False Black Widow, Cobweb Spider, Cupboard Spider
Steatoda grossa
Family Theridiidae (Cobweb Spiders), Order Araneae. Key family characteristics include a rounded, bulbous abdomen, comb-like serrated setae on the hind tarsi used to pull silk over prey, and typically building irregular, messy tangle webs.
Physical Description
Steatoda grossa features a robust, glossy body with a high, rounded, bulbous abdomen that overhangs the cephalothorax. The exoskeleton has a leather-like texture that can appear satin or shiny. Females are notably larger and more robust than the males, which have narrower abdomens and longer legs.
Size Estimate
Body length ranges from 6 to 10.5 millimeters for females and 4 to 6 millimeters for males. The total leg span can reach up to 20 to 30 millimeters, making this a medium-sized cobweb spider.
Coloration & Markings
Base coloration ranges from deep purplish-brown to dark charcoal black, sometimes appearing nearly black. The upper surface of the abdomen features pale, yellowish-white or pinkish geometric markings, often in the form of crescent shapes or triangular chevrons, which fade or become less distinct in older adults.
Web Type
Builds an irregular, messy tangle web, commonly known as a cobweb. These webs are constructed in dark, protected corners near ground level or in structural crevices, consisting of a sticky, three-dimensional scaffold of silk threads designed to intercept crawling and flying insects.
Eye Arrangement
Consists of eight small eyes arranged in two parallel, slightly curved rows of four. The lateral eyes are typically contiguous, while the median eyes are separated, a classic arrangement for the Theridiidae family.
Leg Characteristics
Legs are moderately long, slender, and dark brown to blackish, generally matching the body color without prominent banding. They lack heavy spination but possess specialized comb-like structures on the hind tarsi for manipulating silk.
Venom & Safety Information
Venom Information
Possesses neurotoxic venom designed for subduing invertebrate prey. While it is medically relevant due to its distant relation to the true black widow, its venom is significantly less potent and rarely causes severe systemic effects in humans.
Medical Significance
Minimally dangerous to humans. A bite can cause localized pain, redness, and swelling, occasionally accompanied by mild systemic symptoms like fever or malaise (known as steatodism). Bites are rare and typically occur only when the spider is pressed directly against the skin.
Safety Recommendations
Avoid handling with bare hands in dark areas like woodpiles or crawlspaces. If found inside, it can be safely relocated outdoors using a container and a piece of card. Bites can be treated with cold compresses and antiseptic.
Habitat & Behavior
Habitat & Range
Cosmopolitan distribution across temperate and subtropical regions worldwide, including Europe, North America, Australia, and New Zealand. Commonly found in human-made habitats such as basements, garages, outbuildings, woodpiles, and dark structural voids.
Behavior & Temperament
Docile and non-aggressive, relying on retreat into its web or curling up into a defensive ball when disturbed. It is nocturnal, spending daylight hours hidden in a retreat adjacent to its web.
Diet & Prey
Feeds primarily on crawling and flying insects, including woodlice, flies, mosquitoes, beetles, and other spiders. It occasionally captures larger prey like roaches and wasps by subduing them with sticky silk and potent venom.
Seasonal Activity
Active year-round indoors, but most visible during late summer and autumn when mature males actively wander in search of females during the peak mating season.
Description
The False Black Widow is frequently mistaken for its dangerous relative, the true black widow, due to its dark, glossy appearance and bulbous abdomen. However, it lacks the iconic red hourglass marking, showcasing subtle pale geometric patterns instead. As a common household tenant, it quietly patrols dark corners and crevices, keeping pest insect populations in check without seeking confrontation with humans.
Key Features
Glossy, dark purple-brown to black coloration, bulbous rounded abdomen, absence of a red hourglass marking, and pale yellowish-white geometric or crescent-shaped markings on the dorsal abdomen.
Similar Species
True Black Widow (Latrodectus species): Distinguished by the presence of a distinct red or orange hourglass marking on the underside of the abdomen and a jet-black matte finish. Steatoda triangulosa (Triangle Weaver): Has a distinct triangular pattern on a lighter brownish-yellow abdomen.
Life Cycle
Females produce several silken egg sacs containing up to 200 eggs each over their lifespan. Spiderlings emerge after a few weeks, disperse via ballooning or crawling, and undergo multiple molts before reaching maturity. Lifespan can range from several months up to six years for females.
Ecological Importance
Serves as an effective natural pest controller in domestic and semi-natural environments by regulating populations of household insect pests and invasive arthropods.
Conservation Status
Not evaluated by the IUCN, but populations are widespread, stable, and frequently expanding alongside human settlement and urbanization.
Interesting Facts
Despite being called the False Black Widow, Steatoda grossa can occasionally prey on true black widows if they wander into its web. They are exceptionally long-lived for spiders, with females surviving for several years under favorable conditions.
Photography Tips
Use macro lenses with a ring flash or diffused external lighting to capture the fine details of the pale abdominal markings against the dark, glossy cuticle. Photograph from a slight side angle to highlight the rounded, three-dimensional shape of the abdomen.
Identified: 2026-09-14T18:10:35.210060