Written by Joe Ballenger

As a rule, Widows have pretty nasty venom. I was knocked on my butt for about 3 days after a bite I received from a Southern Widow (Lactrodectus mactans) after mishandling her. These sorts of incidents are extremely rare, and some trials of Widow bite treatments have had to extend nearly a decade in Widow-rich areas to get enough incidents to study. When talking about Black Widow bites, we are not talking about something that’s common.

There are many species of widows, and not all of them are black. The species Steven is talking about, Latrodectus geometricus, is actually a very pretty camo-brown color. It still has the hourglass, though, which I think makes it look rather interesting.

Brown Widows aren’t considered to be particularly harmful. Some severe bites have occurred, so they can potentially hurt people…but the vast majority of bites are minor.

How do we know that something’s not harmful? What does that data look like? How do we prove a negative in science?

Brown Widow vs Black Widow

The first thing I want to do is to point out that Brown Widows have venom that’s actually kind of potent, although they don’t produce a whole lot of it. When I say they’re generally not harmful, that’s not a free pass to be careless around them.

An important measure of venom toxicity is the LD50, the dose which kills half the test animals (mice, in this case). A lower LD50 means the venom is more toxic, whereas a high LD50 means that venom is not very toxic. In mice, caffeine has an LD50 of about 200 mg/kg.

Comparing the toxicity of their venom to that of a closely related species tells us something about their biology. The venom of the Brown Widow happens to be 3x more toxic than the venom of the Black Widow species which hospitalized me in 2012. So the Brown Widow has great potential to harm, and should be handled very carefully if encountered.

Despite having somewhat potent venom, we still think they’re harmless.

Why is that?

Measuring Spider Bite Severity

Figure from Ibister & Fan, 2011

The paper everyone seems to cite is a 1993 study from South Africa where they followed people who had been bitten by Brown Widows found in the area. It was incorporated into a comparison for another paper in The Lancet, which I’m posting above.

According to the 15 case reports we have, the bites from Brown Widows tend to be less harmful than bites of other species. The system-wide effects which send people to the hospital, ranging from rigid paralysis to extremely high blood pressure, never seem to appear with Brown Widow bites. In some cases, you might have stomach cramps…but that’s the worst this case series reported.

We Look Really Hard for Case Reports

I have to admit here that the data for L. geometricus in particular isn’t as great as I’d like it to be.  Of the 15 cases of Latrodectus geometricus bites Mueller followed, only 10 of the spiders were brought in for proper ID. It should be mentioned that L. geometricus was one of four species found in the area.

Besides the fact there’s no way to know for sure if 1/3rd of their study group was really bitten by this species, I’m also not really enthusiastic about a study with a sample size this small. Small sample sizes make it really hard to detect differences between groups with certainty.

That being said, Rick Vetter (same guy from our last spider post) cites this paper, and as recently as 2012 clarified the risk:

Despite the fact that most brown widow envenomations are minor events, the general public assumed that these spiders are significantly toxic. The work of Muller (1993), which reports 15 verified brown widow bites in Africa, shows that they pale in comparison to typical Latrodectus bites. Reports do exist of more substantial brown widow envenomations in Mississippi, Brazil, and Venezuela but, as is typical in the medical literature, single case histories get published because of their extreme symptom expression and, hence, skew literature representation toward the rare and dynamic end of the spectrum.

What he’s saying here is that in the Widow literature, we pretty much have to depend on cases where people report the bites to a medical professional to study them. If the condition is not severe enough for people to bother calling their doctor, we can’t really collect data on that condition. So these 10 cases from South Africa (and the 3 briefly mentioned in this paragraph) likely represent the absolute worst case scenario for Brown Widow bites.

Vetter, and other authors who study Brown Widows, note that Brown Widows like to build their webs in places which would make encounters with people very likely. Vetter continues:

Although brown widows are now abundant in southern California, bites are not common where in our collective experience, we are only aware of one verified brown widow bite, which had minor symptoms.

Rick Vetter makes a living by looking up case reports of spider bites, and seeks out hospital data in a lot of his research. Despite that, he could only locate one bite in his (highly populated) region of California. So if people are getting bitten, it’s almost never severe enough to go to the hospital. When it is severe enough to go to the hospital, the symptoms for Brown Widow bites are less severe than bites from other Widow species.

Professional arachnologists have a very difficult time finding medical records of bites for this species, although bite reports from closely related species are easy to find. The little data we can find on bites of this species indicates they’re not severe. This is despite having somewhat potent venom, and living in areas where encounters should be frequent.

The work of Mueller in South Africa is what we would expect to see in a case study of a very rare event that tends to be minor.

Why Are Brown Widow Bites Minor?

This is a tougher question to answer, but experiments with closely related species give us some clues.

Experiments with Western Widows, very closely related to the Southern Widow, show that Widows in general are reluctant to bite. When harassed by ‘fingers’ made out of jello, they never bite when simply poked. When pinched between the jello ‘fingers’, they chose to bite the fingers 60% of the time. When they do bite, venom appears to be injected only about half the time.

Unfortunately, as far as I can tell, similar experiments do not exist for the Brown Widow. However, anecdotally, Brown Widows tend to be more skittish than the other species and will drop off the web before biting.

The Bottom Line

In science, proving a negative is very difficult…although I’d argue it’s not impossible. We can find bites for black widows in general, but not L. geometricus when using the same methodology. The few bite records we have tend to not be particularly severe, despite the fact that abnormally severe bites tend to be reported more often.

The reasons for this aren’t well understood, and it likely doesn’t boil down to one factor. Anecdotally, these spiders tend to be very skittish and drop off their webs at the first sign of trouble. The relatively mild symptoms indicate Brown Widows are reluctant to inject venom when biting, and this idea is backed up by the relatively low venom production for this species.  It’s also possible they could be more reluctant to bite, or it could be harder for them to bite people…but these ideas haven’t been studied.

Ultimately, their inability to cause harm is inferred by scientists who study hospital records and compare the number of bites and severity of symptoms between species.

As I’ve said before with bees and wasps, these are animals which deserve respect. They’re small and easily scared…but still very powerful. If you see one around your home, it’s still a good idea to avoid them.

Author’s Note:

If you’re interested in the biology of Black Widows, check out Catherine Scott’s Blog and Twitter feed. She’s also in the process of crowdfunding some of her research. If you can afford to donate, this project is worthy of support.

Works Cited