There's a Black Widow in my shed (not)!

Recently, while writing one of my series about woodpile invertebrates, an Australian reader/blogger mentioned the rather larger and more dangerous invertebrates found down under - here in the UK, most invertebrates are harmless to humans and even those that can bite or sting rarely cause major problems unless you are allergic to them. So, when I found a particular spider in my shed, I was reminded of occasional media reports of people finding, or even being bitten by, 'black widows'. Now, we don't have black widows (genus Latrodectus) here, but we do have 'false black widows' (Steatoda) which are also in the family Theridiidae and look superficially similar. Spiders in this family are known collectively as the comb-footed spiders due to the bristles on the tips of their hind legs which they use to tease out silk to form 'tangle' - rather than sticky - webs.

My Steatoda specimen (about 9mm long) showing the bulbous abdomen typical of the Theridiidae - also note the short hairs. The pattern shows a pale, ywllowish arc and spots on a purplish background.

The features above show this to be S. grossa - a female - although the pattern can be highly variable or even absent; Roberts (1993) provides detailed diagrams of female epigynes and male palps if required for identification. This species is found mainly in and around houses in southern England (sometimes in coastal areas of the south-west) and although traditionally considered quite scarce, it appears to be increasing in range and frequency due to the warming effects of climate change. A similar increase has been seen in the non-native S. nobilis (NHM, 2007).

The Natural History Museum (NHM) receive enquiries about bites from Steatoda species, but spider-bites are uncommon in the UK (just a few reported each year); only 12 species are capable of biting humans (including the two Steatoda mentioned so far) out of a total of about 640 species. No-one has ever died of a spider-bite in the UK, and serious effects are rare (to be honest, true black widow bites are only occasionally fatal, though they are very painful and unpleasant). So, what should you do if you find a spider like Steatoda in the UK? Well, apart from temporarily incarcerating this specimen in order to take photos (it's now back in the shed along with other specimens, inlcuding males), I tend to leave them alone. If you want to handle them, it is easy to be careful by collecting them in a suitable container e.g. if you want to put them outdoors (or into the shed). Personally, I'm happy to leave them be, especially as I generally only see them when moving things around in the shed, at which point they flee and hide. Happy spidering!

Here's looking at you - a dorsal view of the pearly eyes of S. grossa.

References

Natural History Museum (2007). The truth about false widow spiders.[accessed 14/11/11]. Lots of snippets of info about this group of spiders in the UK.
Roberts, M.J. (1993). The Spiders of Great Britain and Ireland (compact ed.) (2 vols.). Harley, Colchester. This is the standard comprehensive work on spiders of the British Isles and is excellent, but it isn't cheap! There is a 2009 reprint by Apollo Books.