Spider attack or a web of lies?

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.

Araneus_diadematus_comparison

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' pedipalps that show this is a male

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.

Spider bites

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).

NobleFalseWidow

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

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.

Cellar Spider (Pholcus phalangioides) feeding on a different spider species

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.

Friday mystery object #235 answer

Last Friday I gave you this mystery sound to identify courtesy of curator Cheryl Tipp at the British Library Sound Archive:

It provided a nice break from the usual bones and gave some of the live animal fans among you a chance to identify something that the usual bone fans might miss.

This distinctive call was recognised by stuart petch (@thelightoutside) and Kiki with henstridgesj coming close. It is the call of a male Black Grouse Tetrao [or Lyrurus depending on your taxonomic authority of choice] tetrix (Linnaeus, 1758).

Black Grouse by Vnp 2011

Black Grouse by Vnp 2011

In the Spring, these striking birds take part in something called a lek, where the males competitively show-off in order to impress the females. They strut about in a special arena, flash their tails and make the call you heard in the recording. This allows the females to pick the most impressive specimen to father their offspring, presumably in order to have healthy young.

This kind of mating system is quite common in birds and song plays an important role, so I will be back with another song next Friday!

Friday mystery object #235

This week I don’t really have a mystery object, I have a mystery sound for you to identify from the British Library Sound Archive via the Curator of  Wildlife Sounds, Cheryl Tipp.

Any idea what this is?

Patches is listening carefully

Patches is listening carefully

As usual you can put your thoughts in the comments section below. Have fun!

Friday mystery object #234 answer

Last Friday I gave you this mystery skull to identify, which I discovered in a box of unidentified bits and bobs:

mystery234

It was pretty obvious that it was the skull of a big cat of some kind, with most of you suggesting a Jaguar or Cheetah (either of which would make me very happy as we have the skull of neither in the Horniman collection). Unfortunately it appears to belong to neither.

As I’ve mentioned  before, cats are quite difficult to differentiate from each other as they haven’t been diverging for all that long and their widespread distributions can mean that populations within a particular species can be quite variable in morphology. Leopards are a good example of this, with a (once continuous) range from Korea to South Africa.

Global distribution of the leopard (Panthera pardus) by Tommyknocker

Global distribution of the leopard (Panthera pardus) by Tommyknocker

As it turns out, this specimen is most likely from a Leopard Panthera pardus (Linnaeus, 1758), since it’s from an adult animal (as is clear from the well formed sagittal crest) but is on the small side for a Lion or Tiger and too big for a Cougar or Cheetah. It also lacks the broad post orbital region seen in the Cheetah and Snow Leopard, and it lacks the concave profile of both the Snow Leopard and Jaguar. All of these identification pointers can be found in this handy pdf by Margaret “Cookie” Sims.

Just to show you what I mean about the variability within a species, here’s a second skull from the same box, that also matches the Leopard identification.

Leopards

I expect the big difference in size is largely down to sexual dimorphism, but as you can see the overall proportions are quite different as well. This may be a difference between widely separated populations, or it could just be individual variation – either way it goes to show that cats are hard to identify.

Friday mystery object #234

This week I have a mystery skull from the collections of the Horniman Museum & Gardens for you to have a go at identifying:

mystery234

The family it belongs to may be easy to work out, but as we’ve experienced in the past, the species can be more difficult to establish.

Cryptic answers would be much appreciated, to give the less experienced a chance to work it out. Have fun!

Friday mystery object #233 answer

Last Friday I gave you a variety of mandibles to have a go at identifying. They lacked a scale bar and represented a range of different species that have similarities in mandible shape.

There were some great cryptic suggestions of identities, but it must be said that Jake came through with a really clear and pretty much spot-on list of suggestions. So here are the answers in a handy form that might be useful for reference:

mystery233b

The Sheep and Cow have a distinctive upward inflexion at the end of the mandible, with the Cow’s being so strong that the incisors start above the level of the top of the molar tooth row – unlike the Sheep’s.

This inflexion is much less marked in the Red Deer, which has a narrower body of the mandible, presumably relating to the less intensive chewing of a browser compared to grazers (grass is tough stuff). The Deer also has a notch along the bottom of the jaw, which Jake pointed out as a useful feature.

The Pig mandible tapers less overall, but is thicker at the end with the articulation – presumably because the omnivorous Pig is chewing differently, using the temporal muscles more than the masseter muscles and therefore needing a different area of the jaw for muscle attachment. The teeth are also pretty distinctive. Like the Pig, the Donkey mandible lacks the long and hooked coronoid process, but is also very triangular in shape with quite squared teeth – features typical of an equid.

So hopefully that gives you some pointers for telling some common herbivore mandibles apart when you don’t have a scale bar – a more common problem for some of us than you might think…

Friday mystery object #233

This Friday I have a challenge for you. Can you work out which five different species these mandibles come from?

mystery233

They are all different sizes and the lack of scale bars is deliberate – this is about trying to find useful features from the shape rather than the size, It’s not easy!

You can put your answers in the comments section below. Good luck!