Black widow spiders are arachnids known for their distinct appearance and potent venom. Females are typically shiny black with a red hourglass-shaped marking on the underside of their abdomen. While their venom is potent, bites to humans are rarely fatal. These spiders are shy and non-aggressive, biting only when threatened or their web is disturbed.
Understanding Black Widow Egg Sacs
Female black widow spiders construct egg sacs to protect their developing offspring. These sacs are typically white, tan, gray, or yellowish, and can be pear-shaped, globular, or spherical. Their texture is paper-like, appearing as tightly woven silken cups that are durable and firm to the touch.
Egg sacs usually range from 9.5 mm to 15 mm in diameter, or about 1 centimeter. Each sac can contain 100 to 900 eggs, commonly between 200 and 400. Females fiercely guard these sacs.
Egg sacs are often found in secluded, dark, and undisturbed locations. Common sites include woodpiles, beneath stones, natural debris, and sheltered areas of human structures like barns, garages, basements, and crawlspaces. They are built close to the ground for concealment.
Hatching Timeline and Influencing Factors
The incubation period for black widow eggs generally ranges from 14 to 30 days. This duration can vary depending on environmental factors. Eggs develop into spiderlings within the egg sac during this period.
Temperature and humidity are primary environmental conditions that influence the hatching timeline. Warmer temperatures accelerate development, while cooler temperatures prolong the incubation period.
This duration represents an average, and specific conditions in a given environment can lead to slight variations. The female spider’s ability to provision her eggs equally also plays a role in the synchronized development of the brood, which can impact hatching consistency.
Life After Hatching: Black Widow Spiderlings
After the incubation period, tiny spiderlings emerge from the egg sac. These newly hatched spiderlings are small, often about 1/8 inch, and are typically pale or yellowish-white. They gradually acquire black and red coloration with each molt.
Upon hatching, the spiderlings remain inside the egg sac for a period, often until after their first molt. They must then tear through the silken walls of the sac to escape. A significant aspect of their early life is sibling cannibalism, where spiderlings may feed on their brothers and sisters, a behavior influenced by factors like family genetics and initial size differences among the brood.
Only a small percentage of the hundreds of spiderlings in a sac survive this initial stage, with some sources indicating only a handful or up to a dozen living past 30 days. Surviving spiderlings then disperse from the web, often within a few days. A common dispersal method is “ballooning,” where they release silk threads into the air and are carried by the wind to new locations, helping them to find new habitats and avoid competition.