Pirate spiders

Mimetidae are the pirates of the spider world, but their acts of theivery take place on the webs, rather than ships, of other spiders. The name Mimetidae means “imitator” and is thus a very fitting name for these sneaky spiders.

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A mimetid found at Payne’s Prairie in Florida. Photo: Sean McCann (used with permission).

Common names for this family include pirate spiders and cannibal spiders, for reasons that will soon become clear. They have a worldwide distribution, occurring on every continent except Antarctica, and everywhere in North America except the arctic.

What makes mimetids so fascinating is their predatory behaviour. These spiders don’t build their own webs. Instead, they invade the webs of other spiders – most often spiders in the families Araneidae (orb-weavers), Theridiidae (cobweb weavers), and Dictynidae (mesh web weavers). Here’s a series of photographs showing an interaction between a pirate spider and an orb-weaver in Arizona. (Full disclosure: Sean and I introduced the mimetid ourselves, hoping to witness a predation event).

Below is the web of a trashline orbweaver, Cyclosa turbinata (family Araneidae). The vertical “trashline” that bisects the upper half of the orb is made of old prey carcasses.

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This photo and the rest in this series by Sean McCann.

Here’s a closer look at the trashline. The spider is well camouflaged when she sits right in the centre of the orb-web.

CyclosaHere’s a better view of the spider herself.

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And this is another Cyclosa conica female, for a better idea of what these spiders look like.

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Ok, now back to the pirate spider! This is a male Mimetus hesperus that we found nearby, and introduced onto the yucca right next to the orb-web.

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Soon he entered the periphery of the web and assumed the ‘legs cocked’ posture characteristic of hunting mimetids. He then started carefully plucking the threads of the orb-web with his front pair of legs. This plucking makes the web vibrate in very much the same way it would if an insect had been captured, and resulted in the Cyclosa female orienting toward the source of the vibrations, but remaining in the hub of the web.

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Gradually Mimetus began to move toward the hub of the orb-web, plucking and sometimes even snapping spiral threads (much as would happen if a winged insect was struggling to free itself from the sticky threads). At first it seemed the mimetid was going to be successful in luring the female Cyclosa out onto the web and into its deadly embrace, but after a few steps toward the mimetid she suddenly dropped out of the web on a dragline.

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As Cyclosa hung below, the mimetid made his way to the hub of the web and took up residence. Sean and I were impressed by Cyclosa’s ability to recognize the mimetid as as being dangerous rather than dinner, but disappointed not to see Mimetus succeed in securing a meal. So we put the spider back onto her web. (Sorry Cyclosa!)

As soon as she started moving back toward the hub, Mimetus lunged and bit Cyclosa. Mimetids are equipped with a spider-specific venom that paralyzes their prey almost instantly.  

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The successful pirate then carried its meal back to the periphery of the web to feed. Below you can see that he has Cyclosa by the leg. Apparently mimetids almost always bite the legs of their victims, and when they do paralysis occurs within moments. If they bite another spider’s abdomen, however, the venom takes much longer to work. 

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We left the pirate enjoying his meal on the yucca. He may have gone on to find a new web to invade, or taken over Cyclosa’s web for a while. If we hadn’t interfered, he may have remained in the hub of the web and used it to capture insect prey himself. In addition to this sort of takeover, mimetids are also known to steal prey from the webs of other spiders who are much larger (and thus too big to prey on). They also sometimes eat the eggs of other spiders.

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Mimetus sp. from High Park in Toronto, Ontario. Photo: Sean McCann (used with permission).

Notes on identification:

Mimetids look most similar to orb-weavers (araneids) and cobweb weavers (theridiids) but they can be distinguished from spiders in all other families by the unique pattern of spines on their first two pairs of very long legs.

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Nice example of the characteristic spination on the tibiae and tarsi (first two leg segments) of the first two legs of pirate spiders. Photo: Nicky Bay (used with permission). Check out more of Nicky’s awesome pirate spider photos here.

The eye arrangement is not so diagnostic (it’s quite similar to that of araneids and theridiids) but here’s a great portrait courtesy of the Insects Unlocked project.

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Finally, Mimetids build characteristic egg sacs that are easy to identify to genus even in the absence of the mother (who inevitably abandons her offspring). The sac has a long thin stalk and/or a fluffy coating, depending on the genus, and these two features may help protect the eggs within from parasitoids or predators.

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Egg sac of a mimetid in the genus Ero, found hanging inside a hollow stump in Burns Bog, Delta, BC. Photo: Sean McCann (used with permission)

References and further reading:

Eric Eaton’s blog post on pirate spiders.

Africa Gomez’s blog post on pirate spiders.

Bristowe, W. S. (1958). The world of spiders. London: Collins.

Jackson, R. R., & Whitehouse, M. E. (1986). The biology of New Zealand and Queensland pirate spiders (Araneae, Mimetidae): aggressive mimicry, araneophagy and prey specialization. Journal of Zoology, 210(2), 279-303.

Kloock, C. T. (2001). Diet and insectivory in the “araneophagic” spider, Mimetus notius (Araneae: Mimetidae). The American Midland Naturalist, 146(2), 424-428.

Kloock, C. T. (2012). Natural History of the Pirate Spider Mimetus hesperus (Araneae; Mimetidae) in Kern County, California. The Southwestern Naturalist,57(4), 417-420.

Opportunity makes a thief

Sometimes unexpected things happen when you’re observing spiders. The following series of photos is by Catherine Aitken, who has a wonderful wildlife photography blog: Lardeau Valley Time. She recently witnessed and captured this incredible interaction in her garden, and kindly gave me permission to share her photos here.

Here we see a lovely pink and white flower crab spider (Misumena vatia) peacefully slurping her lunch (an unfortunate hoverfly).

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Photo: Catherine Aitken (used with permission).

But soon an uninvited guest (a foraging western yellowjacket) arrives.

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Photo: Catherine Aitken (used with permission).

A great struggle ensues.

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Photo: Catherine Aitken (used with permission).

The wasp emerges victorious, while the spider retreats.

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Photo: Catherine Aitken (used with permission).

Crab spiders are pretty formidable predators, and I’ve seen them feeding on yellowjackets themselves, as in the photo below. So I found this instance of a wasp stealing a crab spider’s prey rather surprising and fascinating. You never know what wonders you might witness when you spend time watching spiders!

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Xysticus with eastern yellowjacket. Photo: Sean McCann (used with permission).

Castianeira: ant-like spiders

The spider genus Castianeira (in the family Corinnidae) is one of my favourites. These small spiders are rather elusive, but so beautiful! There are currently 128 known species in the genus, so I will only be able to highlight a small number in this post. This should nonetheless provide a glimpse into the diversity of gorgeous forms they take!

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Castianeira dorsata from Arizona. Photo: Sean McCann

Natural History

Common names for the family Corinnidae include “ground sac spiders” (they used to be included in the sac spider family Clubionidae) and “antmimic spiders”. They are not all antmimics, but many species in the genus Castianeira are rather ant-like and are considered generalized ant-mimics.

