A comb-tailed spider

Last weekend, I joined a group of fellow arachnophiles for a day at Burns Bog. We did not achieve our goal of finding the rare ground spider Gnaphosa snohomish (a bog specialist), but instead we met a very common spider that is nonetheless not well known: a comb-tailed spider in the family Hahniidae.

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Neoantistea magna, a common yet mysterious forest-dweller (photo Sean McCann).

A distinguishing feature of spiders in the subfamily Haniinae is the arrangement of the spinnerets in a single row like the teeth of a comb – thus the common name.

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The arrangement of the spinnerets of ‘comb-tailed’ spiders in the subfamily Hahniinae.         (Photo by Tom Murray, licensed under CC BY-ND-NC 1.0)

I generally think of spiders as being one of two basic types: wanderers or web builders. The wanderers include visually hunting ground dwellers like wolf spiders, whereas web building spiders are sit-and-wait predators that rarely leave their silken snares. This is overly simplistic, of course, but asking “web or not?” is often a useful first step in  classifying spiders. The genus Neoantistea, however, gave me a first encounter with members of an intermediate group known as vagrant web builders.

The sheet webs of Neoantistea spiders are tiny – typically less than 5 cm across. They are built in moss or across shallow depressions such as those formed by the tracks left by animals walking on soft ground. The diminutive spiders (their total body length is less than 5 mm) live under their webs, retreating into crevices in the litter or moss when disturbed.

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Small sheet-web of Neoantistea magna (photo Sean McCann).

What makes these spiders unusual for web builders is that although the web can be a useful aid for catching prey, it is not necessary. Neoantistea magna have reasonably large eyes and can recognize and hunt prey just as easily off of their webs as on them (Engers & Bultman 2006).

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A portrait of a male Neoantistea magna, showing the arrangement of the relatively large eyes (photo Sean McCann).

Although it was easy to identify the spiders we found to genus – the distinctive spinnerets leave no doubt as to the family, and of the North American members of the Hahniinae, Neoantistea is the only genus of web builders – determining the species was another matter entirely. Usually spider identification relies on close examination of the genitalia.

To ID this handsome fellow, two of the key features were the tibial apophysis and the patellar spur, tiny protrusions of the pedipalps which are very difficult to see without a microscope (here’s a diagram of the segments of the pedipalps).

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The key to identifying spiders often lies in the features of their elaborate genitalia. Here the large curved outgrowth on the tibia and the hooked spur on the patella of the pedipalp are circled (photo Sean McCann).

Speaking of genitalia, although very little is known about the biology of Neoantistea magna, there is one report of mating behaviour (Gardner & Bultman 2006). During copulation, the male clasps the female with his first two pairs of legs. The robust femur and tibia (see leg segment diagram) on each of these legs are studded with a double row of tubercles, giving them a serrated look. 

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Male N. magna. Note the burly front legs, presumably modified for grasping the female (photo Sean McCann).

Although the female may attempt to disengage from her partner, he is able to maintain a firm hold with his rather spectacularly modified legs and continue copulation.

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Female N. magna, with slender front legs (photo Sean McCann).

Fun with etymology:                                                                                                             The genus name Neoantistea means “new Antistea”. Antistea comes from the Latin word antistes, which means “one who stands in front of a temple, overseer, high priest”. Why were these tiny spiders given such a grandiose name? It’s a mystery.

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References

Engers, W., & Bultman, T. (2006). Foraging Habits of Neoantistea magna (Araneae: Hahniidae).

Gardner, D., & Bultman, T. (2006). Natural History and Reproductive Biology of a Hahniid Spider in Southwestern Michigan.

Opell, B. D., & Beatty, J. A. (1976). Nearctic Hahniidae (Arachnida: Araneae)Bull Mus Comp Zool Harvard Univ.

Tetragnatha revisited: dinner and romance at sunset

This post features photographs by Sean McCann. For more beautiful photography and natural history of arthropods and other wildlife, check out his blog, Ibycter.com

As a sequel to our recent encounter with some long jawed orb-weavers in the genus Tetragnatha (the tiny and cryptic Tetragnatha caudata), this week on an evening walk at Iona Beach, Sean and I observed some neat predation and mating behaviour in another species, most likely Tetragnatha laboriosa.

We made our first observation just as the sun was beginning to set, the beginning of the most active hunting hours for Tetragnatha laboriosa. This female had just captured her first meal of the evening, a bug in the family Miridae.IMG_1953

After biting it, she began wrapping it with silk, which she pulled out of her spinnerets with her last pair of legs (you can see her caught in the act below). IMG_1956

After wrapping the bug lightly with silk, she carried it back to the hub of her orb web and settled down to dine.IMG_1962

Unfortunately for the spider, dinner was interrupted by Sean’s efforts to get a good photograph. The disturbance prompted her to drop her meal and retreat to the vegetation at the edge of her web. Isn’t she just gorgeous?!
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After a minute or so, she went back for her abandoned prey.
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She then carried it off the web to resume her meal in peace. You can see from this image how the lovely coloration of these spiders allows them to blend in with plant stems when they adopt their cryptic stick-like posture.
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Later, when the sun had all but set and we were just about to head home, Sean spotted a pair of spiders (probably the same species, T. laboriosa) mating in a female’s web.
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Mating involves a fair bit of contortion for long jawed orb-weavers. Below you can see the male’s extremely long pedipalp (one of a pair of appendages modified for transferring sperm) engaged with the female’s epigyne (genital opening). The male’s short third pair of legs is used to position his partner’s abdomen. Throughout copulation he maintains a firm grip on the female’s jaws with his own.  IMG_2106

Here is a closer look at the mating position, where if you look closely you can see one of the female’s fangs interlocking with the special tooth on the male’s corresponding chelicera.jaws_clasping

Here is a drawing by B. J. Kaston of what the cheliceral embrace looks like close-up. The male, with larger jaws, is below, and the female above.

