Yellow Sac Spiders
Contrary to popular opinion, it is nearly impossible to determine precisely what type of spider has bitten someone, when no spider has been found. Even though brown recluse bites are quite rare these shy spiders are often blamed for bites. Experts believe sac spiders cause more bites than any other type of spider and their bites are often misdiagnosed as brown recluse spider bites.
The venom of sac spiders contains a cytotoxin—which means it kills cells—like the venom of a brown recluse. Unlike brown recluse bites which can take 2-3 months to heal, most sac spider bites heal much more quickly, although the reaction will vary greatly from individual to individual.
Typical symptoms of a sac spider bite include a stinging sensation followed by redness and mild swelling. In a few cases, the bite may blister and break, leaving a sore that can take several weeks to heal. Sometimes the person will feel mildly ill.
Sac spiders belong to the genus Cheiracanthium and the family Clubionidae. They are quite small and easy to overlook—about 1/4 - 3/8 inch long, with no conspicuous markings. The front legs are longer than the other three pairs. Sac spiders are quite pale. A common house species, the yellow sac spider is pale greenish, tan or straw colored. Other sac spiders are light brown.
Normally, these are outdoor spiders, but sac spiders often invade structures. Their numbers increase significantly in the fall when the weather turns cool and their food supply disappears. If there are small insects available, sac spiders can become established indoors. At night, sac spiders actively hunt their prey—usually small insects. In search of prey, they run quickly waving their forelegs before them. Indoors, they can be observed on walls and ceilings, but drop to the floor to seek cover when disturbed.
Sac spiders construct a silken tube or sac in a protected area, such as within a leaf, under landscape timbers or logs, or at the junction of a wall and ceiling, and they use this sac as their daytime retreat. This is how the sac spider gets its name. These spiders do not build webs. After mating, females lay 30 to 48 eggs and cover them in a thin coat of loosely spun silk. The small, white, paper-like sacs are often found in easily overlooked locations, along ceilings and corners, or behind pictures and shelves. The female may guard these egg sacs and may produce several egg masses during her lifetime.
Inspect for sac spiders by looking for sacs in upper corners of rooms, ceilings, behind pictures, on window molding, blinds or curtains. During the day, sac spiders may be inside these sacs so vacuuming is an excellent method of control. Remove and discard vacuum cleaner bags to prevent re-infestation.