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Castianeira longipalpa, found under a rock near a lot of ants at Iona beach in Richmond, British Columbia. This species is thought to be a generalized mimic of myrmecine or ponerine ants. Photo: Sean McCann

The species shown above and below don’t look especially like any particular species of ant, and their mimicry is “imperfect” – besides having fairly elongate bodies and stripes that might give the illusion of a third body segment, they look a lot more like spiders than ants. At first glance, however, they can easily be mistaken for ants (at least by humans). They are fairly ant-like in size and colour (often red or brown and/or black) and they move around a lot like ants, waving their front legs like antennae, and bobbing their abdomens in ant-like fashion. These spiders are often found in close proximity to ants, which provides some support for the idea that they are in fact mimics. They might benefit by looking ant-like to predators who find ants distasteful (and don’t look too closely).

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Castianeira sp. from near the Soutwestern Research Station in Arizona. Photo: Sean McCann

Some species, like the one from Singapore below, are a little more ant-shaped, but their morphology is not as extremely modified as some other kinds of ant-mimicking spiders (like this one photographed by Alex Wild).

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Castianeira sp. from Singapore. Photo: H. K. Tang, licensed under CC BY-NC-ND 2.0.

Some Castianeira species are thought to mimic velvet ants (Mutillidae), rather than ants. Mutillids are not actually ants but wasps, and the females are wingless and brightly coloured, with extremely painful stings. In this case, harmless Castianeira spiders might benefit by looking like the much more dangerous velvet ants, and thus be avoided by predators (this is called Batesian mimicry).

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Castianeira occidens from South Fork, Arizona. This spider was found running (fast!) across a forest path during the day. Photo: Sean McCann

As with the ant-like species, these spiders tend not to look very much like any particular species of velvet ant. They are generally mutillid-like in their movements and in that they have bright markings on their abdomens reminiscent of the warning colouration (aposematism) of velvet ants, like the one below.

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Velvet ant from French Guiana. Photo: Sean McCann.

One of the most gorgeous spiders I have ever seen is the Castianeira dorsata (below, and at the top of this post) that Sean found wandering around by a stream one evening while we were staying at the Southwestern Research Station in Arizona. These spiders are supposed to be active during the day, so it was interesting to find this one running around in the dark while we were out with our headlamps searching for wolf spiders. Obviously, looking like a velvet ant isn’t going to fool anyone if it’s so dark they can’t see how brightly coloured you are.

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Castianeira dorsata from Arizona. Photo: Sean McCann

I don’t know of any velvet ants that look much like this (although many are bright orange), but I just can’t get over how beautiful this spider is with its sunset-like stripes on the abdomen and a bluish iridescence on the carapace. Even more dramatic is what I like to call the “tiger-striped” Castianeira below. The photo below shows the relative size of Castianeira amoena on a human hand. I imagine they would be as fearsome or even more so than a tiger if they were blown up to a comparable size. In reality however, these spiders are extremely shy, and very fast runners when they are disturbed (as by humans!) – so all the photographs in this post are real treasures.

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Castianeira amoena. Photo: promiseminime, licensed under CC BY-NC-ND 2.0.

Finally, the spider below is notCastianeira, but a species in the related genus Graptartia (also in the subfamily Castianeirinae). I couldn’t resist adding it because it shows such a beautiful example of mimicry, and one that’s much more specific than the examples above. The velvet ant model and spider mimic where found within a few metres of another (one of the criteria for Batesian mimicry is that the model and mimic have to be found in the same place!) and the photographer then managed to move them so that he could capture them together in the photo below. Wow!

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Graptartia granulosa mimic and velvet ant model from Tanzania. Photo: Paul Bertner (used with permission).

Notes on Identification

Usually these spiders are pretty recognizable because of their distinctive colouration, but some of the less bold ones can be confused with other ant-like spiders (like Micaria). Spiders in the genus Castianeira have 8 eyes in 2 rows. The posterior eye row (the upper row, in the photo below) is slightly wider than the anterior (lower) row with all four eyes about the same size. Both rows are slightly procurved (curved toward the front end of the spider). The anterior median eyes (front and centre) can be slightly smaller to much larger than the other two in their row.

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Castianeira cingulata portrait, showing the eye arrangement. Photo: sankax, licensed under CC BY-NC 2.0.

The last (hindmost) pair of legs is always longest, followed by the first (frontmost) pair, and the abdomen is often decorated with bands of white scale-like setae (hairs).

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Dorsal veiw of Castianeira longipalpa from BC. Photo: Sean McCann.

 

References:

Dondale, C. D., & Redner, J. H. (1982). The insects and arachnids of Canada. Part 9. The sac spiders of Canada and Alaska. Araneae: Clubionidae and Anyphaenidae (No. 1724).

Reiskind, J. 1969. The spider subfamily Castianeirinae of North and Central America (Araneae, Clubionidae). Bull. Mus. Comp. Zool. 138(5): 163-325.

Segestriidae: tube web spiders

Recently Sean and I took a quick trip down to Bellingham, Washington, to meet up with a friend. While we were wandering around a beachfront park, we encountered this beautiful spider under the loose bark of a tree. I didn’t recognize it, and we didn’t have our field guide with us, so it wasn’t until we got home that we were able to identify it as Segestria pacifica – a member of the tube web spider family Segestriidae.

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Segestria pacifica. This species is the one member of the family Segestriidae that can be found in British Columbia, but we’ve never encountered it before. (Photo: Sean McCann)

Segestriids are closely related to spiders in the family Dysderidae (which includes the common woodlouse hunter, Dysdera crocata). Like dysderids, segestriids have only 6 eyes. You can see the characteristic eye arrangement in the photo below – it looks like the pair of eyes that should be at the front and centre of the spider’s face are missing.

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Nice portrait of Segestria pacifica showing the eye arrangement and hairy chelicerae. (Photo: Kyron Basu, licensed under CC BY-ND-NC 1.0)

Spiders in this family also have the unusual habit of resting with their first three pairs of legs pointing forward and the last pair pointing back (most spiders do two pairs pointing forward and two pairs pointing back).

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Segestria pacifica resting in the characteristic pose with six legs forward, two legs back. (Photo: Sean McCann)

Segestria florentina (sadly not found in North America) has beautiful irridescent green chelicerae. The function of this striking colouration is not clear – it’s unlikely to be for catching the eye of a potential mate, because these spiders have poor vision and rely mainly on vibratory and acoustic communication. What we do know is that this structural colour is produced by parallel layers of chitin that reflect different wavelengths of light (called a multilayer reflector). The mechanism is the same one that gives these beautiful beetles their green irridescence.

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Segestria florentina female showing off her beautiful irridescent green chelicerae (Photo: Luis Miguel Bugallo Sánchez, licensed under CC BY-SA 2.5 ES)

Segestriids are nocturnal and build their tube webs in crevices, often in the cracks of rock walls, under loose tree bark, or in the ends of broken branches. Several signal threads are arranged radially around the opining of the tube-web. The hunting spider sits near the entrance of the tube, waiting for prey to make contact with one of her trip lines. This contact transmits vibrations through the silk to the spider’s sensitive feet, six of which rest near the opening of the web, allowing her to determine the exact location of the prey.

