Cheiracanthium mildei L. Koch, 1864

Yellow sac spider

The pale spider sleeping in a silk sleeping bag behind your picture frames, blamed for far more bites than it delivers.

Order: Araneae
Family: Cheiracanthiidae
Group: Spiders
Ontario status: Established, introduced; common indoor nocturnal hunter

Why it matters

It eats household insects and its bite, while possible, is a mild local irritation, not the flesh-destroying wound of legend. The vast majority of 'sac spider bites' diagnosed in clinics turn out to be skin infections or other conditions.

Origin

Europe and the Mediterranean; arrived in North America with human trade and now lives in our houses.

Behavior

Nocturnal wandering hunter; does not use a web to catch prey. Fast and defensive when cornered. Rests in its silk sac by day.

Living conditions

Indoors in upper wall corners, behind pictures and curtains, in window frames. Spins a small silk tube, a daytime sleeping bag, and comes out to hunt at night.

Signs of presence

Small white silk tubes (about the size of a thumbnail) in upper corners and behind wall hangings
Pale yellow spiders with noticeably dark jaws seen at night
Egg sacs guarded inside the silk retreat in summer

Weaknesses

Depends on its daytime retreat, removing clutter and sealing behind-wall voids removes its shelter
Desiccates in dry air; does poorly in well-ventilated dry rooms
Eggs are laid inside the silk sac, so vacuuming the sac removes the whole next generation
Active at night, so daytime inspection finds them resting and easy to capture

Myths and facts

Myth Yellow sac spiders cause necrotic, rotting bite wounds.
Reality Verified bites produce mild redness at most. Medicine has never documented necrosis from this species; the reputation comes from misdiagnosed infections, including MRSA.

Research notes

Morphology

Body length 7-10 mm; pale yellow to beige carapace and legs; dark brown to black chelicerae (a useful field mark); abdomen pale with a faint dorsal stripe. Legs long, first pair directed forward.

Diagnostics

Pale spider + dark jaws + silk tube retreat in an upper corner. Confirm against Clubiona (native sac spiders) by genitalia; habitus is usually sufficient for genus-level field ID.

Easily confused with

Native sac spiders (Clubiona spp.) Nearly identical in the field; reliable separation needs microscopic examination, for management purposes the distinction rarely matters.
Brown recluse (Loxosceles reclusa) Recluses have six eyes in three pairs and no dark jaws; sac spiders have eight eyes and prominent dark chelicerae. Different families entirely.

Life cycle

Adult: Adults live about 1 year; overwinter indoors as adults or subadults
Egg: 30-50 eggs per sac, guarded by the female inside the silk tube
Egg to adult: Several months at room temperature
Generations per year: 1-2 indoors
Nymph or larva: Spiderlings disperse after first molt; several molts to maturity

Development

Egg to spiderling to adult through successive molts. Female guards the egg sac inside her silk retreat.

Reproduction

Males wander at night seeking female retreats. Female produces 1-2 egg sacs per season and guards them aggressively, most defensive encounters happen at the retreat.

Distribution

Global: Introduced and established across North America; holarctic
Native range: Europe, Mediterranean basin
Ontario: Established indoors province-wide; common in homes

Ecology

Nocturnal cursorial predator of small insects and other spiders. Synanthropic in North America; outdoor populations on vegetation, indoor populations in wall voids and ceiling-wall junctions.

Resistance

Unknown, not a pesticide target; no documented resistance.

IPM implications

Monitoring: count silk retreats in upper corners during daytime inspection. Management is physical, vacuum retreats and sacs, declutter wall hangings, seal cracks at the ceiling-wall junction. The medical-myth dimension matters: part of the job is reassuring clients with evidence, because fear of this spider drives unnecessary chemical use.

Chemical ecology

Venom contains insecticidal peptides; mammal effects are mild local cytotoxicity at most. The necrosis myth has been tested directly: venom assays and verified-bite case series show no necrotic activity. Silk used for retreats and egg sacs, not prey capture.

Genetics and DNA

No reference genome published as of 2026.

Taxonomy

Order Araneae, family Cheiracanthiidae (formerly placed in Clubionidae/Miturgidae). Authority L. Koch, 1864. Introduced Palearctic species, now holarctic in distribution.

Open questions

Venom proteome incompletely described; true bite incidence in Ontario unmeasured.

References

Foelix, R.F. (2011). Biology of Spiders, 3rd ed., Oxford University Press.
Vetter, R.S., UC Riverside spider-bite myth research (various publications).