Can Cellar Spiders Save You From Brown Widows?

January 10, 2017

It would be hard to find a backyard that doesn't have Brown Widows, Latrodectus geometricus, these days. Since arriving in Southern California about 10 years ago, they have spread everywhere. The most obvious signs of their presence are their characteristic spiky egg sacs, but what isn't characteristic is their coloration. The color of the Brown Widow abdomen varies from very dark to almost white and the mottled pattern may or may not be easily seen. (This is in contrast to adult black widows, L. hesperus, which have uniformly black abdomens and no bands on their legs). These photos, taken in the Nature Gardens of the Natural History Museum, show this variation, even in Brown Widow spiders which are close neighbors and possibly related to each other!

 Photo credit: Kelsey Bailey

 

The variety of abdominal colors and patterns is not the only mystery surrounding the ubiquitous Brown Widow. Biologists have yet to fully understand how they interact with other spiders that live in L.A. Do they eat other spiders? And does anything eat Brown Widows?

In my time observing these spiders, I have seen some curious behavior involving Brown Widows and another common 8-legged beauty called a Cellar spider. Cellar spiders have very thin, long legs and make messy cobwebs. They are often confused with a different kind of spider relative also known as "daddy long-legs," are the subject of a strange urban legend regarding the strength of their venom. 

Photo credit: Ryan Hodnett

 

A few weeks ago I noticed a Brown Widow spider had been hanging out in its web under a window at my house. The spider would come out at dark and hang upside down in its tangled web. Then one night, it disappeared! Instead, a species of cellar spider called Pholcus phalangioides was occupying the brown widow's web. I checked a few days later and the cellar spider was still there, but the Brown Widow was nowhere to be seen. 

Cellar spiders are known to prey on other spiders, including widows. They approach the widow in its web and throw silk over it until the widow is immobilized, then move in for the kill! It is possible that the Brown Widow succumbed to the fatal grip of the cellar spider right outside of my window. The next time you see a Cellar spider in your yard, give it a little spider pat on the back for helping to keep Brown Widow populations in check.

Photo credit: Thomas Bresson

 

 

 

 

(Posted by: Jan Kempf)


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Beneath the Perch

June 23, 2017

Q: What are Those Miniature Spiky Puffballs? A: Brown Widow Egg Sacs

September 22, 2012

Earlier this week, staff found some small circular egg cases on a gate in the North Campus. Upon closer inspection we realized they were brown widow, Latrodectus geometricus, egg sacs. But how did we know this?
 



Earlier this week, staff found some small circular egg cases on a gate in the North Campus. Upon closer inspection we realized they were brown widow, Latrodectus geometricus, egg sacs. But how did we know this?

Two egg sacs, each containing about 100 eggs, notice the geometric design.


Differences between brown widows and western black widows:

Brown Widows

Egg sacs are pale yellow and spiky (BINGO)

Egg sacs contain upto 150 spiderlings (best word ever)

Can lay 20 sacs over their lifespan

Adult females are USUALLY tan with an orange hourglass design on the underside of the abdomen

Lower incidence of medically significant spider bites

 

Western Black Widows

Egg sacs are pale yellow and smooth

Egg sacs contain upto 300 spiderlings

Can lay 10 sacs over their lifespan

Adult females are black (duh!) with a red hourglass design on the underside of the abdomen

Higher incidence of medically significant spider bites

 

Visit UC Riverside's Center for Invasive Species Research site for more information on identifying Brown Widows.

 

Check out this video Sam Easterson made of a brown widow tending her egg cases:

If you want to meet a brown widow up close and personal, all you have to do is visit our Spider Pavilion. The pavilion opens to the general public Sunday September 23. We have both a brown widow and western black widown on display in tightly shut enclosures! Stop by and say hello.

 

(Posted by: Lila Higgins)

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