Broad-faced Sac Spider
Trachelas tranquillus
Trachelidae (Sac Spiders), Order Araneae
Physical Description
The body is oval-shaped with a distinctively reddish-brown to orange cephalothorax that appears very smooth and shiny, coupled with a paler yellowish-gray or tan abdomen. It has robust, powerful chelicerae (jaws) which are often a darker reddish-black.
Size Estimate
Body length typically ranges from 7 to 10 millimeters, with a leg span of roughly 2 to 3 centimeters. The specimen appears to be an adult.
Coloration & Markings
Characterized by a striking contrast: a dark reddish-brown to orange-brown cephalothorax, dark reddish chelicerae, and a pale beige, grayish, or yellowish abdomen that occasionally displays a faint chevron or longitudinal stripe down the center.
Web Type
Do not build traditional catching webs. Instead, they construct small, papery silk retreat sacs under bark, in leaf litter, or occasionally in folds of fabric, which they use primarily for resting, molting, and brooding.
Eye Arrangement
Eight small eyes arranged in two rows of four, typical of wandering spiders, set closely together across the front of the relatively broad cephalothorax.
Leg Characteristics
Legs are robust, moderately short, and uniform in color with the cephalothorax, lacking prominent spines or distinct banding, equipped with paired claws and claw tufts for gripping smooth surfaces.
Venom & Safety Information
Venom Information
Produces a mild cytolytic and neurotoxic venom used primarily to immobilize small insect prey. Its potency against humans is low.
Medical Significance
Generally considered of minor medical significance. A bite may cause localized pain, redness, and mild swelling akin to a bee sting, but systemic effects are extremely rare.
Safety Recommendations
Shake out clothing and shoes left on the floor before wearing them to avoid trapping wandering spiders. If found, relocate safely using a cup and card.
Habitat & Behavior
Habitat & Range
Found widely across eastern and central North America. They inhabit wooded areas under loose bark, beneath logs, and frequently wander into human dwellings, garages, and clothing left in dark places.
Behavior & Temperament
Active nocturnal hunters that roam freely at night in search of prey. They are generally non-aggressive, but may bite defensively if trapped against the skin, such as inside clothing.
Diet & Prey
Carnivorous generalist predator that actively stalks and subdues small insects, other spiders, and arthropods using its strong chelicerae.
Seasonal Activity
Most active during the warm spring and summer months, often seeking indoor shelter or protected outdoor crevices as temperatures drop in autumn.
Description
Trachelas tranquillus, commonly known as the broad-faced sac spider, is a frequent visitor to human habitations due to its wandering habits. Easily recognized by its shiny, reddish-brown head region and pale abdomen, it is a beneficial nocturnal hunter that plays a quiet role in keeping insect populations in check.
Key Features
Smooth, dark reddish-brown cephalothorax, contrasting pale yellowish-gray abdomen, and noticeably darker, robust reddish-black chelicerae.
Similar Species
Cheiracanthium inclusum (Yellow Sac Spider), which is generally paler yellow-green overall without the distinct dark reddish cephalothorax characteristic of Trachelas tranquillus.
Life Cycle
Females construct silk sac retreats where they deposit eggs. Spiderlings emerge in late spring or summer, undergoing several molts before reaching maturity, with a typical lifespan of about one year.
Ecological Importance
Acts as an effective natural predator of household and garden insect pests, helping to regulate arthropod populations.
Conservation Status
Not evaluated by the IUCN, but populations are secure, widespread, and common throughout their natural range.
Interesting Facts
Despite their wandering nature and tendency to hide in dark crevices like clothing or shoes, broad-faced sac spiders are entirely harmless to humans and are rarely encountered except when accidentally compressed.
Photography Tips
Use macro photography with natural side lighting to highlight the smooth texture and distinct color contrast between the dark cephalothorax and pale abdomen.
User Notes
Found in pants pocket
Identified: 2026-08-07T23:14:34.853690