Spiders are very good at exploiting us humans, and using our vehicles and cargo to conquer new territory. One example of this phenomenon is the Brown Widow, Latrodectus geometricus. Exactly where this species is native to is a matter of speculation. It is known from South Africa, the Middle East (Afghanistan), the Mediterranean (Cyprus), Australia, the southern United States, and even Japan.
Here in the U.S., L. geometricus has been well-known in the southeast, but has been extending its range up the Atlantic Coast (to North Carolina so far) and across the entire southern tier of states (Tennessee, Texas, southern California). There have also been reports from Nevada and Colorado. The specimens shown here were imaged along a retaining wall at an undisclosed location in Redondo Beach, California (a friend’s house).
According to arachnologists, despite the virulence of the venom of the Brown Widow, the species should not be considered dangerously venomous to the average, healthy adult person. The spiders apparently inject less venom than black widows, and the effects are more localized than systemic. One can avoid the potential of a bite simply by not placing his or her extremities where they can’t see, and avoiding clutter in yards, sheds, garages, and other situations where the spiders are likely to occur.
The Brown Widow is very likely to be confused with a completely innocuous spider known as the American House Spider, Parasteatoda tepidariorum. The two are nearly identical in appearance, but the House Spider never has a red hourglass marking on the underside, whereas the Brown Widow does. Like most widows, the Brown Widow prefers to sequester itself deep inside cracks or crevices by day, venturing out into its web only at night. House spiders do not have retreats, so are visible at all times, though usually tucked snugly against a wall or other surface during the day. The House Spider also has a much broader distribution, being found across most of the North American continent. Below is a pair of House Spiders, male on the right.
The egg sacs of the Brown Widow are highly distinctive, being spherical but bearing numerous tufts of silk (see images below). Both the Brown Widow and the House Spider belong to the family of "cobweb weaver," Theridiidae, which only adds to the confusion in identifying them. They both build extensive, tangled snares.
The “spread” of the Brown Widow to the west coast is a relatively recent occurrence, the first specimens being noted in the Los Angeles area in February, 2003 as a result of the Los Angeles Spider Survey being conducted by the Natural History Museum of Los Angeles County. Verified accounts of this species from elsewhere along the Pacific Coast would be welcome. It has been suggested that trucks, cars, and recreational vehicles have mostly been responsible for the assisted migration of this species. That seems reasonable, especially in the case of RVs, which often sit idle for long periods, allowing spiders to establish themselves there.
Sources: Brown Widow Spiders
Santana, Fred. 2007. Brown Widow SpidersUF/IFAS Sarasota County Extension
Levi, Herbert W. and Lorna R. and Herbert S. Zim. 1990. Spiders and Their Kin. New York: Golden Books. 160 pp.