False black widow vs Western black widow
These two are easy to mix up. Here's how to tell a false black widow from a western black widow, and whether either one bites or stings.
False black widow
Steatoda grossa
Western black widow
Latrodectus hesperus
≠The key differences
True widow spiders (Latrodectus) have a more elongated abdomen and usually show a red or orange hourglass or dorsal markings; their bites are more likely to cause medically significant symptoms.
More uniformly brown or purplish, no bright red hourglass; body shape less glossy; bites are less medically significant.
⚖︎Side by side
| False black widow | Western black widow | |
|---|---|---|
| To humans | Venomous bite | Venomous bite |
| Size | Females 6-10 mm body length; males 4-8 mm | Females 7-13 mm body length; males 3-6 mm body length |
| Colours | Dark brown, Purplish brown, Pale cream (faint markings) | Black, Red (hourglass), Brown (male/patterns) |
| Family | Theridiidae | Theridiidae |
| Habitat | Human structures (houses, basements, garages), sheds, under eaves, rock crevices and debris piles | Dry, sheltered sites near the ground or in human structures: woodpiles, sheds, crawl spaces, garages, rock crevices, under ledges and in dense vegetation. |
| Range | Originally from the Mediterranean and Macaronesia region; now nearly cosmopolitan, commonly found across North America, Europe and many other parts of the world | Western North America: western United States, southwestern Canada and parts of northern Mexico |
| Active | Active year-round in heated buildings; outdoors most active spring through autumn and mainly nocturnal | Spring through fall; most active at night |
| Pest? | Yes | Yes |
🔍How to spot a false black widow
- Round, smooth, shiny abdomen; body length 4-10 mm depending on sex
- Uniform dark brown to purplish coloration; may show faint pale markings but lacks bright red hourglass
- Builds irregular, tangled cobwebs in sheltered corners, often near the ground or in undisturbed indoor areas
- Compare size and markings to distinguish from related species (see similar species)
🔍How to spot a western black widow
- Shiny black, globose abdomen on females with a red or red-orange hourglass marking on the underside
- Females larger (7-13 mm) and more robust than males (3-6 mm); males are often brownish with dorsal patterns
- Irregular, tangled cobwebs in sheltered, low locations (near the ground, under eaves, inside sheds)
- Smooth, glossy abdomen (not notably hairy) and relatively long, slender legs
?Frequently asked questions
What is the difference between a false black widow and a western black widow?
True widow spiders (Latrodectus) have a more elongated abdomen and usually show a red or orange hourglass or dorsal markings; their bites are more likely to cause medically significant symptoms. More uniformly brown or purplish, no bright red hourglass; body shape less glossy; bites are less medically significant.
Is a false black widow or a western black widow more dangerous?
False black widow: Venomous bite. Western black widow: Venomous bite. Bites are uncommon and usually occur when a spider is pressed against skin. Females are venomous; bites can cause latrodectism (local pain, muscle cramps, sweating, nausea, elevated blood pressure).
Identification from photos is never certain: don't handle insects you can't identify, and get medical advice for bites, stings or allergic reactions.