Steatoda grossa (aka the false widow)

Steatoda grossa female

Some time ago I found a spider in my kitchen that I identified as Steatoda grossa. For most people that means very little, but for me it was rather exciting as they are not considered common in Ohio, where I live. Identifying spiders isn’t as easy as looking at the abdominal patterns, which is what most amateurs do when looking through a field guide. The patterns on spiders can be variable. You must look at the genitalia for most spiders in order to conclusively identify them. For female spiders you must look at the epigynum, which is located on the ventral side of the spider’s abdomen near the epigastric furrow. Viewing this on larger females can be accomplished by placing the spider in a Ziploc bag, placing her so that she is ventral side up, and holding the bag tight enough to reduce movement without squishing her. I was able to do this with my Steatoda and was able to confirm that she was in fact Steatoda grossa. If you want to see an example of the epigynum diagram you can see one on Bugguide.

Steatoda grossa epigynum (viewed through a microscope)

 

Steatoda grossa are members of the Theridiidae family, commonly referred to as cob web weavers, due to the tangle web they weave (Adams and Manolis 2014). Worldwide there are more than 2,200 species of Theridiidae (Bradley et al. 2013). This family includes the Latrodectus (the widows); Steatodas superficially resemble Latrodectus, leading to the common name of “false widow” (Adams and Manolis 2014). Steatodas are not usually black, but as they are dark in color in low lighting they can appear black (Howell), and unlike the true widows, Steatodas lack the hourglass marking on the ventral side of the abdomen. They tend to hang inverted in their cobweb (Adams and Manolis 2014) which gives a clear view of the ventral side to determine that no hourglass is present. The females can live up to six years (Bradley et al. 2013). They are not considered harmful to people (Bradley et al. 2013) but in many ways provide us benefits as these spiders eat a lot of the things that we considered pest species. They like to live in and around our home, and I like to think of them as my personal pest control army.

Steatoda grossa male

 

A short time after I found the female in my kitchen I found a male Steatoda in my bathroom.  I was fairly confident that the male was also Steatoda grossa, but the trick with the Ziploc bag doesn’t work with males as it is the structures of the pedipalps that you need to see to determine species. I placed the male in a vial, and tried to photograph him at various angels to see if I could determine species. Although I didn’t get any images that confirmed 100% he was S. grossa, I felt that I could see enough of the structures to be confident enough to try introducing him to the female.

Steatoda grossa male and female. (Male web building (left), and mating (right)

 

On May 24^th, 2015 I decided to introduce the male to the female. This can be a stressful event as either one of the spiders can decide not to be friendly to the other, and you can end up with both spiders injured or dead from the ordeal. I was very nervous, but gently placed the male in the enclosure with the female. (It is better to introduce the male into the female’s enclosure so that he can get chemical signals from her web regarding her reproductive status.) There were a few tense moments when they approached each other, and then the male retreated to the bottom of the enclosure where he started spinning a small web. The best I can describe it was a platform that he suspended himself inverted beneath, about as big around as he was with his legs outstretched. He carefully attached lines to the webbing of the female. Once he was done he started tugging on the female’s web. I have heard that male spiders will use this to communicate with the female that he is of the same species and eager to mate. It didn’t take long for her to proceed to his location in the enclosure. There was some front leg tapping, and a couple of attempts to pull the female closer that ended with her retreating, but after a couple of tries he successfully wooed the female and mating ensued. I have posted a couple of videos documenting this, the courtship, failed attempt, and successful mating here, and a better view of the mating here.

The first Steatoda grossa egg sac.

Once the mating was complete I removed the male from the female’s enclosure. On June 6^th she created her first egg sac, and not long after that she created a second egg sac. I have been keeping her well fed, mainly on Isopodas, as they are easy to catch and she seems to enjoy them.

 

Steatoda grossa spiderlings with the second (yet to emerge) egg sac.

On July 7^th, 2015, I was thrilled to see that nearly fifty spiderlings have emerged from the first egg sac. They are still very small, and I personally think they are adorable. Now I just need to wait for the second egg sac to hatch, and then I can release them in and around my home, so that the next generation of pest control is in place.  For those of you interested I have since preserved the male in ethanol, and was able to get some decent photo of his palps, to be 100% certain that he is Steatoda grossa. He will be donated to the Ohio State University’s Museum of Biodiversity. I am looking forward to finding many more in and around my house!

Male Steatoda grossa pedipalp

 

 

 

Work Cited

Adams, Richard J., Manolis, Tim D.,. 2014. Field Guide to the Spiders of California and the Pacific Coast States. Berkeley, CA: University of California Press. .

Bradley, Richard Alan., Buchanan, Steve., American Arachnological Society.,. 2013. Common spiders of North America. Berkeley, CA: University of California Press.

Howell, W. Mike., Jenkins, Ronald L.,. 2004. Spiders of the Eastern United States : A Photographic Guide. Boston, MA: Pearson Education.