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Tube-web of Segestria senoculata (Photo: Totodu74, licensed under CC BY-SA 3.0)

“Corolla spiders” in the genus Ariadna live in the Namib Desert, and have modified their tube webs to include a circle of stones around the entrance. Here the trip lines are very short, and are all attached to the small stones circling the web entrance. The function of the stones is essentially the same as the silk signal lines of a regular tube web, which would not be very effective in the desert because of the constantly shifting sands and gravel. The stone circle solves this problem. When prey brushes against one of stones in the circle, vibrations are transmitted to the spider and it rushes out to dispatch them. The corolla spiders apparently preferentially select quartz crystals for their signal stones – these may direct vibrations from prey more effectively than other kinds of stones.

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Fig. 1 from Henschel (1995) showing the stone circle built by corolla spiders in the Namib desert. The drawing shows the position of the six forward-facing legs of the hunting spider resting just inside the mouth of the burrow.

Females in this family don’t build a traditional egg sac but deposit a mass of eggs in the tube web and then cover them with silk. Very little else seems to be known about the natural history of these spiders, although apparently they are quite easy to keep in captivity, and females can live for several years.

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References:

Adams, R. J. (2014). Field Guide to the Spiders of California and the Pacific Coast States. University of California Press.

Henschel, J. R. (1995). Tool use by spiders: stone selection and placement by corolla spiders Ariadna (Segestriidae) of the Namib Desert. Ethology, 101(3), 187-199.

Ingram, A. L., Ball, A. D., Parker, A. R., Deparis, O., Boulenguez, J., & Berthier, S. (2009). Characterization of the green iridescence on the chelicerae of the tube web spider, Segestria florentina (Rossi 1790) (Araneae, Segestriidae)Journal of Arachnology, 37(1), 68-71.

 

Web reduction for rival obstruction!

This post is about a new paper just published online in the journal Animal Behaviour, titled Web reduction by courting male black widows renders pheromone-emitting females’ webs less attractive to rival males by Catherine Scott, Devin Kirk, Sean McCann, and Gerhard Gries! You can read the full text here (free access until 28 August, 2015).           All photos and video are copyright Sean McCann

This short video shows a male western black widow engaged in web reduction behaviour – a common element of the complex courtship rituals males perform on females’ webs. You can see him cutting some silk lines, then pulling silk out of his spinnerets with his last pair of legs, wrapping it around a bundled up section of the female’s web.

Web reduction behaviour is somewhat puzzling. This male black widow is attempting to convince a potentially cannibalistic female several times his size to mate with him. Destroying large areas of her home – which she relies on for both prey capture and protection from predators – is not the most obvious approach. So why do males do it (I know, the title of the paper probably gives this one away), and why do females let them?

Before we address the mystery of web reduction, let’s take a step back and set the scene for this story. It’s a pretty juicy one – there’s attraction, courtship, rivalry, and manipulation! Or maybe not manipulation… we’ll see!

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Black widows are sexually dimorphic: the familiar female is on the left, and the much smaller and more brightly coloured male is on the right. My study species is the western black widow, Latrodectus hesperus.

Island view beach, on the Saanich Peninsula of Vancouver Island, BC, is a beautiful place to visit. The site shown below is on the lands of the Tsawout First Nation, who have kindly allowed me to collect spiders and do field work here over the past few years.

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This coastal sand dune ecosystem supports a great diversity of organisms, but the black widows are the dominant web-building spiders. Females build their tangle-webs under the driftwood logs at a density of 2-3 webs per square metre of available habitat. There are many logs on this beach; correspondingly, there is a huge population of widow spiders.

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At our field site, black widows build their webs under driftwood logs – often several females can be found living under a single piece of wood!

Female black widows almost never leave their webs, so when it comes time to mate, they need males to come to them. The solution to this problem comes in the form of pheromones on their silk. These chemical messages are kind of like scent-based personal ads, that provide the male with information about the female’s mating status and whether or not she’s well fed (sexual cannibalism is rare in this species, but starving females will sometimes eat males!).

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If the female’s silk pheromone was actually a personal ad, it might say something like this!

Adult male black widows have only one goal in life: find a female to mate with (and become the father of as many offspring as possible). Once they mature, males stop capturing prey and abandon their webs for a more a nomadic lifestyle. If they detect an attractive female’s pheromone in the air, they follow it to her web.

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A nomadic western black widow male traverses the sand dunes in search of a mate. Note: “pheromone trail” added for dramatic effect.

Once he arrives on the female’s web, the male begins an elaborate courtship display, dancing on the web to transmit vibratory messages (including “male, not meal!”) to the female. It may take many hours of courtship before copulation finally occurs.

male_courtingSoon after he arrives, the male may start to destroy the female’s web. He cuts out sections, bundles them up, & wraps them in his own silk. Males usually reduce the area of the web by about 50%, and gather the destroyed sections up into loose silk-wrapped packages. These can be rope-like (as in the photo at the top of this page) or a tighter ball that has been wrapped extensively by the male like the one below.

silk_ball_smallNow to address our earlier question: why do the males engage in this behaviour? 

Well, black widows are not the only spiders who do web reduction. Other species in the same family (Theridiidae) and others (including Linyphiidae) have similar behaviour. Watson showed in 1986 that web reduction in the sierra dome spider makes the female less attractive to rival males. He concluded that by bundling up the female’s web, the male decreases the surface area from which the silk-borne sex pheromone is released. Some researchers have assumed that it works the same way in other species. Others have suggested that web reduction could function in communication between the male and the female, perhaps improving vibration transmission through the web, or transmitting a male silk pheromone to the female. It could have one or both of these functions in black widows, but until now, no one had ever investigated!

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The sierra dome spider, Neriene litigiosa (family Linyphiidae).

We set out to determine whether web reduction decreases female attractiveness to male black widows in a natural setting. We were pretty sceptical that reducing the web surface area by only about 50% could limit pheromone emission. Usually you have to decrease the dose of a pheromone by an order of magnitude to see any difference in attractiveness. However, males aren’t just bundling up the female’s pheromone-laden silk – they are also adding their own. We thought maybe it could be the addition of the male’s silk (and associated pheromones) that keeps other males away. We designed an experiment to try to find out.

First, we put a bunch of female spiders in cages and allowed them to establish webs. Then we loaded up the cages and took them, and a batch of males, to our field site.

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This is what (part of) a truckload of black widows looks like. Here, my coauthor Devin is loading the cages containing females and their webs into the back of the lab pickup.

Once we got to our field site, we removed the female spiders from their cages (because we wanted to look at the attractiveness of silk only, not the females themselves) and set up four treatments: intact webs, mechanically reduced webs (with half the silk cut out), male-reduced webs (with about half the silk, on average, bundled and wrapped by the males we brought) and empty cages as controls (to confirm that captured males were actually attracted to the silk in the cages, not just wandering randomly).

