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How to identify it
- Light yellow to cream, with a slightly darker stripe running lengthwise down the top of the abdomen. Penn State says the longlegged sac spider, C. mildei, has a faint greenish tinge to the abdomen, and Kentucky describes the group as pale yellow to pale green.[1][5][2]
- The jaws (chelicerae), the palps and the tips of the legs are dark brown, which stands out against the pale body.[5]
- Small: Penn State gives 5 to 10 millimeters for females and 4 to 8 millimeters for males, and Rutgers says they are typically less than 3/8 inch (10 mm) long.[5][6]
- Eight eyes of about equal size, arranged in two rows of four.[1]
- The retreat is a small silk tube or sac in ceiling corners, behind shelves and pictures, or among leaves outdoors. On light, neutral-colored walls the sacs blend in and are easy to miss.[1][5][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Brown recluse | Recluses have six eyes in three pairs, not eight in two rows, and CDC's NIOSH adds the dark violin-shaped mark on the brown recluse's head.[1][7] |
Where it lives
Lives indoors in house corners and outdoors in shrubbery and foliage.[1][2]
Biology and life cycle
Two species live in North America. Penn State says the agrarian sac spider, C. inclusum, is native to much of the United States except the northernmost states, while the longlegged sac spider, C. mildei, came from Europe and is now found throughout the United States north of Virginia, Utah and California.[6][5]
The agrarian sac spider is met more often outdoors and usually lays its eggs on the undersides of leaves; the longlegged sac spider lives mostly in buildings and lays its eggs almost entirely indoors.[5]
Females lay eggs in June or July in a loose mass inside the silk retreat and stay nearby to guard them. Rutgers says the spiderlings overwinter and become adults the next May to June.[5][6]
Adults run on walls and ceilings at night and quickly drop to the floor if disturbed. Spiderlings also hunt at night and often return to the retreat at daybreak.[1][5]
Rutgers says they are more common outdoors than indoors, where they hide under loose bark, in leaf litter, under rocks, behind shutters and under eaves. They get inside through torn screens, loose or unsealed doors, vents and utility lines, or on plants brought in, and then build retreats where walls meet the ceiling and in basements and crawl spaces.[6]
Damage and signs
- They don't damage the house. The concern is bites, which usually happen when a wandering spider is trapped against skin in clothing, bedding or shoes.[1][5]
- Sources differ on how readily they bite. UC IPM ties bites to spiders trapped against skin, while Penn State says the longlegged sac spider bites readily and has been seen crawling across skin and biting without provocation. Penn State adds that agrarian sac spider bites usually happen outdoors while gardening in summer.[1][5]
Health risk
Verified bites are mild. A 2006 study of 20 verified yellow sac spider bites in the United States and Australia found none that caused skin necrosis, and in an Oregon study of 33 verified spider bites, where the yellow sac spider was the most common biter (10 bites), none of the bites caused significant medical symptoms.[10][11]
Control
Prevention
Shake out or check clothing, towels and shoes before putting them on, and wear gloves and long sleeves when moving stored boxes or other undisturbed materials.[6][7]
Keep them out
Seal cracks in the foundation and gaps around windows and doors, keep screens in good repair, and point outdoor lights away from doors and windows so they attract fewer insects.[1][3][6]
Sanitation
Vacuum sac retreats from wall and ceiling corners and from behind shelves and pictures. Spiders that go through the vacuum hose usually don't survive.[1][9]
Remove clutter, woodpiles, leaf litter and heavy vegetation near the foundation, and vacuum or sweep webs and spiders regularly; vacuumed spiders usually don't survive.[1][3]
Pesticides
Insecticides don't give long-term spider control and generally shouldn't be used outdoors; spiders keep re-infesting treated areas.[1][3]
Common questions
- Are yellow sac spiders dangerous?
- Not usually. In verified bites the symptoms have been minor, and none of the 20 U.S. and Australian cases in a 2006 study caused skin necrosis. MedlinePlus notes that most spider bites are harmless.[10][11][8]
- Do yellow sac spiders cause necrotic wounds?
- The evidence says no. The 2006 study found that the claim rests mostly on venom tests in rabbits and guinea pigs and on repeated citing of unconfirmed reports; of 39 verified bites in the world literature, only one caused mild necrosis, from a European species. A 2011 review in The Lancet adds that most suspected necrotic spider bites are caused by something other than a spider.[10][12]
- What should I do if a spider bites me?
- Wash the bite with soap and water and put a cold, damp cloth or wrapped ice on it to ease swelling. CDC's NIOSH also advises raising the bitten area if possible and not trying to remove venom. If you can do so safely, keep the spider so it can be identified. Call 911 for signs of a severe allergic reaction, such as trouble breathing or swelling of the face or mouth.[7][8]
Sources
- University of California Statewide IPM Program, “Pest Notes: Spiders, Publication 7442.” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnspiders.pdf (accessed 2026-11-11)
- University of Kentucky Entomology, “ENTFACT-622: Common Spiders Found Around Homes and Buildings.” entomology.mgcafe.uky.edu/ef622 (accessed 2026-11-11)
- University of California Statewide IPM Program, “Pest Notes: Widow Spiders and Their Relatives, Publication 74149 (revised March 2017).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnblackwidow.pdf (accessed 2026-11-11)
- iNaturalist, “Cheiracanthium inclusum taxon page (order and family).” www.inaturalist.org/taxa/127401 (accessed 2026-11-11)
- Penn State Extension, “Agrarian Sac Spider and Longlegged Sac Spider (S. Jacobs, updated January 19, 2026).” extension.psu.edu/agrarian-sac-spider-and-longlegged-sac-spider (accessed 2026-11-11)
- Rutgers Cooperative Extension, “Spiders of Medical Importance, FS1121.” extension.rutgers.edu/fs1121 (accessed 2026-11-11)
- CDC / NIOSH, “Venomous Spiders at Work.” www.cdc.gov/niosh/outdoor-workers/about/venomous-spiders.html (accessed 2026-11-11)
- MedlinePlus, U.S. National Library of Medicine, “Insect bites and stings (Medical Encyclopedia).” medlineplus.gov/ency/article/000033.htm (accessed 2026-11-11)
- University of California Statewide IPM Program, “Spiders (Pest Notes, Home and Landscape).” ipm.ucanr.edu/home-and-landscape/spiders/ (accessed 2026-11-11)
- American Journal of Tropical Medicine and Hygiene, via PubMed, “Vetter RS, Isbister GK, Bush SP, Boutin LJ (2006), Verified bites by yellow sac spiders (genus Cheiracanthium) in the United States and Australia: where is the necrosis? American Journal of Tropical Medicine and Hygiene 74(6):1043-1048.” pubmed.ncbi.nlm.nih.gov/16760517/ (accessed 2026-11-11). Cited from the PubMed abstract.
- Toxicon, via PubMed, “McKeown N, Vetter RS, Hendrickson RG (2014), Verified spider bites in Oregon (USA) with the intent to assess hobo spider venom toxicity, Toxicon 84:51-55.” pubmed.ncbi.nlm.nih.gov/24726469/ (accessed 2026-11-11). Cited from the PubMed abstract.
- The Lancet, via PubMed, “Isbister and Fan (2011), Spider bite.” pubmed.ncbi.nlm.nih.gov/21762981/ (accessed 2026-11-11)