The truth about spider bites: “Aggressive” spiders and the threat to public health

This post was written with Chris Buddle, and originally appeared on his blog Expiscor at Scilogs.com

Misinformation about spider bites is everywhere

Spiders are polarizing: people tend to be fascinated or fearful, and for some, Arachnophobia can be quite serious. However, spiders are often feared unnecessarily. They are quickly blamed for almost ALL unexplained bites or lesions!

It doesn’t help that there is an incredible amount of misinformation and fear-mongering related to spiders in the popular media and all over the internet. What’s worse, misinformation about spiders also appears in the medical literature. For example, the journal Emergency Medicine recently published an article on bites and stings that made the following claims:

“The [hobo spider] is considered aggressive and tends to bite even with only mild provocation. The clinical presentation, inclusive of systemic reactions, is similar to that of the brown recluse spider.”

This got us all riled up, and we decided to set the facts straight:

1. The hobo spider is not aggressive.

Here I am, holding an “aggressive” hobo spider. Photo S. McCann.

Exhibit A (photo, right): the allegedly aggressive hobo spider. This female hobo was minding her own business on her webuntil being rudely removed to a human hand and made to pose for photographs.

As for its supposed tendency to bite when provoked, Samantha Vibert, an arachnologist at Simon Fraser University says,

“During my PhD, I’ve studied the ecology and courtship behaviour of the hobo spider, Eratigena agrestis. This work entailed surveying dense populations of the hobo spider in the field and conducting experiments in the lab. Over the course of several years, I’ve handled hundreds of hobo spiders. I am always puzzled when I hear the hobo spider described as an “aggressive” species. Their one and only strategy when disturbed is to run away. The only mildly disturbing thing about them is the speed with which they bolt! When they feel threatened, they’ll abandon their web and make a dash for the nearest dark corner. I can only assume that their bad reputation stems from 1) a misunderstanding of their scientific name (agrestis means “of the field”, and not “aggressive”) and 2) their appearance. For people who dislike spiders, their relatively large size and long legs are not endearing. In fact, this spider is very meek and gentle but sadly misunderstood!”

2. The evidence that hobo spiders cause dermonecrotic lesions is poor and largely circumstantial.

Hobo spiders are European, but they were introduced to North America in the early 20th century and have since become established in the northwestern US and British Columbia, Canada. Hobo spiders live peacefully alongside humans in Europe, where nobody seems to be all that concerned about them. A 2001 study found no support for the hypothesis that the venom of North American hobo spiders differed from that of their European counterparts in its ability to cause necrosis. Most of the evidence tying hobo spiders to dermonecrotic lesions in medical case studies is weak and circumstantial. For example, the mere presence of this species in the house or even the neighbourhood where the supposed bite victim lived has been used to implicate hobo spiders. There only seems to be one case reported in the medical literature with reasonably incriminating evidence – the dead spider was found crushed under a pant cuff. But even in this case, the person had a pre-existing medical condition and did not seek medical attention for more than 2 months after the bite, casting doubt on whether the spider’s venom was actually responsible for necrosis.

hobo

A female hobo spider. Photo by S. McCann.

3. Spider bites are extremely rare, and often misdiagnosed.

It is disappointing to see these myths about spiders propagated among some doctors and the public, but more importantly, it is dangerous. Here is a list of some of the actual conditions that have been misdiagnosed as resulting from spider bites:

  • infections (bacterial, viral, and fungal)
  • cancers (basal cell carcinoma and lymphoma)
  • poison oak and ivy
  • burns
  • Lyme disease (resulting from tick bites)
  • vascular disorders
  • pyoderma gangrenosum

Arachnologists Robb Bennett and Rick Vetter advise doctors that,

“The criterion standard for spider-bite diagnosis should be a spider caught in the act of biting or otherwise reliably associated with a lesion (and properly identified by a qualified arachnologist). Unless this standard is met, a working diagnosis of a spider bite should not be considered. Any of the above conditions are more likely.”

In spite of there being no brown recluses in Canada, it is common to hear about people who have been diagnosed with brown recluse bites here. Most of these folks never saw a spider or felt a bite, but had their mysterious lesion diagnosed as a spider bite by their doctor, who they trust as an expert. This comic nicely sums up why the vast majority of the time, it is safe to assume that a spider did not bite you.

male_recluse_Matt_Bertone

The poor brown recluse is always getting blamed for unexplained lesions. Photo by M. Bertone, reproduced here with permission.