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Four treatments: intact web, scissor-reduced web (50% of silk removed entirely), male-reduced web (courting males bundling up about 50% of silk on average), and no-web control.

We then turned the cages into traps that would capture any wild males attracted to the silk inside by surrounding them with sticky strips. Then we set the traps out on the beach in groups of four – one for each of the treatments.

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The experimental setup. Each of the four traps contains a different treatment, and the white sticky strips surrounding the cages trap males that are attracted to the silk inside the cage.

We set the traps out at sunset (black widows are nocturnal) and waited to see what would happen. Soon the data started strolling in. We checked the traps every three hours, collecting and preserving any captured males. The sheer number of males out on the prowl was incredible – some webs attracted more than 10 males overnight!

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Close-up of a trap containing a pheromone-laden female’s web, with a male black widow captured outside on the sticky strip.

After 24 hours, just by looking at the vials full of male spiders we had captured, the results were clear (if you want to see statistics, you can read the paper!). Male-reduced webs caught only about one third as many males as intact webs, so web reduction does in fact decrease attractiveness! As we suspected, however, removing half of the silk entirely did not significantly reduce a web’s attractiveness – we captured almost as many males outside scissor-reduced webs as intact webs. (A few spiders got trapped outside the empty control cages – they may have blundered into the sticky strips on their way toward an an attractive web.)

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Beautiful data. It’s not often that the raw data tell the whole story, but here they do!

Evidently, when a male black widow reduces a female’s web, whatever he is doing is much more effective at decreasing its attractiveness than removing half of the pheromone-laden silk entirely. And he’s not actually removing any of the female’s silk – he’s just bundling it up into a ball. There are a couple of potential explanations for how web reduction works. Perhaps the female’s pheromone is not evenly distributed on the web, and the male targets the pheromone-rich silk for web reduction. Then, by wrapping those sections up in his own silk, he creates a barrier that limits the emission of the female pheromone. Another possibility is that the male’s silk has its own pheromone on it, one which other males detect and avoid. Or it could be a combination of both these mechanisms – we’re still not sure. We did another experiment to test the second idea, but the results neither supported nor completely ruled it out (see the paper for more details!). We will try to get to the bottom of this in the future.

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Time for a cute male widow interlude! Look at him peeking out from behind that blade of grass.

For now though, let’s come back to the questions we set out to answer. Why do male black widows do web reduction? It allows them to monopolize the female, by making her web less attractive to other males. Courtship and mating last several hours, so if a male reduces the female’s web as soon as he arrives, he can decrease the likelihood of rival males arriving and interrupting. This may be very important at our field site, where competition for access to females appears to be fierce – during our second experiment, we had one intact web attract over 40 males in a single night! So web reduction is good for the male, because it helps him to avoid competition.

What about the female? Losing her web may be costly – she has to spend time and energy rebuilding it. However, we think she might actually benefit from web reduction too, and that the benefits may outweigh the costs. Sure, her web is important for prey capture and protection, but it’s also really attractive. So attractive, in fact, that even if she doesn’t add more pheromone, it will continue emitting its “come-hither” message for several days. Given number of males we saw arriving at each web during our experiment (40 in one night! even 10 is pretty extreme!), remaining attractive once she has already found a mate might not be so great. Having a choice between multiple males might be a good thing, but the female really only needs to mate once to fertilize all of her eggs. By “muting” her chemical signal though web reduction, the male might be doing her a favour: allowing her to rebuild her web without attractive pheromones (female sex pheromone production shuts off immediately after mating in black widows, but they don’t ever take down their existing webs). Rather than having to waste time and energy chasing off superfluous suitors, this may allow her to get on with the business of producing egg sacs!

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Female western black widow guarding her egg sac. These spiders are very protective mothers!

References and further reading (also linked in the text)

Baruffaldi, L., & Andrade, M. C. (2015). Contact pheromones mediate male preference in black widow spiders: avoidance of hungry sexual cannibals? Animal Behaviour, 102, 25-32.

MacLeod, E. C., & Andrade, M. C. (2014). Strong, convergent male mate choice along two preference axes in field populations of black widow spidersAnimal Behaviour, 89, 163-169.

Salomon, M., Vibert, S., & Bennett, R. G. (2010). Habitat use by western black widow spiders (Latrodectus hesperus) in coastal British Columbia: evidence of facultative group living. Canadian Journal of Zoology, 88(3), 334-346.

Stoltz, J. A., McNeil, J. N., & Andrade, M. C. (2007). Males assess chemical signals to discriminate just-mated females from virgins in redback spidersAnimal Behaviour, 74(6), 1669-1674.

Watson, P. J. (1986). Transmission of a female sex pheromone thwarted by males in the spider Linyphia litigiosa (Linyphiidae). Science, 233(4760), 219-221.

A tiny tarantula

This is another post about spiders that Sean McCann found and photographed on his recent trip to Guyana

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When you think of tarantulas (mygalomorph spiders in the family Theraphosidae), the first thing that comes to mind is probably a huge hairy spider like the famous face-sized Goliath bird-eating spider.

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Sean with a male goliath bird eating spider on his head in Guyana. Note: don’t try this at home. Photo: Jonathan Meiburg.

But not all tarantulas are large! Here is a tiny tarantula from the rainforest of Guyana.

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To get a better idea of the size of this spider, here’s a shot of Sean’s traveling companion, Jonathan Meiburg, photographing the little beastie.

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This tarantula is less than 2 cm long and likely not yet mature, but even full-sized it will be less than 3 cm long.

We suspect that this is a Neostenotarsus species, but there is not much information available about these tarantulas. What I have been able to find indicates that they are fairly well known in the pet trade, but information on their biology is lacking.

UPDATE: Thanks to Stuart Longhorn for informing us that this spider is most likely in the genus Catanduba, not Neostenotarsus (likewise for the images linked above).

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The wee spider strikes a defensive pose.

The size and colour of this tarantula reminded Sean of a velvet ant when he first saw it.

IMG_2662What do you think? 

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A velvet ant (actually a wingless wasp!) also found in Guyana.

 

Who eats spiders?

Spiders are awesome predators. Although there are a few exceptions, spiders are all professionals when it comes to eating other animals. But spiders have predators too! Who eats spiders? You might be surprised.

Birds are important predators of spiders, particularly in forest canopies.

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A white-throated toucan eating a large spider in Guyana. Photo: Sean McCann.

Insectivorous birds such as great tits eat a variety of arthropods including insects and spiders. Early in their nesting season, spiders comprise up to 75% of the food great tit parents bring to their chicks!

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Great tit (Parus major) carrying a false black widow spider – probably a meal for one of its offspring. Photo: Cliff, used with permission.

Spiders are also major predators of other spiders. Some wolf spider (family Lycosidae) species may even be some of their own most important predators. Wandering hunters like wolf spiders and jumping spiders tend to be opportunistic predators and will take most anything they come across, including other spiders.

