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 for flinging silk over prey, and typically building irregular, tangled webs.
Physical Description
The spider has a glossy, bulbous, globular abdomen and a compact, rounded cephalothorax. The body appears smooth and satin-like rather than heavily hairy. Females are significantly larger and more robustly built with a rounded abdomen, while males are smaller, narrower, and have longer legs relative to their body size.
Size Estimate
Body length ranges from 6 to 10.5 mm for females and 4 to 6 mm for males. Total leg span can reach up to 20 to 30 mm. This specimen appears to be an adult female based on the large, rounded, glossy abdomen and proportions.
Coloration & Markings
Deep dark brown to glossy black coloration, sometimes displaying purplish or reddish undertones. Unlike true widows, it lacks bright red hourglass markings, though it may occasionally feature faint, pale yellowish or grayish chevron/crecent markings on the dorsal abdomen, especially in younger individuals or males.
Web Type
Builds an irregular, sticky tangle web often referred to as a cobweb. These webs are constructed in dark, sheltered, undisturbed microhabitats near ground level, such as under logs, rocks, or within crevices.
Eye Arrangement
Features eight small eyes arranged in two horizontal rows of four each, typical of the Theridiidae family. The eyes are relatively uniform in size, set on slightly raised tubercles.
Leg Characteristics
Legs are moderately long, slender to moderately stout, dark brown to black, matching the body color. They generally lack prominent spines or heavy banding, though they feature specialized comb-like claws on the fourth pair of legs used for manipulating silk.
Venom & Safety Information
Venom Information
Neurotoxic venom, but significantly milder than that of true widow spiders (Latrodectus species). While not medically dangerous to healthy humans, a bite can cause localized pain, swelling, redness, and occasionally mild systemic symptoms such as malaise or fever.
Medical Significance
Low medical significance. Bites are uncommon due to their timid nature. Symptoms are usually localized and resolve within a few days without professional medical intervention, though secondary bacterial infection is always a minor risk with any puncture wound.
Safety Recommendations
Avoid direct bare-handed contact when moving logs, rocks, or cleaning dark, undisturbed storage areas. Wear gloves in woodpiles and basements. If found inside, it can be easily relocated outdoors using a container and a piece of card.
Habitat & Behavior
Habitat & Range
Cosmopolitan distribution across temperate and subtropical regions worldwide, commonly found in North America, Europe, and parts of Australia and New Zealand. Prefers dark, humid environments such as under logs, bark, stones, hollow trees, and frequently around human structures like basements, sheds, and crawlspaces.
Behavior & Temperament
Docile and non-aggressive. It relies on its web for catching prey and will typically retreat, curl up into a defensive ball, or drop from its web when disturbed rather than bite. Bites usually only occur if the spider is directly trapped, squeezed, or pressed against human skin.
Diet & Prey
Feeds primarily on crawling insects, woodlice, and other spiders, including potentially medically significant species like true widows. It uses its sticky cobweb and strong silk to subdue and hoist heavier prey items.
Seasonal Activity
Active year-round in protected microhabitats or indoors, with peak visibility and mating activity occurring during the warmer spring and summer months. Overwinters as adults or older juveniles in sheltered locations.
Description
The False Black Widow (Steatoda grossa) is frequently mistaken for its notorious cousin, the Black Widow, due to its dark, glossy, bulbous appearance. However, it is a harmless and beneficial neighbor that quietly patrols dark corners and under logs, keeping pest insect populations in check. With its smooth, satin-dark body and preference for secluded retreats, it represents a fascinating member of the cobweb spider family.
Key Features
Glossy, bulbous, globular dark brown to black abdomen; compact cephalothorax; lack of any bright red hourglass markings; presence of faint pale markings on the upper abdomen in some specimens; found in irregular tangle webs close to the ground.
Similar Species
True Black Widow (Latrodectus spp.), which possesses bright red or orange markings (such as an hourglass) on the underside of the abdomen. Other Steatoda species (like Steatoda borealis or Steatoda triangulosa) which feature distinct geometric or triangular patterns rather than solid dark coloring.
Life Cycle
Females produce papery, brownish egg sacs containing dozens to hundreds of eggs, which are hidden within the web or retreat. Spiderlings hatch and disperse via ballooning or crawling. Molting occurs multiple times throughout growth. Lifespan for females can reach several years.
Ecological Importance
Acts as an effective biological control agent by consuming significant quantities of crawling pests, household insects, and potentially invasive arthropods, contributing to the balance of local micro-ecosystems.
Conservation Status
Not evaluated by the IUCN, but populations are secure, abundant, and widely distributed globally, often benefiting from human structural environments.
Interesting Facts
Despite their ominous common name, False Black Widows are known to occasionally prey upon true black widow spiders that wander into their webs. Their glossy, rounded appearance has evolved to mimic dangerous spiders, serving as a form of Batesian mimicry to deter potential vertebrate predators.
Photography Tips
Photograph from a slightly elevated side angle to capture the glossy texture and spherical shape of the abdomen. Use diffuse natural lighting or a ring flash to highlight dark coloration without creating harsh reflections. Maintain a steady macro focus on the dorsal and lateral patterns.
User Notes
Found under a log
Identified: 2026-08-11T15:45:33.962545