One of the few American spiders that does bite people indoors with any regularity. It stings sharply and can leave a red mark for a few days. It was long blamed for necrotic wounds; that link has not held up.
We identify, we don't diagnose. If you've been bitten or stung and symptoms develop, or you're worried, seek medical attention.
Size5–10 mm body
MarkingsPale cream, yellow or light green, almost translucent · Dark mouthparts that stand out against the pale body
WhereWall and ceiling corners, curtain folds, and in leaf litter outdoors.
What it is not
A yellow sac spider is most often confused with a species that is medically significant. If you are trying to rule those out, check these:
Pale cream, yellow or light green, almost translucent
Dark mouthparts that stand out against the pale body
No web — makes a small silk sac in a wall or ceiling corner
Walks with the front legs held forward
Where you’ll find it
Across most of the United States, and one of the few spiders genuinely common inside occupied homes rather than in basements and garages.
Wall and ceiling corners, curtain folds, and in leaf litter outdoors.
How it behaves
It builds no capture web. At night it hunts across walls and ceilings, and by day it rests in a small silk tube tucked into a corner, a curtain fold, or where the wall meets the ceiling. Those little white silk sacs are the usual first sign that you have them. It is one of the very few American spiders that bites people with any regularity indoors, almost always because it has ended up inside clothing or bedding.
What a bite looks like
A red mark with mild local swelling, sometimes with two faint puncture points. It can stay sore and slightly raised for a few days. It was long blamed for necrotic wounds; that link has not held up under investigation.
Sharper than most spider bites, often described as a sting that wakes people up, followed by an itchy ache.
We identify, we don't diagnose. See a doctor if the site spreads, becomes increasingly painful, or shows signs of infection.
A skin mark cannot confirm which animal caused it. Read the bug bite and sting guide for the limits and the symptoms that should end an online search.
You’ve found one indoors
Check bedding and clothing before use if you are finding them regularly, and clear the silk sacs from wall and ceiling corners. Because they hunt rather than sit in a web, sealing gaps and reducing insect prey does more than anything aimed at the spider itself.
Common questions
Do yellow sac spiders cause necrotic wounds?
The evidence does not support it. They were blamed for years, largely because they are common indoors and recluses were being over-diagnosed, but investigation has not borne the link out.
Why do I keep finding small white silk sacs on my ceiling?
Those are its daytime retreats. Each one is a spider resting up until dark. Clearing them is harmless and is the simplest way to reduce numbers.
Are yellow sac spider bites common?
More common than most, because the spider hunts across the surfaces people sleep and sit on, and it bites defensively when trapped against skin. The bite is painful but not medically significant.
Jean Martins · CC-BY