Yellow Sac Spider

Cheiracanthium inclusum — the yellow sac spider, also called the agrarian sac spider — is one of the most common and most-encountered indoor spiders in temperate North America and one of about 200 species in family Cheiracanthiidae. The species is widespread across the entire US, southern Canada, and parts of Latin America. Adults are 5-10 mm body length, pale yellow to greenish-tan with darker palps and chelicerae. The species does NOT build snare webs — instead, it constructs small silken tubular retreats ('sacs') in tight crevices, behind picture frames, in the corners of ceiling-and-wall junctions, in folded leaves, and on the underside of porch ceilings. The spider remains in the silken sac during the day and emerges at night to actively HUNT prey across nearby surfaces. The species is one of the most consequential spiders in modern medical entomology because YELLOW SAC SPIDER BITES ACCOUNT FOR THE MAJORITY OF UNIDENTIFIED 'SPIDER BITE' REPORTS in temperate North America. The bite causes a brief sharp pain at the moment of envenomation plus a small red welt that resolves in 1-3 days; severe necrotic skin lesions are rare and atypical (despite occasional misattribution). Multiple medical studies (Vetter et al., 2008 and others) have shown that the majority of suspected 'brown recluse bites' across the US are actually attributable to yellow sac spider bites or other causes (skin infections, allergic reactions, MRSA infections). The species' more unusual behavior is the documented attraction of yellow sac spiders to GASOLINE VOLATILES (specifically the hydrocarbons emitted from vented fuel systems). Yellow sac spiders crawl into vehicle fuel-system vent tubes and other plastic ductwork, build silken sacs inside, and CHEW through the plastic to expand their habitat. Mazda recalled approximately 65,000 Mazda6 sedans in 2014 due to documented yellow sac spider damage to fuel-system vent tubes (the spiders' silk + plastic damage caused engine performance issues and potential fuel-leak fire risks). Subsequent recall events have occurred for other vehicles with similar vent-tube architecture. The species is one of the most-encountered and most-misidentified indoor spiders in modern North American residential pest management.