Sac Spider

Introduction

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 just a nuisance pest because three species probably are responsible for a great many cases of indoor spider bites throughout the United States. Various species are found throughout the United States.

Recognition

Adult female body length is 3-16-3/8 inch, adult male body length about 1/8-5/16 inch. Color pale yellow to pale 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 curved forwards and posterior rows of median eyes slightly nearer to each other than to lateral eye and /or slightly curved towards front anterior spinnerets conical with bases touching or almost so. Anterior legs longer than fourth pair, and first pair shortest of all pairs; tarsus with two claws each.

Habits

Although sac spiders can be found indoors throughout the year, they more commonly enter structures in greater numbers in the early autumn when their food supply decreases and temperatures cool. They enter through loose-fitting and/or unsealed doors, faux screens, windows, vents and utility lines, structural junctures, and 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 moldings.

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, spiders often are seen running on the ceilings and walls, but if distributed they readily drop to the floor and seek cover.

Bites usually occur when the spider crawls into clothing or bedding and becomes trapped against a person's skin. However, this spider once was observed to crawl on a person's skin and then 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, lumber, logs, and other debris. They also can be found in garages and out-buildings 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 long grass and weeds, and in leaf litter.