Spider Sunday: A Common "Indoor" Spider

At this time of year, there is one spider that is commonly encountered indoors in the eastern United States and southeast Canada. It has a bi-colored body that makes it conspicuous and different from most other spiders one is likely to see around their residence. Trachelas tranquillus, is sometimes known as the “Broad-faced Sac Spider,” a member of the family Corinnidae.

This is a spider of average size, adult females measuring 7-10 millimeters in body length, males 5-6 millimeters. It occurs from Nova Scotia and Minnesota south to Kansas, Oklahoma, and northern Georgia, preferring deciduous woodlands. In nature, specimens are often found in retreats fashioned from curled leaves; or they are seen occasionally under stones.

I have seen this species hunting actively at night, prowling around porch lights where it may be hunting various insects attracted there. Trachelas does not spin a web, but does lay down draglines to help secure itself during climbs. Females probably also impregnate their draglines with pheromones (scents) so that males can track them down.

© Ken Lebo 2012

Mating usually happens in summer and early fall, after both genders have matured (nice images on the highlighted link). Females create a lens-shaped egg sac, the bottom flat against a substrate and the top slightly convex. The whole package is roughly ten millimeters long and usually attached beneath loose bark, or under a rock.

Besides killing live prey, Trachelas may scavenge on dead insects. Perhaps this is why it prospers when other spiders have either perished or gone into hiding in autumn. This scavenging habit may also influence the outcome in those rare instances when the spider bites a human. Secondary infections from its bite have been recorded, originally interpreted incorrectly as caused by the spider’s venom.

© Sarah Rose

The fact that Trachelas hunts regularly in and around homes and other buildings means it is more likely to have interaction with people. Still, verified cases of bites are infrequent, and usually result in only localized pain and swelling. People sensitive to arthropod venoms, or prone to allergic reactions, should seek medical attention for any arachnid bite (or sting in the case of scorpions).

Further complicating this picture is the confusion of Trachelas with another spider that is completely innocuous, though more intimidating. The Woodlouse Hunter, Dysdera crocata, is a specialized predator of woodlice, known commonly as “sowbugs,” “roly-polies,” or “pillbugs.” This species has exceptionally long jaws and fangs it uses to turn over its armored prey. While it looks dangerous, it is not.

Dysdera crocata © Nick Richter 2010

Dysdera is not native to North America, having been introduced from Europe at some point in our history. It has also become accustomed to prowling around human dwellings, so may be found indoors. It does not climb as well as Trachelas, however, and in my experience the Woodlouse Hunter prefers to hug baseboards.

Dysdera crocata © Sarah Rose

I encourage my readers to appreciate all spiders, but also act responsibly. It is a good idea to try and exclude spiders from entering your home and workplace by repairing worn weatherstripping on doors, mending holes in window screens, and sealing any cracks and crevices that could offer passage for insects and arachnids. Be careful when bringing objects indoors from outside, too, like firewood, gardening implements, children’s toys, and shoes and clothing left outdoors overnight.

Sources: Cox, Shelly. 2011. “Ground Sac Spider,” MoBugs blog.
Eaton, Eric R. and Amanda Howe. 2012. “Trachelas tranquillus (Ground Sac Spider),” Spiders.us.
Jacobs, Steve. 2002. ”Broad-faced Sac Spider,” Penn State University fact sheet.