A sac spider is a small, pale spider in the genus Cheiracanthium that gets its name from the silk tube, or sac, it builds as a daytime retreat. Unlike web-building spiders, sac spiders don’t spin webs to catch prey. They hunt at night, roaming walls, ceilings, and vegetation to catch insects. Two species are common in the United States: Cheiracanthium inclusum, which is native to most of the country, and Cheiracanthium mildei, a European species now established across much of the northern U.S.
How to Identify a Sac Spider
Sac spiders are small. Females measure 5 to 10 mm in body length, roughly the size of a pencil eraser, while males run 4 to 8 mm. Their coloring is pale yellow to cream, sometimes with a faint greenish tinge on the abdomen (more common in C. mildei). Both species have dark brown jaws, foot tips, and pedipalps, which creates a noticeable contrast against the lighter body. A slightly darker stripe runs lengthwise down the top of the abdomen.
One distinctive feature is their front pair of legs, which are noticeably longer than the others and used to grab prey. They have eight small eyes arranged in two rows of four. Their body color can actually shift depending on what they eat: individuals that feed heavily on house flies tend to look more grayish, while those eating red-eyed fruit flies take on a reddish tint.
Where Sac Spiders Live
C. inclusum is found across most of the United States except the northernmost states. It’s common outdoors in gardens, shrubs, and leaf litter. C. mildei, the introduced European species, has spread throughout the U.S. north of a rough line from Virginia to Utah to California. It’s more commonly found indoors and is the sac spider most people encounter in their homes.
During the day, sac spiders retreat into a small silk tube they construct in a sheltered spot. Indoors, you’ll find these tubes tucked into the corners where walls meet ceilings, behind picture frames, inside folded curtains, or in storage boxes. Outdoors, they build them under bark, inside rolled leaves, or beneath stones. The sac is a flat, white or off-white silk pouch, usually less than an inch long.
Hunting and Behavior
Sac spiders are nocturnal hunters. Once night falls, they leave their silk retreats and actively roam in search of small insects and other arthropods. They don’t build webs to trap prey. Instead, they rely on their long front legs to seize insects on contact, making them active predators rather than sit-and-wait ambush hunters. This roaming behavior is why they’re one of the spiders people most commonly encounter indoors at night, sometimes walking across walls or ceilings.
Sac Spider Bites
Sac spiders are one of the more frequently blamed spiders when people wake up with an unexplained bite, partly because they’re active at night and commonly found in bedding or clothing left on the floor. A bite typically causes localized redness, mild swelling, and a stinging or burning pain that lasts roughly three days before fading.
For years, sac spiders were considered capable of causing necrotic skin lesions, similar to brown recluse bites. This belief is now seriously questioned. While Cheiracanthium venom does cause tissue death in guinea pigs, the evidence in humans is largely circumstantial. A prospective study of 20 confirmed Cheiracanthium bites found that none of the patients developed necrotic wounds. Researchers reviewing the medical literature identified only a single verified case where a confirmed sac spider bite produced a small necrotic lesion. The only spider venom conclusively shown to cause skin ulceration in humans is that of the brown recluse, which contains a specific enzyme (sphingomyelinase D) that sac spider venom lacks.
Most sac spider bites heal on their own within about a week. Basic first aid is straightforward: clean the area with mild soap and water, apply a cool compress for 15 minutes at a time to reduce swelling, and take an over-the-counter pain reliever if needed. If the bite itches, an antihistamine can help.
Sac Spiders vs. Brown Recluse
Because sac spiders are pale brown and roughly similar in size, they’re sometimes confused with brown recluse spiders. The differences are easy to spot once you know what to look for. Brown recluses have six eyes arranged in three pairs, while sac spiders have eight eyes in two rows. Brown recluses also carry a distinctive violin-shaped marking on their back, which sac spiders lack entirely. Sac spiders tend to be more uniformly pale with those characteristically dark-tipped legs and jaws, and their front legs are proportionally longer. If you’re in a region where brown recluses don’t naturally occur (most of the northern and western U.S.), a small pale spider on your ceiling is far more likely to be a sac spider.
Keeping Sac Spiders Out of Your Home
Because sac spiders follow their prey indoors, the best prevention strategy targets both the spiders and the insects they eat. Seal cracks in your foundation and gaps around windows and doors. Check that window and door screens fit tightly, since good screens keep out both spiders and the insects that attract them.
Inside, regular sweeping, mopping, and vacuuming removes both spiders and their silk retreats. Keep clutter off floors, especially in garages, basements, and storage areas. If you store items in boxes, place them off the floor and away from walls, and seal boxes with tape to prevent spiders from moving in. Outdoors, keep the area along your foundation clear of leaf litter, heavy vegetation, and debris. You can knock down visible silk sacs from exterior walls with a broom or garden hose. If you find a sac spider inside, vacuuming it up or catching it in a glass jar and releasing it outside are both effective and simple approaches.