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

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.

Brown Widow egg sacs
Brown widow egg sacs

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 up to 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 up to 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 widow on display in tightly shut enclosures! Stop by and say hello.

(Posted by: Lila Higgins)