Black spider in Okemos, Michigan?

[rune2766]

Hi everyone!

I was wondering if anyone could give me a hand in identifying this guy? I'm over in Okemos, MI (city just east of Lansing, MI) and recently moved into a townhouse that backs up to some woods. I found a few of these in the basement of the townhouse. They vary in size from 1/2"-1" in length including legs. The largest one I saw was about 1" with a very large rounded abdomen. I tried some basic searches but nothing conclusive. Most of search returns were for the Northern Black Widow (which the largest one I saw had a striking resemblance to the photo located on the Michigan DNR site about the black widow).

Thanks so much for the help!

[Ungoliant]

Summary: I think this is a false black widow (Steatoda grossa) and not a true widow spider (genus Latrodectus).

This spider is female and belongs to the cobweb weaver family (Theridiidae). To figure out whether she is a false widow or a true widow, you have to look at her markings. Did she have any red or orange triangular/hourglass markings on the underside of her abdomen?

If she doesn't have any markings, she is a false black widow (Steatoda grossa). These spiders are commonly mistaken for black widows, because their bodies are dark, shiny, and similar in shape. Bites from these spiders may produce symptoms that are similar to (but much less severe than) widow bites.

If she had hourglass markings on the underside of her abdomen, she is most likely a black widow spider (genus Latrodectus). There are three different species of black widows in the U.S. You can distinguish between the species by the geographic location of the spider (not all species occur in all areas) and the markings.

The northern black widow (Latrodectus variolus) has an incomplete (or broken) hourglass marking on the underside of the abdomen. The back side of the abdomen often has a row of red spots. The absence of red spots on your spider suggests that she is not a northern black widow.

The southern black widow (Latrodectus mactans) and the western black widow (Latrodectus hesperus) are similar in appearance. Both have complete hourglass markings on the abdomen. The rest of the body is solid black, although there may be a red spot on the tip of the abdomen. It's difficult to tell these species apart just by looking. Often, location is key; I'm not sure if either if these species is normally found in Michigan. (Michigan.gov only mentions the northern black widow.)

In case you were wondering, male widow spiders and juvenile widow spiders of both sexes are harmless. only the mature female is considered medically significant. Even if it is a widow, you are unlikely to be bitten unless you molest the spider. These spiders keep to themselves and only bite in self-defense as a last resort, such as when handled, pinched, or squeezed. Deaths in healthy adults from widow bites are extremely rare, especially since the advent of antivenom.

[Eric]

Rune2766:

I agree with Ungoliant as to the identification.

Eric

[rune2766]

Thank you for all the information! I didn't get close enough to check to see if there were any markings, the other couple that I saw didn't have the broken hour glass or red markings across the abdomen so it is probably the false black widow that you described.

I did have a couple questions regarding juvenile and male black widows and false black widows. If they essentially look the same in just a quick look, how does one differentiate between the two? Also, is it common to find multiple black widows or false black widows in such a small location like a basement?

[Ungoliant]

I did have a couple questions regarding juvenile and male black widows and false black widows. If they essentially look the same in just a quick look, how does one differentiate between the two? Also, is it common to find multiple black widows or false black widows in such a small location like a basement?

Male and juvenile widows look quite different from mature females. The male's abdomen is not as large or as round as the female's. Adult male widows normally still have "hourglass" markings (along with other markings).

Like the female, the male northern black widow (Latrodectus variolus) has red or orange spots on the back of its abdomen. The white marks that are also present distinguish it from the female.

Unfortunately, it is often difficult to determine the sex of immature spiders just by looking at them. With juvenile widows of both sexes, their coloration becomes more "adult" with each molt.

BugGuide has male and female photos for each species that you can review as examples.