One of the more common types of public enquiry I get as a natural history curator is about spiders. Every year in September people start reporting large spiders that they don’t recognise and which therefore *must* be invasive species or tropical escapees from packets of bananas. Every year I either reassure (or disappoint) those people by explaining that the spider they have discovered is a common species in its fully-grown adult stage, sometimes looking a bit on the bloated side because it’s full of eggs.
Garden Spider (Araneus diadematus) full of eggs. Inset: comparison against non-gravid Garden Spider abdomen – note the characteristic cross pattern.
This year has been a bit more active on the spider enquiry front, because there has been a lot of scaremongering about spiders in the tabloid press. Beyond the usual September influx of sightings when adult male spiders become more active as they seek females to mate with, there has also been an increase in the number of requests for identification throughout the year, with people unnecessarily concerned about the Noble False Widow (Steatoda nobilis) spreading steadily northward as the global climate warms.
Male Giant House Spider (Eratigena atrica) on the lookout for females. Inset: note the ‘boxing glove’ shaped pedipalps that show that this is a male
With all this frenetic arachnological activity I thought it might be time to lay to rest some common misconceptions about spider bites and offer a reality check to counter some of the over-hyped reports of injuries blamed on spiders.
Spiders in Britain are pretty harmless critters. So far there have not been any reported deaths in the UK from spider bites ever. You are literally more likely to be killed by being struck by lightning.
This is partly because spiders very rarely bite people, despite what you might be told by the press. The UK is full of things that do bite however, such as mosquitoes, midges, horseflies, ticks, bed bugs and fleas. All of these unpleasant beasties have a reason for biting – they are after your blood. Spiders are not. They will generally only bite when they feel very threatened.
Another reason why spiders in the UK are not life-threatening is simply that their venom doesn’t pack enough punch. The most venomous spider living in the UK is the Noble False Widow, which can give a painful bite, similar in intensity to the sting of the Common Wasp (Vespula vulgaris).
Noble False Widow spider – a little visitor in my kitchen
Keep in mind that a Common Wasp is far more likely to sting you than a Noble False Widow is to bite you, since the spiders are not aggressive – they largely keep to themselves and they don’t try to mug you for whatever sugary food or drink you have available.
A few other spiders in the UK can also bite, sometimes painfully, but again their venom is meant for subduing small invertebrates rather than humans, so although you may get some pain and perhaps swelling some time after being bitten, that may persist for a few days, it shouldn’t cause you any serious problems unless it gets infected.
Spider bite or MRSA?
Now this is where it gets interesting. Cellulitis caused by bacteria like Staphylococcus aureus (or ‘staph’ for short) can commonly be mistaken for spider bites. Any bites, stings, scrapes, cuts or even hair follicles in the skin can open the underlying tissue to infection, leading to necrosis of the skin, similar to that caused by the bite of certain spider species – in particular the Brown Recluse (Loxosceles reclusa), which it’s worth noting does not occur in the UK.
Cellulitis – not a spider bite. Image by James Heilman, MD
In areas of the USA where the Brown Recluse is also not present, it still gets blamed for a large number of injuries that are actually caused by staph bacteria that are resistant to Methicillin antibiotics (this is the strain of bacterium more commonly known as MRSA). In one study assessing the possibility that MRSA had become established in a prison community, half of the study group incorrectly assumed that the symptoms of their MRSA infection were symptoms of having been bitten by a spider.
In fact, it seems that spiders regularly get blamed for injuries that they don’t cause. One American study reported 216 medical diagnoses of Recluse bites in Western states in 41 months, despite the fact that the spiders don’t naturally occur in those areas. In fact there had only ever been 35 confirmed sightings of Recluse spiders in the regions studied, which really doesn’t add up: how can non-existent spiders be biting people?
Why blame spiders?
It seems that there are a few factors at play here, not least irrational arachnophobia fuelled by irresponsible reporting in the press. People are told about ‘deadly’ spiders and then when they get an infection they assume that it’s actually a reaction to a spider bite. They then tell the media they’ve been bitten by a spider and the vicious circle is complete.
People also assume that a bite has been caused by a spider because they have seen a spider nearby. However, that spider is probably an innocent bystander that’s more interested in eating the bloodsucking pest that actually caused the bite – assuming that it’s a bite at all and not cellulitis caused by something else.
A Cellar Spider (Pholcus phalangioides) intent on feeding on a another spider species and not remotely interested in biting humans
Finally, the effects of spider venoms are often very poorly understood, because they are so frequently conflated with a variety of other dermatological problems and the rarity of confirmed bites means that little research has ever been carried out. That means medical staff are operating from a poor information base and may rely on poor information when making a diagnosis. The danger here is that significant medical issues (such as the spread of MRSA) may be overlooked because spiders are being unfairly blamed.
So if you read a case in the media about some horrible ‘spider bite’ it’s sensible to remain a bit sceptical. If the spider wasn’t seen actually doing the biting, the chances are the injury wasn’t actually caused by a spider at all. More likely it’s an infected wound, maybe following a bite or sting from something else with a taste for human blood or the sweet things we eat ourselves.