False Black Widow, Cupboard Spider, Bumpy-backed Spider
Steatoda grossa
Family Theridiidae (Cobweb Spiders), Order Araneae
Physical Description
A medium-sized spider with a shiny, bulbous, egg-shaped abdomen and a smaller cephalothorax. The abdomen is typically wider than the head region, and the body lacks the dense hairy texture common in wolf spiders. Females have a significantly more robust abdomen than males.
Size Estimate
Body length 6-10mm (females), 4-6mm (males); total leg span roughly 15-25mm. This specimen appears to be an adult female based on the abdominal volume.
Coloration & Markings
Dark purple-brown to nearly black base. It features a characteristic pale, crescent-shaped band on the anterior portion of the abdomen (the side closest to the head). Some individuals show additional faint chevrons or spots along the dorsal midline. Unlike true widows, it never has a red hourglass.
Web Type
Messy, three-dimensional tangle or cobweb. These are typically built in dark corners, under furniture, or in cupboards. The silk is notably strong and sticky.
Eye Arrangement
Eight eyes in two horizontal rows. While difficult to see without macro photography, the arrangement is typical of Theridiids, with lateral eyes that almost touch.
Leg Characteristics
The legs are relatively long and slender, typically reddish-brown to dark brown without prominent banding. The first pair of legs is usually the longest, utilized for manipulating silk and prey.
Venom & Safety Information
Venom Information
Contains 'alpha-latrotoxins' similar to true widows but much less potent. The venom is neurotoxic but generally only causes localized symptoms in humans.
Medical Significance
Low to moderate danger. Bites are uncommon and comparable to a bee sting, though some individuals may experience 'steatodism,' characterized by localized pain, sweating, and mild nausea lasting a few hours. Not considered life-threatening.
Safety Recommendations
Use gloves when cleaning out dark cupboards or garages. If found in the home, they can be easily relocated using a cup and a piece of paper. Bites should be cleaned with soap and water; seek medical advice only if severe allergic reactions occur.
Habitat & Behavior
Habitat & Range
Cosmopolitan distribution; found globally. They are highly synanthropic, preferring dark, undisturbed indoor environments like basements, crawl spaces, garages, and cupboards.
Behavior & Temperament
Generally shy, sedentary, and non-aggressive. They spend most of their time hanging upside down in their webs. If disturbed, they usually retreat to a corner or drop to the floor rather than biting.
Diet & Prey
Generalist predators feeding on roaches, earwigs, crickets, and even other spiders. They use their sticky webs to ensnare prey much larger than themselves before wrapping them in silk.
Seasonal Activity
Active year-round in temperature-controlled indoor environments, though they may be more visible in late summer and autumn when males wander in search of mates.
Description
The False Black Widow is a master of the shadows, often mistaken for its more dangerous cousins. With a high-gloss finish and a crescent moon marking, it is an architectural specialist of the indoor world, spinning complex silk traps in the dark corners of human dwellings.
Key Features
Glossy bulbous abdomen, dark brown/black color, pale crescent-shaped band on the front of the abdomen, absence of red markings (no hourglass), and tangling cobweb habitat.
Similar Species
Latrodectus mactans (Black Widow) - distinguished by the red hourglass and deeper black color. Steatoda nobilis (Noble False Widow) - has more intricate 'skull' patterns on the abdomen.
Life Cycle
Females produce round, white to off-white papery egg sacs containing 40-100 eggs. They can live up to 6 years in stable indoor environments. Mating involves the male vibrating the female's web to signal he is a mate rather than prey.
Ecological Importance
Critical for indoor pest control. They effectively reduce populations of disease-carrying insects and other household pests without the need for chemical intervention.
Conservation Status
Not evaluated by IUCN, but populations are stable and widespread due to their success in human-modified environments.
Interesting Facts
Steatoda grossa is a known predator of true Black Widows. They are also being studied for their silk strength and have been used in research involving the effects of various chemicals on web-spinning behavior.
Photography Tips
Focus on the dorsal (top) side of the abdomen to capture the crescent marking. Use a flashlight or diffuse flash to bring out the color in the dark body, as they often appear as simple black blobs in low light.
Identified: 2026-05-25T16:22:13.347359