Yellowlegged Sac Spider

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Compiled by Whitney Cranshaw, Colorado State University:

Colorado Insects of Interest

Yellowlegged Sac Spider

Scientific Name: Cheiracanthium inclusum (Hentz), C. mildei C.L. Koch

Class: Arachnida
Order: Aranae (Spiders)
Family: Miturgidae

Identification and Descriptive Features: Yellow-legged sac spiders of the genus Cheiracanthium are generally yellowish but may be pale grayish-tan. There are no conspicuous markings and only fine hairs cover the body. The 8 eyes are arranged in two straight rows.

Legs of yellow-legged sac spiders are long and delicate, with the front pair somewhat longer than the others. Full grown the body is about 3/8 inch long and with legs extended are about 3/4-inch.

Figure 2. A male yellow-legged sac spider.
Distribution in Colorado: Cheiracanthium mildei, a native of the Mediterranean, is now widely distributed in North America and is a common both indoors and outdoors throughout Colorado. State records for C. inclusum, also an introduced species, are limited to Elbert and Alamosa counties, but it likely is more widespread.

Life History and Habits: Yellow-legged sac spiders can be commonly found among the dense vegetation of shrubs, trees and fields. They hunt at night and do not use webs for prey capture instead locating prey during wandering searches. A wide variety of insects (including eggs) and other spiders may be eaten.

Figure 3. Yellow-legged sac spider adjacent to silk tube retreat.
Silk is used to create a tube-like retreat within which they spend the day. Outdoors these are typically located under rocks, leaves or other sheltering debris. Eggs, primarily produced during early summer, are also laid within the retreat. The eggs are produced in a loose mass of about 40 eggs, covered with thin silk and covered. The female remains with the eggs until the spiderlings emerge and will actively defend the eggs.

Silk is used extensively for subsequent dispersal, both in ballooning and for bridging between plants. The spiders have a one year life cycle with winter is spent in the adult or subadult stage.

Cheiracanthium mildei is also well adapted to indoor life and is one of the most common spiders found breeding in buildings in Colorado. They do have long fangs and are capable of producing a painful bite; this species is sometimes suspected of being the most common cause of spider bites that occur indoors. If bitten the wound should be disinfected to prevent secondary infection bacteria, which sometimes develops after bites from these species and can produce a larger wound.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.