The Sac Spider

Common Name:
Sac Spiders or Clubionids
Scientific Name:
Chiracanthium spp. & Trachelas spp.


The common name comes from the retreat they build which is a flattened silk tube or sac where they spend most daylight hours; they do not build webs. These spiders are more than nuisance pests because 3 species are probably responsible for a great many cases of indoor spider bites throughout the United States. Various species are found throughout the United States.


Adult female body length 3/16-3/8" (4.9-10 mm), adult male body length about 1/8-5/16" (4-8.5 mm). Color pate yellow to pate green, with chelicerae and palp tips brown, and legs and abdominal midline slightly darker, or with cephalothorax orange brown to reddish top and bottom, abdomen pale yellow to light gray with anterior median area slightly darker, and anterior legs darkest with other legs becoming increasingly paler towards rear pair. With 8 subequal eyes in 2 rows, front row slightly curved forwards and posterior row's median eyes slightly nearer to each other than to lateral eyes and/or slightly curved towards front. Anterior spinnerets conical with bases touching or almost so. Anterior legs (I st pair) longer than 4th pair, or 1 st pair shortest of all pairs; tarsi with 2 claws each.


Ground spiders (Gnaphosidas) with front row of 4 eyes slightly curved to rear and middle 2 eyes slightly larger and dark (vs. other 6 eyes pale) and anterior spinnerets cylindrical in shape and distinctly longer than others.


  1. Trachelas tranquillus (Hentz). Adult female body length 5/16-3/8" (7.5-10 mm), male about 3/16" (5.5 mm); color orangish to reddish and yellow, cephalothorax (carapace/top and sternum/bottom) orange brown to reddish, abdomen pale yellow to light gray with anterior median area slightly darker, and anterior legs darkest with other legs becoming increasingly paler towards rear; Ist pair of legs shortest of all pairs of legs; found in New England and adjacent Canada southwest to northern Alabama and Georgia, and west to Minnesota and Kansas.

  2. Yellow sac spider, Chiracanthium inclusum (Hentz). Adult female body length 3/16- 3/8" (4.9-9.7 mm), male 1/8 - 5/16" (4-7.7 mm); color light yellow with chelicerae and palp tips brown, and legs and abdominal midline slightly darker; lst pair of legs shorter than 4th pair; found throughout the United States except for most northern tier of states.

  3. Yellow sac spider, Chiracanthium mildei L. Koch. Adult female body length 1/4-3/8" (7-10 mm), male 3/16 - 5/16" (5.8-8.5 mm); color light green to yellow-white with chalicerae and palp tips brown, and legs and abdominal midline slightly darker; lst pair of legs shorter than 4th pair; introduced from Europe, found in New England, New York, New Jersey, Alabama, Missouri, and Utah.


In the New England states, the yellow sac spider females lay their eggs in June and July in a loose mass covered only with a thin white silk sac. The egg sac is kept in her retreat indoors but it is often located in a rolled leaf outside. The female usually stays nearby on guard. Outside, the spiderlings overwinter and become adults the following May and June. They live about one year.

In the New England states, T. tranquillus females lay their eggs in the autumn in a white lens-shaped egg sac, about 3/8" (IO mm) in diameter and 1/8" (2.4 mm) high. The egg sac contains 30-48 eggs. Since the egg sacs can be found until November (October in Kansas), they probably overwinter with spiderlings emerging the following spring. In the New England states, adult males are found from late July through August and aduli females may overwinter.

The 3 species of sac spiders detailed above are of medical concern because of the many spider bites attributed to them. It is estimated that the 2 yellow sac spiders (C. inclusum and C. mildei) may be responsible for many bites on people indoors. However, few bits cases are reported, apparently because the spider is rarely caught in the act, captured, and/or properly identified. Their venom is a cytotoxin and primarily affects the tissues at the bite site. The bite may not be felt by some, but most people report a sharp pain at the time of the bite. The bite seldom results in more than localized redness, a burning sensation, and slight swelling at the bite site which may last for 24-48 hours.

In more severe bites or reactions, the localized burning sensation lasts about 30-60 minutes and residual pain for up to 3 days. During the following 1-8 hours, the reddenec area may enlarge and fill with puss. Occasionally, a systemic reaction may occur which is characterized by fever, malaise, stomach cramps, and nausea. Ulceration may occu over a week and result in a well-defined eschar (hard crust or scab) or pit developing This ulceration normally heals itself in several weeks.

First aid consists of using hydrogen peroxide or alcohol to cleanse the bite area anc prevent secondary infection, applying an ice pac to reduce swelling, keeping calm, anc seeing a physician if systemic symptoms occur. Remember to attempt to collect the responsible spider and take it with you for identification purposes.


Although sac spiders can be found indoors throughout the year, they mor( commonly enter structures in greater numbers in the early autumn when their foo( supply decreases and temperatures cool. They enter through loose fitting and/ol unsealed doors, faulty screens, windows, vents and utility lines, structural junctures, an( plant materials brought inside. They may stay in crawl spaces and structural voids feeding on the resident arthropod population, or they may enter the living space through gaps around ducts, light and electrical outlets, plumbing, and molding.

Once indoors, they may build their silk retreats in the upper corners and the ceiling wall junctions of rooms and rest there during the day; in basements and crawl spaces retreats are found where joists and band boards meet the subflooring. At night, sa( spiders are often seen running on the ceilings and walls, but if disturbed they readily drof to the floor and seek cover.

Bites usually occur when the spider crawls into clothing or bedding and become,, trapped against a person's skin. However, C. mildei was once observed to crawl on E person's arm and then to bite repeatedly without provocation.

Outside, sac spiders build their silk retreats under items lying or piled on the ground such as stones, landscape timbers, firewood piles, lumber, logs, and other debris. They can also be found in garages and outbuildings such as sheds and dog houses. On the outside of buildings, they can be found at structural junctures, upper corners of windows, behind shutters, and under eaves and overhanging shingles. Other places include on long grass and weeds and in the leaf litter.