The Black Widow refers to about 32 species in the genus Latrodectus, including the Southern, Northern, and Western black widows of North America. These spiders are known for their neurotoxic venom and the distinctive red hourglass marking on the female’s abdomen. Contrary to popular belief, the female black widow does not carry her young on her back. Her reproductive strategy relies instead on a stationary, protective casing for her offspring.
Reproduction: The Black Widow Egg Sac
The female black widow protects her eggs by creating a tough, spherical egg sac. This silken structure is typically white, tan, or gray with a papery texture, measuring approximately 10 to 15 millimeters in diameter. The female suspends this protective globe within her tangled, irregular web, often in a secluded, dark location such as a woodpile, under a stone, or in a hidden corner.
A single egg sac can contain a large number of eggs, with counts ranging from around 20 to over 900, though the average is often cited to be about 250 eggs. The mother remains highly protective, exhibiting increased defensive behavior when the sac is present. She guards the sac fiercely for the incubation period, which lasts two to four weeks, until the spiderlings hatch inside the enclosure.
The spiderlings undergo their first molt while still inside the sac, emerging as miniature versions of the adult, though initially pale or yellowish-white. The female may produce several of these sacs throughout the warmer summer months. This entire process of confining and protecting the young inside a stationary structure is the black widow’s form of parental care.
Spiderling Dispersal and Survival
Once the spiderlings emerge from the egg sac, they face a challenging environment. The numerous hatchlings remain near the web briefly, but a lack of food and space quickly triggers fratricide. The young spiders are cannibalistic, and stronger individuals often consume their weaker siblings for nourishment.
This intense competition means only a small fraction of the hundreds of hatchlings survive the initial days. The surviving spiderlings must then disperse quickly to find their own territory and avoid further cannibalism. The primary method they employ for this rapid relocation is a specialized form of aerial travel called ballooning.
During ballooning, the spiderling climbs to a high point, raises its abdomen, and releases a strand of silk into the air. The fine thread catches an air current or is lifted by an electrostatic field, acting like a parachute. This allows the young to travel distances ranging from a few feet to several miles, ensuring they establish new, independent webs far from their birth site.
Mistaken Identity: Spiders That Carry Their Offspring
The notion that black widows carry their young on their backs is a case of mistaken identity, likely confusing them with another common group of spiders. The species most famous for this behavior are Wolf Spiders (Lycosidae), which exhibit a distinct form of maternal care. A female wolf spider first attaches her large, globular egg sac to her spinnerets and drags it until the eggs hatch.
Once the spiderlings emerge from the sac, they instinctively climb onto their mother’s back. They remain in a dense cluster for several days to a week, clinging tightly to specialized hairs on her abdomen as she continues to hunt and move. This visible mass of young riding on the mother’s back is the behavior most often confused with the black widow.
Other arachnids, such as Huntsman spiders and scorpions, also display parental care, adding to the general confusion. However, the black widow’s strategy is fundamentally different. It relies on the immobility and strength of a silken cocoon to protect the eggs, followed by the aerial dispersal of the solitary young. The black widow’s red marking is on the underside of its abdomen, unlike the Redback Spider, a close relative whose prominent red stripe is on the dorsal side.