Yellow Sac Spider, Long-legged Sac Spider
Cheiracanthium mildei
Cheiracanthiidae (Sac Spiders), Order Araneae
Physical Description
A medium-sized spider with a slightly elongated cephalothorax and a bulbous, oval-shaped abdomen. They are known for their pale, translucent appearance and lack of conspicuous body hairs compared to wolf spiders.
Size Estimate
Body length 5-10mm (0.2-0.4 inches); leg span approx 2.5cm. This appears to be a mature adult based on leg-to-body proportions.
Coloration & Markings
Typically pale yellow, greenish-tan, or straw-colored. Often features a slightly darker vertical 'heart mark' stripe on the anterior of the abdomen; feet (tarsi) are often darker/black due to dense hairs.
Web Type
Does not build hunting webs; instead constructs small, white, silk-tube 'sacs' in corners, wall-ceiling junctions, or folded leaves for resting and egg-laying.
Eye Arrangement
Eight eyes arranged in two horizontal rows. The eyes are relatively small and equal in size, though not clearly visible in this specific photo.
Leg Characteristics
The first pair of legs is significantly longer than the others. They have dark claws at the tips (scopulae) which allow them to climb smooth surfaces like walls and ceilings with ease.
Venom & Safety Information
Venom Information
Contains a mild cytotoxic venom. While previously thought to cause necrotic lesions similar to brown recluses, modern research has largely debunked this, showing it causes localized pain and swelling.
Medical Significance
Low to moderate danger. Bites are painful (similar to a bee sting) and can cause redness, itching, and localized swelling. Most symptoms resolve within 72 hours; seek medical attention only if systemic symptoms occur.
Safety Recommendations
To avoid bites, shake out clothing or shoes left on the floor. For relocation, use the 'cup and paper' method to move the spider outdoors. They are generally harmless if not squeezed.
Habitat & Behavior
Habitat & Range
Found throughout North America and Europe. They are synanthropic, frequently inhabiting human dwellings, specifically in the upper corners of rooms, behind furniture, and in window sills.
Behavior & Temperament
Active nocturnal hunters that roam looking for prey rather than waiting in a web. They are typically shy but can be defensive if trapped against skin inside clothing or bedding.
Diet & Prey
Generalist predators feeding on various small insects, such as flies, gnats, and other spiders. They have been observed feeding on nectar as well.
Seasonal Activity
Active year-round indoors, but most likely to be noticed in autumn as temperatures drop and they seek warmth or during the summer mating season.
Description
The Yellow Sac Spider is a ubiquitous indoor resident known for its pale complexion and impressive agility on vertical surfaces. This spider is a silent pest controller that roams the walls of homes at night.
Key Features
Pale yellow/straw color, long front legs, black 'feet', and the tendency to build silk retreats in high corners of rooms.
Similar Species
Often confused with the Brown Recluse (Loxosceles reclusa), but lacks the 'violin' mark and has 8 eyes instead of 6 arranged in dyads. Also similar to Ghost Spiders (Anyphaenidae).
Life Cycle
Females lay 30-48 eggs in a silk sac and guard them. Spiderlings emerge in late summer. Most individuals live for roughly one year, with mating occurring in the summer months.
Ecological Importance
Highly effective at controlling indoor insect populations, including fruit flies and gnats, reducing the need for chemical pesticides in homes.
Conservation Status
Not evaluated by IUCN; common and widespread throughout its range with no known threats.
Interesting Facts
Yellow sac spiders are thought to be attracted to the smell of volatiles in gasoline and have been known to cause car recalls by building nests in fuel system vent lines.
Photography Tips
Focus on the front of the cephalothorax to capture the eye arrangement and the long front legs. Use a side profile to show the characteristic abdomen shape and translucent coloring.
Identified: 2026-06-15T04:57:26.645478