Diagnostic characters
- Comb of six to ten curved serrated bristles on the tarsus of leg IV, visible under magnification
- Globose, high-arched abdomen that overhangs the cephalothorax
- Legs generally without strong spines, giving a smooth appearance
- Tangle web with vertical gumfooted lines under tension
- Female far larger than male in Latrodectus and Steatoda
Habitat and seasonality
Ubiquitous. Structural voids, wall and ceiling corners, under furniture, in woodpiles, under stones and debris, inside meter boxes and utility housings, and in low vegetation.
Adults of synanthropic species persist year-round indoors. Outdoor populations peak in late summer and autumn when females reach maturity and produce egg sacs.
Medical significance
The genus Latrodectus is the only medically significant group in the family. Latrodectus venom is a neurotoxin causing latrodectism. All other North American theridiids are harmless to humans.
Placement and scope
Theridiidae sits within the Araneoidea superfamily alongside the orb weavers, and molecular work supports the interpretation that the theridiid tangle web is a derived form of the ancestral orb rather than an independent origin. The family is one of the largest in the order Araneae and is present on every continent except Antarctica.
In North America the family is dominated numerically by small genera such as Theridion, Parasteatoda, Enoplognatha, and Steatoda. The five Latrodectus species present north of Mexico account for a small fraction of the family's diversity but for essentially all of its clinical relevance.
Web architecture
The characteristic theridiid web is a tangle of dry structural silk with a set of vertical lines running down to the substrate. Each of these gumfooted lines terminates in a droplet of viscid glue and is held under tension. When a walking insect contacts the glue, the line breaks free of the substrate and lifts the prey off the ground, where it remains suspended and unable to gain purchase.
This design allows a small sedentary spider to capture prey substantially larger and stronger than itself, which is the mechanism behind the widow spider's ability to subdue beetles and even small vertebrates.
Medical relevance
Latrodectus venom contains alpha-latrotoxin, a presynaptic neurotoxin that triggers massive exocytosis of neurotransmitter at nerve terminals. The resulting syndrome, latrodectism, presents with severe muscle cramping, diaphoresis, hypertension, and abdominal rigidity, typically without significant local tissue damage.
Steatoda species, sometimes called false widows, are regularly submitted for identification as widows. Bites are documented but produce a mild, self-limiting local reaction. Confusing the two genera is the most common consequential misidentification within the family.
Families most often confused with Theridiidae
| Family | How to separate it |
|---|---|
| Pholcidae | Cellar spiders have extremely long thin legs and a cylindrical rather than globose abdomen |
| Linyphiidae | Sheet weavers build a horizontal sheet and hang beneath it rather than in a tangle |
| Nesticidae | Cave cobweb weavers have a paracymbium on the male palp and are largely restricted to caves and deep crevices |
Profiled species in this family
| Species | Scientific name | Medical significance |
|---|---|---|
| Southern Black Widow | Latrodectus mactans | high |
| Western Black Widow | Latrodectus hesperus | high |
| Brown Widow | Latrodectus geometricus | moderate |
| Northern Black Widow | Latrodectus variolus | high |
| Red Widow | Latrodectus bishopi | high |
| Common House Spider | Parasteatoda tepidariorum | low |
References
- [1]World Spider Catalog, version 26 - Natural History Museum Bern. Accessed 2026-06-14.
- [2]Spiders of North America: An Identification Manual, 2nd ed. - American Arachnological Society. Accessed 2026-06-14.
- [3]Common Spiders of North America - University of California Press. Accessed 2026-06-14.