I noticed a spider sitting on one of my orchid plants. I wouldn’t have bothered photographing it, but it was well posed for a shot and my new iPhone is wonderful for macrophotography. I posted the photo on Facebook where my biologist daughter identified it as Steatoda nobilis, the noble false widow spider. I’ve written in the past in this column about widow spiders, but false widows were new to me. A bit of research on the internet turned up a fascinating tale of biological globalism.
S. nobilis is native to Madeira and the Canary Islands but has been introduced widely, initially to coastal areas of the British Isles and the Mediterranean littoral, travelling on imported fruit and plants. There are questionable reports of it arriving in England as early as 1879, and it was permanently resident in Britain by 1907. Specimens were collected in the harbour of Hamburg in the 1950s, but there was no record of established populations in Germany until 2013. Surprisingly, it was recognized nowhere in the New World until a 2011 report by Dr. Richard Vetter who identified a specimen collected in Ventura County. By 2020 it had been found in all the coastal counties as far north as Marin. In 2014 there were the first reports of its occurrence in Chile and Ecuador.
Most of the specimens collected round the world have been in or near human habitations in urban environments. It is described as “synanthropic,” but perhaps that is in part because the individuals who have collected and reported them live in cities. It seems to enjoy structures such as fences, light fittings, and window frames that provide a rigid architecture suitable for building a web. It has colonized much of England and Ireland, with outliers as far north as the Orkneys. Collections elsewhere in the world have been mainly within 10 kilometres of the coast. Though this may be explained partly because of its early introduction to the British Isles, press headlines such as “Britain’s only venomous spiders are invading our homes” and “UK shudders as venomous spider creeps across Britain” have also contributed. There is a close correlation between newspaper coverage and the number of S. nobilis recordings.
Some observers have attributed its recent spread to global warming but, if that were the explanation, one would not have expected Britain to be colonized earlier than the countries of southern Europe. Others have suggested that a mutation, perhaps originating somewhere in the British Isles, has augmented its ability to spread into new environments.
So, is the false widow really venomous? The venom of S. nobilis contains ‘latrotoxins,’ neurotoxins similar to those produced by widow spiders of the genus, Latrodectus. Only a handful of case reports on S. nobilis bites are to be found in the medical literature, few of them with serious consequences. A study from Ireland suggested that some of the severe local reactions were due to the introduction of bacteria at the site, rather than to the venom itself. It is likely that mild envenomations are very much underreported, but considering how common the spider is in Britain, and the anxiety provoked by the press, their scarcity is reassuring. I suspect that the situation is similar to that of the brown widow, Latrodectus geometricus, in San Diego. I can find webs in the handles of our trash barrels any time I care to look, and when I do see a spider it’s usually a brown widow, but I’ve never experienced what I thought might be a spider bite.
The venom of black widow spiders is some 15 times more potent than that of a rattlesnake, and it is capable of producing serious symptoms, even death, in humans. This would seem a gross example of ‘overkill’ for a predator whose normal prey is insects and other small arthropods. However, there are convincing reports of S. nobilis capturing and feeding on animals as large as a bat and a lizard. These may be more than chance occurrences because analysis of components of the venom have shown that alpha-latrotoxin, found in both Steatoda and Latrodectus, is “vertebrate-specific,” meaning that its neurotoxic effects are absent in invertebrates.
I’m surprised that our local media haven’t jumped on this story which offers the possibility of clickbait headlines similar to those that captivated the British reading public.