Black Widow vs False Widow
Latrodectus mactans vs Steatoda grossa
The hourglass settles it. A true black widow is glossy jet black with a red hourglass on the underside of the abdomen. A false widow is purplish-brown rather than black and has no hourglass at all. Only the first is medically significant.
The one thing that settles it
A red hourglass on the underside. You have to see the belly, which is awkward but is the whole answer. Both spiders hang upside-down in their webs, so the underside is often already facing you. No hourglass and a brownish cast means a false widow. On some true widows the mark is split, faint or orange rather than red — but it is there.
Side by side
| Character | Black Widow | False Widow |
|---|---|---|
| Danger | Dangerous | Take care |
| Colour | Glossy jet black, almost lacquered | Purplish-brown to near-black — glossy, but never truly black |
| Underside marking | Red hourglass. Sometimes split or orange, but present | No hourglass. Sometimes a pale crescent across the front of the abdomen |
| Danger | Medically significant — severe muscle pain and cramping, antivenom exists | Stings and aches for a few hours. No lasting harm |
| Size | 8–13 mm body for the female | 6–10 mm body |
| Web and posture | Tangled, strong, low to the ground. Hangs upside-down | Tangled, in a low dark corner. Also hangs upside-down |
Why they get confused
They are genuine relatives, which is where the name comes from. Both are round-bodied, glossy, dark, build the same kind of tangled web in the same kind of low sheltered gap, and hang upside-down in it. At a glance in a garage the silhouette is identical. The false widow also preys on other spiders, including true widows, so finding one does not mean the other is nearby.
If you still can’t tell
Do not rule out a widow just because you cannot see an hourglass — the mark is underneath, and a spider that will not turn around has not told you anything. A glossy black, round-abdomened spider in a tangled ground-level web is worth treating as a widow until you can see otherwise. That costs you nothing; the reverse error is the one that matters. Photograph it through the underside of a glass jar if you need to be sure.
Common questions
Is a false widow bite dangerous?
No. It stings sharply at the time and can ache for a few hours, but it is not medically significant and causes no lasting harm.
Why is it called a false widow if it is not a widow?
It is a close relative in the same family, and looks the part — round glossy abdomen, tangled web, hangs upside-down. What it lacks is the hourglass and the venom.
The spider is brown. Does that rule out a widow?
No, and this is the trap. The brown widow is a true widow and is not black. If it is tan or brown with a mottled, patterned abdomen and banded legs, check the underside for an orange hourglass.
Full records
- Black Widow Dangerous
- False Widow Take care
Where this was checked
- iNaturalist — Latrodectus mactans checked 2026-07-30
- iNaturalist — Steatoda grossa checked 2026-07-30
- University of California IPM — Spiders checked 2026-07-30
Last reviewed 2026-07-30. We identify, we don’t diagnose — what we can and can’t tell you.