Summary
The western black widow occupies the arid western half of the continent and is the widow species responsible for the overwhelming majority of bites reported in California, Arizona, and the Mountain West.
Quick answers
- Is Western Black Widow dangerous to humans?
- Yes. Western Black Widow is one of the few species in North America whose bite is established as medically significant. Adult-female bites can warrant medical evaluation.
- How big is an adult western black widow?
- Adult female body length is 8–16 mm; males are 3–6 mm. Leg span reaches 25–40 mm.
- Where does Western Black Widow live?
- Rodent burrows, packrat middens, under rocks, in irrigation valve boxes, BBQ grills, and outdoor storage. Established U.S. populations occur in 17 states.
- What does Western Black Widow eat and when is it active?
- Insects, scorpions, and other arthropods entangled in the cobweb. Nocturnal; sedentary. Year-round in low desert.
Identification
- Adult female: glossy black with red hourglass (often complete) on underside
- Slightly larger on average than L. mactans (body to 16 mm)
- Spherical abdomen, long thin legs
- Males small (3–6 mm), tan with white or red markings
- Builds web close to the ground, often in protected outdoor cavities
Morphometrics
| Measurement | Female | Male |
|---|---|---|
| Body length | 8–16 mm | 3–6 mm |
| Leg span | 25–40 mm | - |
| Lifespan | 1–3 years | - |
| Trait | Value |
|---|---|
| Web type | Tangled, three-dimensional cobweb with gumfoot threads. |
| Eye arrangement | Eight eyes in two rows of four. |
| Eggs per sac | 100–500 |
| Egg sacs per year | 4–10 |
Geographic range
Native range: Western North America from southern British Columbia south to Mexico, east to the Great Plains.
U.S. states with established populations: Arizona, California, Colorado, Idaho, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, Wyoming.
State pages covering Western Black Widow
ArizonaCaliforniaColoradoIdahoKansasMontanaNebraskaNevadaNew MexicoNorth DakotaOklahomaOregonSouth DakotaTexasUtahWashingtonWyoming
Habitat & behavior
Habitat: Rodent burrows, packrat middens, under rocks, in irrigation valve boxes, BBQ grills, and outdoor storage.
Activity: Nocturnal; sedentary. Year-round in low desert.
Diet: Insects, scorpions, and other arthropods entangled in the cobweb.
Venom & medical significance
| Venom property | Value |
|---|---|
| Type | Neurotoxic (alpha-latrotoxin) |
| LD₅₀ | 0.9 mg/kg (mouse) |
| Note | Clinically indistinguishable from L. mactans envenomation. Same antivenom is effective. |
Bite symptoms & first aid
Symptoms
- Latrodectism - cramping pain spreading from the bite
- Rigid abdominal muscles, sweating, hypertension
- Symptoms peak within 12 hours, resolve in 1–3 days with care
Commonly confused with
- Southern black widow
- Brown widow
- False widow
Key facts
- Western black widows account for the majority of widow bites in the U.S. western states; California reports several hundred bites per year. [1]
Sources
- [1]Black Widow and Other Widow Spiders Pest Notes - UC Statewide IPM Program (Publication 7449). Accessed 2026-06-14.
- [2]Widow Spiders - Colorado State University Extension. Accessed 2026-06-14.