Answers from the Wild Side: Is this a Black Widow?

Widow or no? — Sarah Andresen

Is this a black widow? It’s living in a bat house just outside our barn door in Chilmark! Are they common on the Vineyard?

The Pelikan Brief:

Wow. Yes, it appears to be a southern black widow, Latrodectus mactans. I’ve never heard a reliable report of one occurring naturally on M.V., though some sources say the range of the species extends as far north as Mass. A close relative, the northern black widow, L. variolus, is a lot more likely in our region and probably occurs here, though I’ve never found one on M.V.

The rest of the answer:

Yes, indeed it is a black widow, and a very interesting find! The small size, shiny black body, round abdomen, and especially the red “hourglass” mark this as a female black widow. Interestingly, though, there are several species of black widow, and this is not the one that is normally expected on the Vineyard. The complete hourglass marking, with the two lobes clearly connected, marks this as a southern black widow, which is generally believed to be rare at best in our region. Much more likely is the northern black widow, which is quite similar but features an “hourglass” that is broken into two parts. The northern black widow seems to be sparsely distributed but reasonably common in the Cape and Islands Region. The only ones I’ve ever seen have been in natural settings, not in human-made structures.

I’m afraid I can’t say just what is going on: southern black widows may be extending their range northward and colonizing our region. (Like many spiders, I imagine they can disperse as baby spiders by “ballooning” on the wind on strands of silk). They may have been here all along, but were overlooked. Or this individual arrived from elsewhere on a load of vegetables or landscaping material. Hopefully, a pattern of sightings (or an absence of sightings) in the near future will clarify the picture.

Black widows generally are famous as the most venomous spiders in North America, one of very few arthropods on the continent that is capable of killing a human being. (I set aside the special case of fatal allergic reactions to wasp or bee stings.) Being small spiders, they can inject only a tiny quantity of venom. But the venom is incredibly potent, acting on the victim’s nervous system to cause cramping, paralysis, and on rare occasions death when the muscles necessary for breathing shut down. There is an effective antivenin, and the thing to do if you think you’ve been bitten by a black widow is seek medical care immediate and, if possible, bring the spider with you so its identity can be determined.

But don’t panic! Black widows are timid spiders that rarely bite humans. The main use for their jaws and venom is to subdue small insects that blunder into the widow’s untidy web system. Given the chance when disturbed, a black widow, like other spiders, will hide rather than bite, and it is only when a black widow is faced with squishing that it bites in self-defense. Moreover, fatalities are very rare: one source reports that only about one percent of black widow bites are fatal, and that fatalities almost always involve children or elderly victims who are frail and have a small body size.

Black widows like dark, shady, enclosed spots (a classic black-widow bite scenario, back in the old days, involved spiders getting sat on in outhouses). Pay attention when you’re working or reaching into such places, and you will have little reason to fear this spider, which is (in my opinion) best thought of as an interesting and generally beneficial part of our ecosystem.