Agrarian Sac Spider and Longlegged Sac Spider

Miturgidae—Prowling Spiders

Agrarian Sac Spider

(Cheiracanthium inclusum)

and

Longlegged Sac Spider

(Cheiracanthium mildei)

Sac spiders can be found walking about on foliage; under leaf litter, stones, and boards; on buildings under the windowsills and siding; and in the corners of walls and ceilings within homes. C. inclusum is indigenous to much of the United States (except the northernmost states), while C. mildei, an introduced species from Europe, is now found throughout the United States north of Virginia, Utah, and California. 

Description

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 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.

Life History

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, eventually venturing 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.

Medical Importance

These two spiders 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. 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 10 hours. Some patients may exhibit systemic reactions with fever, malaise, muscle cramps, and nausea. Although it was thought until recently that necrotic lesions were associated with bites from Cheiracanthium species, studies of a large number of verified human bites have failed to confirm these lesions happen.

Reference

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