False Black Widow, Cobweb Spider, Cupboard Spider
Steatoda grossa
Theridiidae (Cobweb Spiders), Order Araneae. Key family characteristics include a rounded, bulbous abdomen, comb-like serrated bristles on the fourth tarsus used for pulling silk, and generally poor eyesight.
Physical Description
The spider features a glossy, bulbous, egg-shaped abdomen and a smaller, distinct cephalothorax. The body is relatively smooth with fine, sparse hairs. It exhibits sexual dimorphism, with females being significantly larger, rounder, and broader than the slimmer males.
Size Estimate
Body length ranges from 6 to 10.5 mm for females and 4 to 6 mm for males, with a leg span reaching up to 20 to 25 mm. The specimen appears to be an adult or sub-adult based on body proportions.
Coloration & Markings
Base coloration ranges from deep brownish-purple to dark charcoal black, sometimes appearing nearly black. It often features subtle pale yellowish or greyish markings, sometimes forming crescents or chevron patterns on the dorsal abdomen, though these can be faint.
Web Type
Builds an irregular, messy tangle web, commonly known as a cobweb, typically constructed in dark, sheltered crevices near ground level, corners, or in cluttered indoor spaces like garages.
Eye Arrangement
Consists of eight small eyes arranged in two horizontal rows of four, typical of the Theridiidae family. The eyes are roughly equal in size, dark, and set on a slightly raised ocular tubercle.
Leg Characteristics
Legs are relatively long, thin, brownish, and taper toward the ends. They lack prominent spines or heavy banding, though they possess specialized comb-like setae on the hind tarsi for manipulating sticky silk threads.
Venom & Safety Information
Venom Information
Contains neurotoxic components (latrotoxin-like compounds, though less potent than true widows). Envenomation is uncommon and generally mild, causing localized pain, redness, and swelling, occasionally accompanied by mild systemic symptoms.
Medical Significance
Minimally dangerous to humans. While a bite can be painful and cause localized discomfort, it is not medically significant and rarely leads to severe systemic illness (steatodism). Washing the area and applying a cold compress is usually sufficient.
Safety Recommendations
Avoid direct handling with bare hands. When cleaning dark corners or garage floors, wear gloves. If relocation is necessary, gently guide the spider into a container using a piece of cardstock and release it outdoors.
Habitat & Behavior
Habitat & Range
Cosmopolitan distribution, found worldwide in temperate and subtropical regions. Highly synanthropic, commonly inhabiting human-made structures such as garages, basements, sheds, under rocks, woodpiles, and dark crawlspaces.
Behavior & Temperament
Timid and non-aggressive. They spend most of their time hanging upside down in their webs. When disturbed, they typically retreat into a nearby crevice, drop to the ground, or curl into a defensive ball rather than bite.
Diet & Prey
Carnivorous, feeding primarily on crawling and flying insects such as flies, beetles, cockroaches, woodlice, and occasionally other spiders. Captured prey is wrapped tightly in silk and subdued with venom.
Seasonal Activity
Active year-round indoors and in sheltered structures, with peak visibility and mating activity occurring during late summer and autumn when males wander in search of females.
Description
Steatoda grossa, widely known as the False Black Widow, is a frequent dweller of human habitations, particularly damp and undisturbed locations like garages and basements. Often mistaken for the true black widow due to its dark, glossy appearance and bulbous abdomen, it lacks the iconic red hourglass marking. It serves as an effective, natural pest control agent by subduing bothersome insects in and around homes.
Key Features
Glossy, bulbous, dark brownish-purple abdomen; lack of a red hourglass marking; irregular cobweb construction; distinct teardrop-to-oval body shape typical of comb-footed spiders.
Similar Species
True Black Widow (Latrodectus species), which possesses a bright red hourglass marking on the ventral abdomen and a shinier jet-black finish. Steatoda borealis and Steatoda triangulosa, which have more distinct geometric or triangular patterns on their backs.
Life Cycle
Females produce papery, brownish egg sacs containing up to 200 eggs, hidden within the web structure. Spiderlings hatch after several weeks, disperse via ballooning or crawling, and undergo multiple molts over a 1 to 2 year lifespan.
Ecological Importance
Acts as an effective biological control agent by consuming large numbers of household pests, nuisance insects, and potential disease vectors, helping regulate invertebrate populations around buildings.
Conservation Status
Not evaluated by the IUCN, but populations are secure, widespread, and heavily established globally due to human-facilitated dispersal.
Interesting Facts
False black widows are known to occasionally prey on true black widows and other potentially dangerous spiders by invading their webs, throwing silk over them, and subduing them.
Photography Tips
Use macro photography with natural side lighting or a diffused flash to highlight the subtle abdominal patterns and glossy texture. Maintain a safe distance and photograph from a lateral angle to capture body proportions and leg structure clearly.
User Notes
Garage floor
Identified: 2026-09-30T14:31:03.049261