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Wolf spider (Lycosidae) eating a female pholcid that was carrying her egg sac – a nutritious bonus snack! Photo: Sean McCann

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Another wolf spider (Lycosidae) eating a ground spider (Gnaphosidae). Photo: Sean McCann

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A large female jumping spider (Phidippus) eating a much smaller zebra jumper (Salticus scenicus) Photo: Sean McCann

Some species even specialize on other spiders! Cellar spiders (Pholcus phalangiodes) not only capture insects and spiders using their own webs, but also enter the snares of other spiders and trick the owners into thinking they are prey by vibrating the silk lines. When the resident spider approaches what it thinks will be its next meal, it soon becomes dinner itself. The cellar spider’s incredibly long legs allow it to keep a safe distance while it subdues much larger spiders by wrapping them with silk.

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Cellar spider (Pholcus phalangiodes) wrapping a much larger spider (possibly a lycosid) with silk in preparation for a very filling meal. Photo: Karla Thompson, used with permission.

Perhaps the most fearsome enemies of spiders are spider wasps in the family Pompilidae. Huge ‘tarantula hawks’ in the genus Pepsis (they can be up to 8 cm long with a 10 cm wingspan!) can take even very large tarantulas. First the wasp stings the spider, paralyzing it almost immediately. But things get much, much worse from there.

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Tarantula hawk (Pepsis sp.) in French Guiana. This enormous wasp is 3 inches (8 cm) long! Photo: Sean McCann.

The wasp drags the paralyzed tarantula into a burrow, and lays a single egg on its abdomen. The spider remains paralyzed and trapped underground until eventually it meets its gruesome demise when the wasp larva emerges and begins to eat it alive.

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A spider wasp dragging its victim (a paralyzed tarantula) into a burrow where it will become food for a baby wasp. Photo: David Crummey, licensed under CC BY 2.0

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Poor spider! Photo: David Crummey, licensed under CC BY 2.0

So birds, spiders, and wasps are regular spider-eaters, but many others animals also partake now and then. Here are a few occasional predators of spiders: 

A toad’s diet may be up to 5% spiders.

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Southern toad, Anaxyrus (Bufo) terrestris, eating a spider. Photo: Scott Beazley, licensed under CC BY 2.0

Lizards also eat spiders. In the Bahamas and the Caribbean, islands without anoles have 10 to 30 times as many spiders as islands without these lizards, suggesting that they can be rather important predators.

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A lizard eating a spider. Photo: Jo Garbutt, licensed under CC BY 2.0.

Monkeys and humans also sometimes eat spiders! Fried tarantulas are a delicacy in some places in the world, including Cambodia. Although entomophagy (eating insects) is becoming popular as a more sustainable alternative to meat, I have to say that I’m not a big fan of the idea of eating tarantulas, which are slow-growing and long-lived creatures.

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Fried tarantulas in Phnom Penh, Cambodia. Photo: Matthew Stevens, licensed under CC BY-NC-ND 2.0.

Finally, fish, bats and shrews also eat spiders occasionally. This certainly isn’t an exhaustive list, but I hope it gives you an idea of the range of animals that eat spiders. Stay tuned for future posts on some of the surprising things that spiders eat!

References

Edgar, W. D. (1969). Prey and predators of the wolf spider Lycosa lugubrisJournal of Zoology, 159(4), 405-411.

Foelix, R. (2010). Biology of spiders. Oxford University Press.

Gunnarsson, B. (2007). Bird predation on spiders: ecological mechanisms and evolutionary consequences. Journal of Arachnology, 35(3), 509-529.

Jackson, R. R., & Brassington, R. J. (1987). The biology of Pholcus phalangioides (Araneae, Pholcidae): predatory versatility, araneophagy and aggressive mimicry. Journal of Zoology, 211(2), 227-238.

Naef‐Daenzer, L., Naef‐Daenzer, B., & Nager, R. G. (2000). Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability. Journal of Avian Biology, 31(2), 206-214.

Nyffeler, M., & Knörnschild, M. (2013). Bat predation by spiders. PLOS ONE, 8(3), e58120.

 

How to tell if a spider is not a brown recluse

I’ve been meaning to make some posts with information on how to identify common spiders for a while, and I will start working on these soon. In the meantime, this post will address one of the most common spider identification questions in North America (north of Mexico): is it a brown recluse?*

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A female brown recluse, Loxosceles reclusa. Photo: Alex Wild, used with permission.

The brown recluse spider (Loxosceles reclusa) is arguably the most feared and most misunderstood spider species in North America. So, here we will find out how to tell if a spider is not a brown recluse. But before we do, it’s important to note that even if you do find a brown recluse, it’s not that big a deal.

Arachnologist Rick Vetter is an expert on the brown recluse spider who has done a ton of research on where they are (and aren’t) and how dangerous they really are (hint: not as dangerous as you think), as well as spending a lot of time dispelling myths and misconceptions held by both the public and the medical community. His website is full of excellent resources, and is the source of most of the information here. I encourage you to peruse his website and the articles linked in this post yourself, but I will highlight a few of the more compelling reasons that the brown recluse hysteria is unwarranted.

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Exhibit A: brown recluse bites are rare even where the spiders are abundant.

A family lived in a house full of brown recluses (more than 2000 of them!) for half a year and not a single bite occurred. Even in places where brown recluses are common, bites are very rare. Of those rare bites the vast majority of bites can be effectively treated with RICE (rest, ice, compression, and elevation) without any dire consequences. The small percentage of bites that are very serious are the ones that get all the attention in both the medical literature and the media, which has led to the misconception that recluse bites are always severe, require hospitalization, result in extensive scarring, and so on. Furthermore, misdiagnoses of all manner of other (more serious) conditions as brown recluse bites are rampant throughout North America (even in areas where the spiders do not occur), adding fuel to the already raging fire.

Now that all that’s out of the way, here is a series of questions to determine if a spider is NOT a brown recluse**. (Some of these may seem silly, but many of the spiders below that are not brown recluses are regularly misidentified as brown recluses by non-experts. Many people aren’t aware just how many different kinds of spiders there are, and for some, seeing brown, any marking vaguely reminiscent of a violin, and 8 legs is enough to conclude that a creature as a brown recluse.)

For those who want to skip the fine print, if the answer to question 1a or 1b is “yes”, it’s probably not a brown recluse. If the answer to any one of the remaining questions is “yes”, it’s definitely not a brown recluse. 

1a. Are you in Canada (or Alaska)?

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The range of the brown recluse spider does not extend into Canada. If you are in Canada, you are extremely unlikely to encounter a brown recluse spider. (Also see notes under 1b.)

1b. Are you in a state outside of the range of the brown recluse?

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Here is a map of the known ranges of all of the species in the genus Loxosceles in North America. If you are anywhere outside the red-outlined region, you are very unlikely to encounter a brown recluse.

Brown recluses are occasionally found outside this range – sometimes they hitch a ride with people moving around the country (in boxes that have been stored in basements, for example). But even in these cases they will typically remain in the building into which they are introduced because they are very poor dispersers. Just because one or a few brown recluse spiders have been found in a new area does not mean that their range has expanded or that they are abundant there. (Note: A brown recluse spider bite diagnosis in an area outside their range does NOT mean that brown recluse spiders have been found there. Many doctors erroneously diagnose spider bites in the absence of any evidence, namely a spider that has been identified as the culprit.)

