Joseph Berger, Bugwood.org
Cheiracanthium inclusum is indigenous to much of the United States (except the northernmost states), while Cheiracanthium mildei, an introduced species from Europe, was found throughout much of the Northeast as of 1978. It is likely that C. mildei has substantially increased its range since that time.
Both species are of similar size (females 5 to 10 millimeters; males 4 to 8 millimeters) and coloration. C. inclusum is a light yellow to cream color with dark brown jaws (chelicerae), tips of the tarsi, and palps. C. mildei has a slightly greenish tinge to its abdomen and a pale yellow cephalothorax. The chelicerae, tarsi, and palps are similar to those of C. inclusum. Both spiders have a slightly darker dorsal stripe running lengthwise down the abdomen.
Sac spider retreats may be found outdoors under objects or indoors in the corners of walls and ceilings. These retreats are silken tubes or sacs in which the spiders hide during the daytime. In homes with light, neutral-colored walls and ceilings, the retreats may go unnoticed, as they are small and blend in with the background coloration.
The agrarian sac spiders deposit their eggs in June or July. The eggs are loosely deposited within a silken retreat, and the female remains nearby to guard them. C. inclusum is more often encountered outside; the majority of these spiders deposit their eggs on the undersides of leaves or other foliage. C. mildei is more often encountered within human-made structures and oviposits almost exclusively indoors. The young spiderlings will often remain within the silken retreat for a short period and will eventually venture out at night in search of food. The young will frequently return at daybreak to hide within the protection of the retreat.
Prowling spiders are "active hunters," searching for prey rather than capturing it within a web. It is during these nighttime forays that the spiders encounter humans and bite when they become trapped between a person's skin and sheets, clothing, shoes, and so forth.
These two spiders probably account for a significant number of human bites. People usually incur C. inclusum bites outdoors while gardening in the summer. C. mildei will readily bite, despite their small size, and they have been observed crawling across the human skin surface and biting without provocation. Although most of these bites are painful at the outset, they normally do not result in any serious medical conditions. For C. inclusum victims and some individuals sensitive to C. mildei, the bites will exhibit the symptoms described below.
The bite is usually very painful and burning at the outset, with developing erythema, edema, and intense itching. The burning sensation associated with the bite will last for an hour or more, and a rash and blistering will occur during the next ten hours. Some patients may exhibit systemic reactions with fever, malaise, muscle cramps, and nausea. Although there is evidence that guinea pigs and rabbits develop necrotic lesions after bites from Cheiracanthium species, the evidence for a similar reaction in humans is unclear.
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Authored by: Steve Jacobs, Sr. Extension Associate
March 2002 Revised 2015