Even where there are brown recluse spiders, they hardly ever bite people. There was a family home in Kansas that had 2,055 brown recluse spiders collected from it over a 6-month period, and no one living in the house got bitten. Still not convinced? In Florida, medical professionals diagnosed 124 brown recluse bites over 6 years. That’s 124 people who accidentally got near enough to the alleged spiders to be bitten, while going about their everyday business (only one of them ever produced an actual brown recluse for identification). Arachnologists – we’re talking people who actively go around seeking outspiders in likely spots – found only 5 brown recluses in Florida over the same 6-year period and a total of only 70 brown recluse spiders over 100 years. The numbers simply do not add up.

Did we mention that spiders hardly ever bite people?

Sometimes spiders do bite people, and a few species are legitimately considered medically significant. In North America, black widows as well as brown recluse bites are serious and certainly may require medical attention. However, black widows in particular have an undeserved bad reputation. They are not aggressive, rarely bite, and even when they do, they often don’t inject any venom. Most news articles about black widows refer to them at least once as deadly – often in the headline. According to the American Association of Poison Control Centers, 2,246 black widow bites occurred in all of the United Stated in 2012 (that doesn’t mean they were verified as actual black widow bites, just that they got reported, so this is almost certainly an overestimate). Only 21 of these reported bites resulted in major, but non-lifethreatening, symptoms, and no one died. Compare those zero deaths with the 36,166 traffic fatalities in the US in 2012. Cars are not regularly described as deadly but perhaps they should be. It would be more reasonable to fear automobiles than black widows. Arachnophobia is of course a legitimate condition, but there really are a lot of worse things to worry about.

A black widow, photographed by S. McCann.

4. There are serious consequences of spider bite misinformation and misdiagnosis

Assuming every unknown lesion is a spider bite can prevent accurate diagnosis, and delay proper treatment. Inappropriate treatment based on the misdiagnosis may be ineffective or worse, harmful. Overzealous diagnoses of spider bites can also lead to arachnophobia and reckless, unwarranted attempts to rid homes of spiders. Unnecessary exposure to pesticides is probably much riskier than sharing your home with spiders.

In sum, we hope this post is seen as a “Public Service Announcement” (or better yet, a “Public Spider Announcement”) and that we can help dispel some myths about spiders. We should be celebrating their incredible biology and natural history, and we should look to spiders as our allies in controlling pest insects, or taking down mosquitoes.

We should look at spiders in awe, rather than in fear.

 

 

Dinner or date?

A comparison of the vibrations transmitted by courting males and ensnared prey in two web-building spider species.

Today, I am excited to publish my first blog post about some of my own spider research! Our paper, “A meal or a male: the ‘whispers’ of black widow males do not trigger a predatory response in females”, has just been published in Frontiers in Zoology (freely available online).

This study is part of the PhD work of my friend and collaborator Samantha Vibert. In fact, we did some of the data collection and analysis for this paper during my very first semester in our lab, when I was working as an undergraduate research assistant. That was when I first began to really look closely at spiders and their incredible behaviour. My experience working with Sam that summer sparked my passion for the complexity and beauty of all of the various aspects of the private lives of spiders, which so often go unnoticed by humans.

Here is a plain-language summary of the paper, written with Samantha Vibert, and with photos by Sean McCann:

Spiders are fascinating but largely overlooked creatures, with sophisticated signalling systems involving chemical, vibratory, tactile, and in some species visual communication. A spider’s web is essentially an extension of her exquisitely tuned sensory system, allowing her to quickly detect and respond to vibrations produced by entangled prey. Not only is the web a highly effective prey-capture device, but it is also the dance floor on which prospective mates must demonstrate their desirability. The first moments after a male spider steps onto a female’s web may present a great risk, since spiders are often cannibalistic. We were interested in how a dancing male spider avoids a potentially deadly case of mistaken identity. One way that he might deal with this challenge is by transmitting vibratory signals that are very different from the vibrations produced by ensnared prey.

webs are good for this

Spider webs are highly effective prey-capture devices, so how does a courting male avoid the fate of flies like this one?

Our study species were the western black widow and the hobo spider, which are both found in British Columbia.

widow web

A western black widow (Latrodectus hesperus) hanging from her tangle-web under a log at Island View Beach on Vancouver Island.

Black widows are in the family Theridiidae, and build complex, three-dimensional tangle-webs, while hobo spiders (family Agelenidae) build dense sheet-webs. Female black widows are much larger than males, while hobo spider males and females are closer in size.

Hobo web

A hobo spider (Tegenaria agrestis) female on her sheet web at Iona Beach, in Richmond, BC.

The purpose of our study was to describe some of the vibratory courtship signals of males in these two species, and to determine which aspects of these vibrations might allow females to discriminate between prospective mates and their next meals.