2. Is it on a web out in the open?

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Some brown web-building spiders that are not brown recluses. Clockwise from top left:       common garden spider (Araneus diadematus) on orb-web, dome-web spider male (Neriene radiata) false widow spider (Steatoda grossa) on cobweb giant house spider (Eratigena atrica) on funnel-shaped sheet web. Photos: Sean McCann.

If you find a brown spider on a web out in the open, it is not a brown recluse. Unlike the various brown web-building spiders shown above, each with their different types of web, brown recluse spiders do not use silk for prey capture. They do build small irregular silk retreats in which they hide during the day. These retreats are made low to the ground and out of sight in cracks and crevices or under objects like rocks.

Update (8/06/2015): I should mention that house spiders in the family Agelenidae are probably the most likely spiders to be mistaken for brown recluses in Canada. While females will usually be found on their webs, males are often found out and about when searching for females. They all look pretty similar to the one pictured below, but see this post for more information house spiders and hobo spiders.

Eratigena atrica

Female giant house spider (Eratigena atrica – formerly Tegenaria duellica). These spiders are often mistaken for recluses, but note the pattern on the abdomen. Photo: Sean McCann.

3. Does it have stripy or spiky legs, or more than one colour on its abdomen?

Stripy_legs

Stripy legs + patterned abdomen = not a brown recluse. Photo: Sean McCann.

If you find a spider that has stripes or large spines on its legs, it is not a brown recluse. If it has a patterned abdomen, it is not a brown recluse.

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Stripy legs with large spines + patterned abdomen = not a brown recluse. Photo: Sean McCann.

Brown recluses have plain brown abdomens and plain brown legs with fine hair but no large spines.

4. Does it have extremely long and skinny legs?

Pholcus_phalangiodes

Cellar spider, Pholcus phalangiodes (family Pholcidae). The dark spot on the cephalothorax looks a bit like a violin,  but do not be fooled. This is not a brown recluse. Photo: Sean McCann

If it has extremely long skinny legs like the spider in the image above, it is a cellar spider (or daddy-longlegs), not a brown recluse. Despite looking very dissimilar to brown recluses, these spiders are often mistaken for brown recluses because of the “violin” mark on the back. Having a violin-shaped marking is not, by itself, a good way to determine if a spider is a brown recluse.

4. Is it really big? 

Brown recluses are not huge spiders. If its body length (not including legs) is more than 0.5 inches or about 1.25 cm, it’s definitely not a brown recluse.

5. Does it have 8 eyes? 

This is the dead giveaway, provided you are close enough to the spider to count its eyes. If it has 8 eyes (like most spiders), it is not a brown recluse. Below are some 8-eyed spiders that are sometimes mistaken for brown recluses.

Fishing spider

Fishing spider (family Pisauridae) actually a close relative of fishing spiders in the family Trechaleidae, with 8 eyes. Also see: stripy, spiky legs. Photo: Sean McCann.

 

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A wolf spider (family Lycosidae). Wolf spiders have 8 eyes and spines on the legs. Photo: Sean McCann.

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A sac spider (family Clubionidae) A ground spider (family Gnaphosidae, genus Drassodes). It looks a bit like a brown recluse, but again, has 8 eyes, some larger spines on the legs, and a dark stripe on the abdomen. Photo: Sean McCann.

 

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Huntsman spider (family Sparassidae). These spiders are fairly frequently mistaken for brown recluses. Note the 8 eyes in 2 rows, and spines and darker dots on the legs and abdomen. Photo: Sean McCann.

Update (12/06/2015): Another 8-eyed spider that can easily be mistaken for a brown recluse (and is common in the southern states) is the male southern house spider. It has a similar violin-like marking on the back, but several other features that distinguish it from brown recluses. The 8 eyes are all tightly clumped together, it has conspicuous spines on the legs, and its pedipalps (the two small leg-like appendages at the very front end of the spider) are extremely long and stick straight out in front of its face (compare to a male brown recluse spider here).

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Male southern house spider (Kukulcania hibernalis). Photo: Sam Heck, licensed under CC BY-NC 2.0.

Some other spiders that are not brown recluses, like the woodlouse hunter Dysdera crocata, also only have only 6 eyes, but they are arranged differently (not to mention D. crocata is red or pinkish or orangish in colour, not brown).

Dysdera crocata on white

Female woodlouse hunter, Dysdera crocata. Her 6 eyes are all in a tight bunch in the centre of the cephalothorax, and her massive fangs are much larger than those of a brown recluse. Also, these spiders are not brown. Photo: Sean McCann.

Brown recluses only have 6 eyes, arranged in 3 pairs.

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This is a brown recluse. It has only six eyes. Also note the fine hairs on the legs, but no spines, and the plain brown abdomen. Photo: Alex Wild, used with permission.

If you answered no to all those questions (or all but questions 1a and 1b and you’re really lucky!) AND the spider looks just like the one in the image above, then you’ve found a brown recluse. If not, then it’s another kind of spider that is totally harmless. (The only other medically significant spiders in North America are black widows). Either way, please remain calm. Spiders are not out to get you, and will leave you alone if you leave them alone. Here are some tips for avoiding brown recluse bites if you do live within their range. Still not sure about any of this? Please feel free to tweet at me (I’m @Cataranea on twitter) or comment here if you have any questions and I’ll be happy to try to answer them.

 

*I’ve also started answering this question on twitter with the hashtag #notabrownrecluse. This campaign, with the goal of educating people about the brown recluse spider, is a blatant ripoff of inspired by wildlife biologist David Steen (he’s @AlongsideWild on twitter), who tweets snake identifications using the hashtags #NotACottonmouth and #NotACopperhead. For more about his awesome twitter outreach, check out this excellent article: ‘This snake scientist is the best biologist on twitter‘.

**This guide is based on the following resources:

Vetter, Rick. (1999). Identifying and Misidentifying the Brown Recluse Spider. Dermatology Online Journal, 5(2). link

Vetter, Rick. (2009). How to Identify and Misidentify a Brown Recluse Spider. Web Resource. link

Pseudoscorpions! Small, strange arachnids

Yes, this is a blog about spiders, and no, pseudoscorpions are not spiders. But they are members of the class Arachnida, like spiders, and fascinating, like spiders! I encountered them for the first time on a recent trip to the Okanagan with Sean, so here’s a post about some of their natural history.

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Two tiny pseudoscorpions Sean and I found while doing some rock flipping on a hillside near Vaseux Lake, BC. Photo: Sean McCann

Pseudoscorpions are really weird, and really awesome, creatures. Their name means “false scorpion” (which is also a common name for this order of arachnids), because superficially they look a lot like scorpions (members of another arachnid order), minus the “tail” with its stinger on the end.

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Drawings of a pseudoscorpion and a scorpion from JH Comstock’s book. The illustrations are most likely by Anna C. Stryke, but possibly by Mrs. Comstock. Note that the scale is different for each drawing.

