Black Widow Spiders

Black Widow female--Photo by Manny Rubio Arizona-Sonora Desert Museum digital library

Black widow spiders (Latrodectus hesperus) are normally shy and sedentary, but will react if they feel threatened. According to some authorities, their venom is 15 times more potent than rattlesnake venom. But black widows bites are fatal less than one percent of the time because their small fangs can’t penetrate the skin very deeply and often the spider does not inject venom. Only the bite of the female is potentially dangerous to humans.

Black Widow female--Photo by Manny Rubio  Arizona-Sonora Desert Museum digital library
Black Widow female–Photo by Manny Rubio
Arizona-Sonora Desert Museum digital library

The female black widow has a body length up to three-quarters of an inch. She is shiny black to dark brown and has a red, hour-glass mark on the bottom of the abdomen. The male is half that size, usually medium brown with cream-colored makings on legs and abdomen.

According to the Arizona-Sonora Desert Museum:

Different species of Latrodectus are found throughout most of North America, more commonly in warmer climates. Black widows are common around man-made structures such as garages, lawn furniture, and woodpiles. They also live in a variety of natural habitats.

The black widow preys mainly upon insects that it traps in the web. The web is irregular and strong to the touch in comparison to other webs. Some species of spider wasps prey upon black widows. Black widows are shy, sedentary, and largely nocturnal. They are not aggressive, but will bite in self-defense.

The female mates only once in her lifetime, retaining sperm for future egg-laying. The smaller male is sometimes eaten by the female following mating, hence the name “widow.” This characteristic, however, is not limited to black widows, but can occur after mating in many arachnids, most of which are highly predatory. The female lays approximately 300 eggs at one time and encases them in a round, cream-colored egg sac made of her silk. One spider produces several sacs within its one- to two-year lifespan, but only one sac at a time. The spiderlings disperse by ballooning.

The black widow is one of two species within our region that is potentially dangerous to humans (the brown spider is the other). The bite can kill a human, but this is rare. More often, the bite is painful and causes serious reactions, including nausea, dizziness and abdominal cramps.

The silk of the black widow and some orb-weaving spiders is the strongest among spiders and stronger than steel wire of the same weight. ASDM notes: “A spider may have up to six types of silk glands, each producing a different kind of silk. It is actually a complex strand of proteins that is produced as a liquid, and solidifies under tension. Silk is used to build webs, catch food, line burrows, protect eggs, detect prey (as trip lines), and even to aid in dispersal.” Spiders avoid being caught in their own webs by having oil on their legs.

Black widows and other spiders prey mostly upon insects that get trapped in their web. The venom serves to paralyze and liquify the prey, so the spider can suck out the insides. Black widows are preyed upon by wasps and lizards.

Most spiders have up to eight eyes, but only the jumping spiders rely on sight to hunt. Most spiders have hairs on their bodies, especially their legs. These hairs allow the spider to feel and “hear” by displacement of air around the hairs.

See also:

Mist of the Sharpshooters

Green lynx spider

Desert Bees

Venomous Centipedes and Cyanide-Oozing Millipedes

The Gentle Desert Tarantula

The Cochineal, a little bug with a valuable product

Vinegaroons and Sun Spiders

It’s time for scorpions

Pepsis wasps have the most painful sting