First, we recorded the vibrations transmitted through the web by courting males in both species using a laser Doppler vibrometer. At the same time, we video-recorded the male’s courtship behaviour. This allowed us to describe and analyze the different kinds of vibrations that were transmitted through the web during specific behavioural elements of each male’s courtship display. We then recorded the vibrations produced by the struggles of two types of common prey insects (house flies and crickets), on both black widow and hobo spider webs.

We found that male and prey vibrations differed more in the black widow than in the hobo spider. Hobo spider male vibrations contrasted with prey vibrations only in terms of their duration – the courting male moves around almost continuously on the female’s sheet web, while prey struggles are generally brief and intermittent. Black widow male courtship vibrations were also longer than prey vibrations on tangle webs (for the same reason), but they were also distinctive based on their generally lower amplitude and higher dominant frequencies.

To our surprise, we also found that most courtship behaviours in both species did not generate the kind of very stereotyped, complex and distinctive “songs” that have been reported in several other spider species. These species tend to court on substrates like leaf litter and plants, which most likely transmit vibrations quite differently than webs. Some male orb-weavers also produce highly rhythmic patterns during their vibratory courtship displays. So our finding leads us to wonder to what extent web architecture and complexity might constrain the transmission of the male courtship signals, and therefore the design of these signals.

One very interesting exception to the rule turned out to be the vibrations generated by the male black widow’s abdomen tremulations (an up-and down waggle of the abdomen, performed as the male hangs upside down from the female’s web). These vibrations were always very distinct from anything produced by prey: they were long-lasting and of very low amplitude, like a constant humming.

Here’s a short video of a male western black widow vibrating his abdomen on a female’s web (Supplemental File 1 from Vibert et. al 2014):

To learn more about these particularly stereotyped, ‘whisper-like’ male signals, we built our own custom web vibrator by modifying a loudspeaker. We were then able to play recorded vibrations of a male’s abdomen tremulation or a fly’s struggles back to females and observe their responses. Black widow females were much less likely to respond aggressively to vibrations played back at the “whisper-like” low amplitude of male abdomen tremulation, but attacked when we turned up the volume to levels typical of prey vibrations. This was the case regardless of which type of vibration we played. So we speculate that the males vibrate their abdomens either to avoid triggering a female’s predatory response, or even to turn it off.

Is it possible that the females that didn’t attack low-amplitude vibrations simply couldn’t detect them? We don’t think so. First, spiders are specialists when it comes to detecting even faint vibrations, and second, some females actually responded with courtship behaviour: abdomen ‘twitches’ which are similar to the male’s abdominal movements, but more emphatic. These abdomen twitches undoubtedly transmit their own vibrations through the web, and it would be very exciting to further investigate the female’s side of the vibratory ‘conversation’ during courtship.

Abdomen vibration seems to be a relatively common type of courtship behaviour and has been described in several spider families (‘abdomen wagging’ in an orb-weaver, and what has recently been described as ‘twerking’ in jumping spiders are a couple of examples). If indeed the “whispers” caused by these vibrations are involved in lowering female aggression, this might explain why such behaviour is fairly common among spiders.

The orb-weaver Argiope keyserlingi’s courthip also involves abdomen vibration, but in this species another vibratory signal was recently implicated in reducing the risk of cannibalism. The ‘shuddering’ of a courting male delays the female’s predatory response. One of the common features of black widow abdomen tremulation and these ‘shudders’ is that they are the first courtship behaviour performed by males after they enter a female’s web.

widow pair

A male western black widow courting a large, potentially dangerous female. Abdomen vibration is performed on and off throughout the male’s courtship display, starting just after the male steps onto the web, and featuring prominently during attempts to approach and mount the female.

Very little is known about the kinds of vibratory courtship signals that male web-building spiders transmit to females through their webs, except for in orb-web weaving species. We hope that this new information about vibratory communication in tangle-web and sheet-web building spiders will contribute to better overall understanding of the function and evolution of web-borne vibratory courtship signals.

References:

Vibert, S., Scott, C., and Gries, G. (2014). A meal or a male: the ‘whispers’ of black widow males do not trigger a predatory response in females. Frontiers in Zoology, 11(4).  doi:10.1186/1742-9994-11-4

Wignall, A. E., & Herberstein, M. E. (2013). The Influence of Vibratory Courtship on Female Mating Behaviour in Orb-Web Spiders (Argiope keyserlingi, Karsch 1878). PloS one8(1), e53057. doi:10.1371/journal.pone.0053057

Wignall, A. E., & Herberstein, M. E. (2013). Male courtship vibrations delay predatory behaviour in female spiders. Scientific reports3doi:10.1038/srep03557

The real house spiders of Vancouver

(and other West Coast cities)

UPDATE: The scientific names used in this post are out of date. The giant house spider is now Eratigena atrica, and the hobo spider is Eratigena agrestis. Tegenaria domestica remains the same!