Pseudoscorpions are tiny. The one below (photographed under a microscope) is only about 1 mm long! The largest pseudoscorpions can get as long as 10 mm. Their small size means they can live in tight spaces, like between floorboards, under tree bark, or even under the elytra (hardened forewings) of beetles – but more on this later.

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Pseudoscorpion found in Vero Beach, FL. Photo: Sean McCann

Pseudoscorpion morphology is strange. Like other arachnids, pseudoscorpions have two main body segments: a cephalothorax (the front part, a combined head-and-thorax bearing all of their appendages), and an abdomen. They may have one or two pairs of simple eyes on the sides of the cephalothorax (or none at all) and their vision is generally poor. As well as four pairs of legs, they have enormous chelate (pincer-like) pedipalps that they use for capturing prey and sensing their environment.

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A neobisiid pseudoscorpion nicely displaying its chelate jaws and pedipalps. Photo: Marshal Hedin, licensed under CC BY-NC 2.0.

Order Pseudoscorpionida

Pincer-like chelicera (singular of chelicerae) of a pseudoscorpion, bearing spinnerets on the movable finger. From Comstock’s book.

 

Their jaws (called chelicerae) are also like miniature pincers. Like spiders, pseudoscorpions can produce silk. Unlike spiders, who have abdominal silk glands and spinnerets, pseudoscorpions’ silk glands are in their cephalothoraxes, and their spinnerets are on the tips of their chelicerae! (The spitting spiders in the family Scytotidae are exceptional among spiders in also having silk glands in their cephalothoraxes, and “spitting” the silk out of their fangs along with venom.)

Pseudoscorpions use silk to build retreats* or cocoons for moulting, overwintering, and sometimes brooding their young.

Also like spiders, many pseudoscorpions use venom to subdue their prey, which includes mites and other tiny arthropods. The venomous pseudoscorpions are in suborder Iocheirata, which means “poison hands”. Their venom glands are in their modified pedipalps, with openings in the tips of one or both of the fingers of their claws.

 

To me, pseudoscorpion anatomy is all topsy-turvy (at least compared to spiders):        their silk comes out the wrong end and their venom comes out of clawed “hands” instead of fangs. 

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Pseudoscorpion – modified from the illustration in Comstock’s book.

Pseudoscorpions also have pretty strange reproduction. Males deposit a spermatophore (a package of sperm) on the ground, which a female must then pick up and insert into her reproductive opening. Males of different kinds of pseudoscorpions have various methods of ensuring that a female finds and uses their sperm. Some are carefree: the male deposits the spermatophore, walks away and (figuratively) crosses his fingers and hopes that a female will encounter it by chance. Other tactics are rather more direct and reliable: the male engages the female in an elaborate “mating dance” and eventually pulls her over his spermatophore to ensure that she picks it up. In one species, Serianus carolinensis, males only produce spermatophores when a female is nearby, then they spin special silk webs that direct her to the package.

Once a female’s eggs are fertilized, she keeps them in a brood pouch under her abdomen (or sometimes next to her in a silken brooding chamber). This brood-sac contains food for the developing pseudoscorpion embryos, which grow and moult into protonymphs (juveniles that look just like adults, but smaller) before emerging.

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A female Arctic pseudoscorpion, Wyochernes asiaticus, with her brood pouch. Photo: Crystal Ernst (used with permission)

Pseudoscorpions love books! Although I would like to think the little pseudoscorpion in the photograph below enjoys reading, what they really like about books is the booklice that sometimes live in them. Because pseudoscorpions can sometimes be found living between the pages of books and feeding on booklice, one common name for them is “book scorpions”.

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“Book scorpion” enjoying some literature while waiting for booklice. Photo: Sean McCann

Finally, pseudoscorpions are hitchhikers! Because they are so small and don’t fly, pseudoscorpions can’t get very far on their own. To overcome this obstacle, they hitch lifts on other organisms – usually larger arthropods like beetles and flies. This particular kind of symbiosis – in which the individual doing the carrying is apparently unharmed – is called phoresis.

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A longhorn beetle, Xylotrechus sagittatus, with phoretic pseudoscorpions hitching a ride on its leg. Photo: Sean McCann

Phoresis is a fantastic word that comes from Greek: phor means “carry, bear; movement” but it can also mean “thief”. Phoretic pseudoscorpions latch onto the bodies of their transportation with their pincers to steal a free ride. Some pseudoscorpions are stowaways under the elytra of comparatively gigantic harlequin beetles, and feed on phoretic mites and find mates while they travel. Piotr Naskreki has a wonderful blog post with pictures of these tiny ecosystems on a beetle’s back.

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A closer view of the cluster of tiny travelers. Photo: Sean McCann

Want to find out more about pseudoscorpions? Of course you do! Here are some references and further reading from around the web:

10 Facts about Pseudoscorpions. Fantastic blog post by arachnologist Chris Buddle, who also created this great photographic key to the pseudoscorpions of Canada.

Comstock JH (1912). THE SPIDER BOOK: a manual for the study of the spiders and their near relatives, the scorpions, pseudoscorpions, whip-scorpions, harvestmen, and other members of the class Arachnida, found in America north of Mexico, with analytical keys for their classification and popular accounts of their habits. Doubleday, Page & Company. (Available from the Biodiversity Heritage Library, with wonderful illustrations)

Harvey MS (2013). Pseudoscorpions of the World, version 3.0. Western Australian Museum, Perth.

Pseudoscorpions page on the Massey University Guide to New Zealand Soil Invertebrates website. *Includes photos of pseudoscorpions’ silk retreats!

Pseudoscorpions page on the Encyclopedia of Life.

The order of the Pseudoscorpiones. Nice summary of pseudoscorpion biology by F. Schramm, with lots of photos and scholarly references.

Zeh DW & Zeh JA (1992). On the function of harlequin beetle-riding in the pseudoscorpion, Cordylochernes scorpioides (Pseudoscorpionida: Chernetidae). Journal of Arachnology, 47-51.

Sex pheromone on the silk of black widow females – more complicated than we thought

The first paper from my MSc has just been published online in the Journal of Chemical Ecology! This study was a collaboration with colleagues Sean McCann (bioassay designer, photography/videography master, and all-around awesome assistant), Regine Gries (analytical chemistry wizard), Grigori Khaskin (synthetic chemist extraordinaire), and my supernatural supervisor Gerhard Gries. If you don’t have access to the journal, you can read the accepted manuscript here.

Here’s the story of the paper.                                                                                             Note: all photos and the video are copyright Sean McCann.

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A female western black widow (Latrodectus hesperus) on her web. The silk is impregnated with sex pheromones that attract males and trigger courtship behaviour.

