteatoda tepidariorum

American house spiders are generally dull brown in coloration, with patterns of differing shades often giving a vaguely spotted appearance (particularly noticeable on the legs). Their average body size is a quarter-inch (6 mm) long, but they can be an inch (2.5 cm) or more across with legs outspread. Their size and coloration allow the spiders to blend into the background and escape notice.

Like some other species of the family Theridiidae, P. tepidariorum is similar in body shape and size to widow spiders, which have venom that is classified as potentially dangerous.

This species can live for more than a year after reaching maturity. Each egg sac contains from 150-200 eggs, with a single female producing 15-20 egg sacs in their lifetime. The spiderlings remain in the mother's web for several days after coming out of the egg sac.[2]

American house spiders usually feed on small insects and household pests such as flies, mosquitoes, ants and wasps. They can randomly attack grasshoppers, butterflies, cockroaches or other spiders depending on their size. If the prey is too agile, the spider will try shooting web at it from a distance before pulling the thread toward itself. Bigger females can also attract baby skinks inside their web by leaving fly remains hanging in it. Once its food dries out, the spider usually drops it to the floor in order to free space in its web, instead of destroying and rebuilding it or changing its location.

Three spider species usually prey upon them: the pirate spiders of the genus Mimetus (Mimetidae), as well as two jumping spider species ‒ Phidippus variegatus and Metacyrba undata.[2] The latter one also often falls prey to its own food when it gets trapped in the tangling web after missing the jump on its target.

Female eating a Green Leafhopper (Cicadella viridis)

A male and female (the female is the larger of the two)

Closeup of a female P. tepidariorum

The assassin bug Stenolemus lanipes (Emesinae) apparently feeds exclusively on spiderlings of this species, but can also become prey of the adult spider.[3]

A male and female often share the same web for long periods, and several females often build their webs in close proximity. However, females will sometimes fight when they encounter each other.

As these spiders live in constant proximity to humans, they are not usually aggressive and will even let a human hand approach their web. Like any other spider, however, they are afraid of bigger foes, and, in most cases, will retreat behind an obstacle (such as a dried leaf or prey remains) upon perceiving more than usual disturbance to their web. Further disturbance may lead to the spider dropping down on a thread, then running away from the web. If the distance is not considerable, it will usually return to its web within a couple of days. Otherwise, it will start a new one.

American house spiders possess poor vision and cannot detect any movement more than three to four inches away. If cornered, they will feign death as last resort.

Unlike most house spiders, which will wander the dwelling by spring if male, or seek refuge from the first cold by autumn if female, P. tepidariorum species of both sexes can be encountered in mid-winter, in wall corners or underneath the windowsill. Some species will build their webs there, other can remain almost motionless until prey is abundant.

American house spiders will bite humans only in self-defense, when grabbed and squeezed. Although the bite of this species is relatively less severe compared to other theridiids, it has to be considered. Bites of P. tepidariorum can cause severe pain with a median duration of 16 hours, and in some cases systemic effects; which is very similar to the symptoms of steatodism (i.e. bites of false widows in the genus Steatoda).[4]

Steatoda and Parasteatoda species can inflict bites with similar, but far less severe, painful symptoms to those of black widows.[5] It is also powerful enough to kill the same species of spider on occasion. The synanthropic habits of several species may play a role in cases of human biting.[6]