Living in Vancouver, I frequently hear about a HUGE hairy spider that was trapped in the bath or lurking in the bedroom or scuttling across the kitchen floor. More often than not, it’s described as a wolf spider (family Lycosidae). I’m pretty confident that every single time, the spider being referred to is actually a member of the family Agelenidae, and in particular, the genus Tegenaria. I’m almost as certain that the species in question is Tegenaria duellica, commonly called the giant house spider or European house spider.

NOT a wolf spider. Female Tegenaria duellica (Agelenidae) Photo: Sean McCann*

Sometimes the slightly better informed giant-spider-spotter will have taken a closer look at the spider, or photographed it, then done some online research and concluded that it’s the ‘dangerous’ hobo spider, Tegenaria agrestis. 

Female hobo spider, Tegenaria agrestis. These spiders may also be found in houses on the west coast of BC, but are generally not as large as T. duellica.

There is a lot of flat-out wrong information on the internet about hobo spiders and their supposed ability to cause necrotic lesions. Some people mistakenly refer to T. agrestis as aggressive house spiders. (1) They aren’t. And, (2) agrestis means ‘of the field’. As for ‘dangerous’, arachnologists Robb Bennett and Rick Vetter

“know of no authentic Canadian report of hobo spider envenomation.”

See their paper on the misdiagnosis of spider bites in Canada for more reasons not to fear hobo spiders. Having done a bit of work with Tegenaria agrestis, both in the field (on Vancouver Island) and in the lab, I can personally confirm that they are far more likely to be running away from humans than attempting to bite them (just like other Tegenaria species).

duellica?

A charming female Tegenaria. Neither aggressive nor at all interested in human flesh.

Both hobo spiders and giant house spiders can be found living alongside humans on the west coast. They are not easy to tell apart, but Rick Vetter and Art Antonelli have prepared an excellent identification guide that will tell you at the very least if it’s NOT a hobo spider (not that there would be any cause for alarm if it was one). In a house in southern BC, that leaves you with T. duellica (larger) or T. domestica (smaller).

male T duellica palps

If it has long pointy palps like this guy, it’s NOT a hobo spider, but either T. duellica or T. domestica.

Even without the (fairly recent) hype about the ‘medically significant’ hobo spider, humans seem to generally fear and despise Tegenaria. In The World of Spidersa delightful book in which the author’s passion for his subjects fairly leaps off every page–W.S. Bristowe admitted that he had no affection for house spiders. In their defence, however, he wrote that,

“A Tegenaria cannot deliver a painful bite. Its unpopularity arises from its leg span, its rapid movements and general creepyness.”

Male Tegenaria duellica

Male Tegenaria duellica. Long-legged and disposed to ‘general creepyness’. 

The movement of these spiders is indeed disconcerting. Males are pretty much all leg, and they can run FAST. Bristowe noted that Tegenaria atrica can run a distance equivalent to 330 times her body length in 10 seconds. This means a human-length (let’s say 5’9″, or 1.75 m) Tegenaria could run the 100-metre dash in 1.73 seconds. Pretty speedy.

But apart from their occasionally-startling dashing to-and-fro (mainly in the fall when males go on the prowl, abandoning their webs to search for females), house spiders in general are fabulous to have around. They are great at taking care of all manner of arthropod home-invaders and are really fun to observe.

House spiders, like this Tegenaria that set up shop just inside the door of our lab, help out by snacking on any bugs that might also enjoy living inside buildings.

If you really can’t stand having them in your home, relocate them outside, and with luck they’ll find a nearby crevice in which to build the retreat of their funnel-shaped webs. You probably won’t often catch them out on their silk sheets, as they will run and hide in their retreats at the slightest disturbance.

outside

Usually house spiders wait for prey just inside their tube-like silk retreat. We enjoy attempting to entice this Tegenaria (who lives just outside our lab) to come out and say hi by dropping small insects onto her sheet web. The speed with which she can dart out and snag prey is quite stunning!

If you’re still not convinced that house spiders are good neighbours rather than enemies, you could always try Bristowe’s method for curing fear of Tegenaria: eat one**.

*Sean McCann, a house spider enthusiast, provided all the photographs for this post. He’d like it to be noted that in addition to being incredibly fast, house spiders also *sound* creepy (if you’re lying on the floor and one runs past your head).

**It didn’t actually end up curing his fear of house spiders, but he did win a lot of money as a result.