Yellow Sac Spider
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There are two species of sac spiders commonly found in California: Cheiracanthium inclusum and Cheiracanthium mildei. The first is a native species of spider commonly encountered in gardens, while the second is thought to have been introduced from Europe during the 1940s and is more likely to be found inside homes. Both species are widely distributed throughout the United States.
Sac spiders are small spiders, about ¼ - ½ inch long (5-10 mm). They are yellow, white or even greenish, with the legs and cephalothorax darker than the abdomen. There may be a slightly darker stripe on the upper middle of the abdomen. Males are similar in size to females, but are darker and more slender. These spiders have eight eyes, arranged in two horizontal rows.
Both species of yellow sac spiders are nocturnal. They wander about at night, often in areas where flies are common, hunting and feeding. Cheiracanthium mildei often builds a resting sac in the corners of walls and ceilings, or behind shelves and pictures. It may also been seen running on walls and ceilings at night, and may drop to the floor quickly on a silken thread if disturbed. Cheiracanthium inclusum is one of the most commonly encountered spiders in gardens, hiding under rocks and bark during the day, and wandering to the end of twigs and branches at night in search of roosting flies. In late summer and early fall, both species of sac spiders may migrate into buildings or automobiles, where they weave protective, silken, cocoon-like webs in which to overwinter. These are immature spiders that will molt once more before reaching adulthood in the spring.
Sac spiders are known occasionally to inflict bites on humans. Bites usually occur when the spider becomes trapped against a person's skin in clothing or bedding. Typical symptoms of a bite include an immediate stinging sensation followed by redness and mild swelling. Sometimes a blister may form at the site of the bite, often breaking, leaving a sore that heals over a period of several weeks. This spider's jaws cannot pierce the skin of everyone and may not inject venom on every bite, so there is a large margin of safety.
Sac spiders tend to be transported easily, particularly in agricultural products. Thus, they are widely distributed, and it is not uncommon for them to be found in produce originating from California.