When I started my MSc, one of the goals for my research was to “find the pheromone” of the western black widow. What does that mean exactly? Well, we already knew that female black widows (spiders in the genus Latrodectus) produce sex pheromones that are somehow incorporated into the silk of their webs. These are sort of like chemical personal ads – they can provide information about things like the species, sex, age, mating history, and body condition of the individual producing them. When a male black widow matures, his only goal in life is to find a female to mate with. He abandons his web and follows his nose (not literally – we don’t really understand much about how spiders smell but their “noses” are most likely on their legs and pedipalps!) to a nearby female’s web. Given a choice among multiple available females, male black widows will go for a well-fed virgin based on the smell of her silk alone. Once he arrives at her web, he contacts the silk and “tastes” (again with receptors on his legs/pedipalps) the pheromone, which triggers courtship behaviour. We wanted to find out the chemical structure of the female’s sex pheromone.

We had a pretty good idea of what to expect, because other researchers had already identified a pheromone of the Australian redback spider (Latrodectus hasselti). It looks like this:

Lhasselti_pheromone

N-3- Methylbutanoyl-O-(S)-2-methylbutanoyl-L-serine methyl ester. Contact pheromone of Australian redback spider (Latrodectus hasselti) females.

Male western black widows are actually attracted to the webs of redback females, implying that the structure of the pheromone is similar, if not identical, in these two species. (It’s not necessary for males to discriminate between Australian redback and North American western black widow females in nature, because they never encounter one another, so it wouldn’t be that strange if they shared the same pheromone). So we set out to analyze the silk of our western black widow females, and see if we could find a similar compound.

better frame

We persuaded females to provide us with clean silk by allowing them to build webs on glass frames for three days. We then collected the silk and extracted it for use in behavioural experiments and chemical analysis.

Some silk collection, extraction, and analytical chemistry* ensued (I’ll leave it to you to read the paper for details if you’re interested), and just as we had hoped, our western black widow females had a compound on their silk that was very similar to the redback pheromone above:

Lhesperus_pheromone

N-3-Methylbutanoyl-O-methylpropanoyl-L-serine methyl ester. Candidate pheromone of western black widow females (Latrodectus hesperus).

Not only is this chemical similar to the redback pheromone, it is also present in small amounts on redback females’ silk. So it seemed like an ideal candidate for the western black widow pheromone, and provided a potential explanation for the attraction between the two species. Now all we had to do was make a synthetic version* of the pheromone and test it on actual males.

Before we could determine whether the compound we had found was in fact the pheromone we were looking for, we needed to come up with a way of comparing its effects with the real thing. We knew that contact with a female’s silk triggers courtship behaviour, but black widow courtship is long and complex, and involves several different kinds of behaviour, some of which are very subtle. The male’s courtship dance sends vibrations through the web to the female, possibly providing information about his quality and identity (including that he is a potential mate, not a meal!). It also involves the production of copious amounts of silk by the male. This male silk carries its own pheromones, and is deposited all over the web and onto the female herself in the form of a “bridal veil” during courtship.

male_wrapping_Web

Male black widow (L. hesperus) engaging in silk-wrapping on a female’s web during courtship. Here the male is wrapping a section of web that he has destroyed during web reduction behaviour, which I will discuss in a future post.

We designed an experimental setup to assess male responses to silk pheromones. We constructed this high-tech device out of bamboo barbeque skewers, laboratory labeling tape, and a paper cup filled with floral foam. The skewers form a “T” and at each end of the horizontal arm we slid on little envelopes made of squares of filter paper folded in half and stapled. This simple and inexpensive device was one of the big successes of the project.

Tsetup

Our simple and inexpensive T-rod for testing male behavioural responses to contact silk pheromones.

The T-rod design makes it easy to compare an individual male’s response to a test stimulus on one side (for example, a female’s silk wrapped around the paper envelope) to a control (blank paper) on the other.

Slide1

Wrapping silk around a filter paper for behavioural experiments.

A male spider is introduced at the base of the “T” and climbs up to the top. Once he gets to the intersection, he can decide whether to go left or right. Males almost always began the experiment by investigating both sides of the “T”, but they spent much longer in contact with the silk-wrapped paper than the blank paper. Not only that, but they spent much of their time wrapping the female silk-wrapped paper with silk of their own – obvious courtship behaviour.

wrap1

A male black widow silk-wrapping on a filter paper with silk extract on it.

Knowing that males would respond to female silk in this way on the T-rod, we were now ready to confirm that the behaviour didn’t depend on the structure of the silk itself, and to see if males would respond to our synthetic candidate pheromone in the same way as they would respond to the real thing.

We prepared female silk extract using methanol as a solvent (this is the same idea as vanilla extract, but instead of extracting the flavour of vanilla beans into ethanol, we extracted the chemicals on the silk into methanol) and applied it to one of the filter papers on the T-rod, and methanol alone to the other.

Slide1

We tool silk from a glass frame like the one above and submerged it in methanol to extract the pheromones into the liquid, which we then used in behavioural tests.

Males responded in exactly the same way to silk extract as they did to silk itself, spending most of their time on the filter paper impregnated with extract, and wrapping it extensively with silk. Here’s a video showing what that looks like (first at full speed, and then slowed down):

This told us that a pheromone that can be extracted from the silk triggers courtship behaviour, and the structure of the silk itself is not necessary. But when we tested male responses to our candidate pheromone (dissolved in methanol, using methanol alone as a control), things were not so clear-cut. Males spent more time on the pheromone-impregnated paper than methanol alone, but they didn’t prefer it as much as they had preferred the silk extract to methanol. A few males engaged in silk-wrapping when they made contact with our compound, but not the majority, like we had seen for the extract. This means that although our “pheromone” elicited some male activity, by itself it is not enough to consistently trigger courtship behaviour. It seems to be a pheromone component – meaning that the pheromone is a mixture of one or more compounds in addition to the one we identified, and more work will need to be done to figure out what they are.

It would have been nice to be able to say we found the pheromone. But our results suggest that the chemical communication system of black widows is more complicated than we originally thought, and even more fascinating.

In the study that identified the redback pheromone, the researchers measured male activity (the amount of time they spent moving around when in contact with a filter paper impregnated with pheromone), not courtship behaviour. It could be that this pheromone too is only one component of a more complex chemical cocktail. Like our pheromone component, it may be responsible for eliciting searching behaviour, but not quite enough on its own to consistently trigger courtship behaviour by males.

If multiple compounds are involved in these spider pheromones, they might each have different functions. We don’t yet know whether the same pheromone that attracts males is responsible for triggering courtship, or if different compounds provide different kinds of information, about things like a female’s mating status and feeding history. We have learned that the scent-based sexual communication system of black widows is likely more sophisticated than we originally thought, and that there is much more to discover!

L hesperus pair

Male and female western black widow on a female’s web.

*Neither the analytical chemistry nor the synthesis of the candidate pheromone were trivial tasks – rather they required the expertise and generous efforts of my very talented coauthors Regine and Grigori. I hope they will forgive me for glossing over the details here! 

Here’s the full citation for our paper:

Scott C, McCann S, Gries R, Khaskin G & Gries G. 2015. N-3-Methylbutanoyl-O-methylpropanoyl-L-serine Methyl Ester – Pheromone Component of Western Black Widow Females. Journal of Chemical Ecology. DOI: 10.1007/s10886-015-0582-x