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Fig. 876 from Kaston 1948. Interlocking jaws of Tetragnatha pallescens (which looks very similar to T. laboriosa) during mating.

The female’s fangs get locked in underneath the special large tooth that protrudes from each of the male’s chelicerae.  tooth_landscape

As if we hadn’t had enough excitement already with the chance to closely witness such an intimate encounter, moments later we spotted two additional males waiting in the periphery of the female’s web. We were in for quite a show!

Here is one of the males that was waiting in the wings, posing elegantly and displaying his long jaws and even longer pedipalps. We’ll call him bachelor #2. IMG_2120

Not long after we spotted them, one of the lurking males made his move, lunging at the mating pair with his jaws held wide.  IMG_2108

A bit of a tussle ensued, after which the mating spiders disengaged. The attacking male pursued the mated male off the web and all the way to the substrate below. The female, apparently rather perturbed by this rude interruption, also left the web. One of the two rival males, apparently dominant, soon ascended back toward the web via his dragline. IMG_2112

Just as the winner of the first brief battle returned to the web, the third male entered the ring, and a second chase ensued. This cycle repeated a couple of times, until at last only one male returned victorious to the periphery of the web.IMG_2129

Bachelor # 2 (or was it #3?) settled down to wait at the edge of the web, while the female made her way back to the hub.     IMG_2140

It turns out that female T. laboriosa only mate once as a rule, and if copulation is interrupted as we observed, it’s a toss-up whether or not she will be willing to pick up where she left off (LeSar & Unzicker 1978). We couldn’t stay to see if our champion was able to successfully mate, but we wished him the best of luck!IMG_2138

Phidippus and Salticus

Two spiders, both alike in family,

Phidippus johnsoni, the red-backed jumping spider. (Photo: Sean McCann)

Salticus scenicus, the zebra jumper. (Photo: Sean McCann)

In fair Victoria, where we lay our scene,

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Uplands park, Victoria, BC, is among the many homes of these common jumping spiders (family Salticidae). (Photo: Colin McCann)

From ancient grudge break to new animosity,

Where spider blood makes spider fangs unclean.

Notes
Jumping spiders (like all spiders) are generally predators of insects and other arthropods. Spiders are the second most common prey items of Phidippus johnsoni (27% of their diet, just after dipterans, coming in at 30% of the total count), and comprise 5% of the diet of Salticus scenicus.

References:
Jackson, R. R. 1977. Prey of the jumping spider Phidippus johnsoni (Araneae: Salticidae). J. Arachnol. 5 :145-149.
Okuyama, T. 2007. Prey of two species of jumping spiders in the field. Appl. Entomol. Zool. 42 (4): 663–668.

The zebra jumper’s stripes may serve as camouflage in many settings, but in this case they were no match for the excellent visual hunting abilities of a fellow salticid. (Photo: Sean McCann)

 

What happens when you poke, prod and pinch black widow spiders? You might be surprised…

This post originally appeared on Chris Buddle’s blog Expiscor at Scilogs.com.

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A stunning female western black widow (Photo: S. McCann)

People seem to have a particular fear mixed with fascination when it comes to venomous animals, and whenever I talk about my work with black widows I am invariably asked questions like, “have you been bitten yet?” The answer is, of course, no. Spiders almost never bite people. I’m always quick to relate that in my experience black widows are not aggressive, even when I go around poking and prodding them with my bare hands.

Replicated experimental results always carry more weight than anecdotes, however, so I am delighted to share this recent paper: Poke but dont pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperus.

Hey look!! An actual peer-reviewed research paper about the poking, prodding, and pinching of black widows, confirming that they are reluctant to bite, even when threatened. Not only that, but the study provides some cool data suggesting that these spiders are capable of assessing risks to make decisions about how to defend themselves.

Here are the details:

David Nelson and his coauthors wanted to know if black widows change their defensive behaviour depending on the level of threat they are faced with. To find out, they used gelatin ‘fingers’ to place spiders in three different threatening situations: a ‘low threat’ attack was a single poke with one finger, a ‘medium threat’ was a series of prods simulating a more persistent attacker, and the ‘high threat’ was three a series of long pinches of the spider’s entire body between two fingers, as might be experienced if being grasped by a predator.

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Figure 1 from Nelson et al. 2014

They found that the spiders engaged in several distinct defensive behaviours during these experimental attacks: retracting the legs toward the body, moving (often retreating), ‘silk-flicking’ (drawing sticky silk out of the spinnerets with last pair of legs and flinging it toward the attacking finger), ‘playing dead’ (curling up into a ball), and biting.

During low-threat, single pokes, no bites occurred. Most spiders were completely non-confrontational, simply moving away, and only rarely flicking silk.  When the threat level escalated to persistent prodding, the spiders changed their defensive behaviour: roughly half of them flicked silk, some played dead, and only one spider (out of 43) attempted to bite the offending finger. Silk-flicking is much safer than biting for a black widow – she can maintain her distance while flinging sticky silk to subdue or slow down her attacker. Biting, on the other hand, requires getting up close and personal with the assailant in order to pierce it with her tiny fangs, making her much more vulnerable to injury.

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See the red tips of this black widow’s puny fangs? It’s a lot safer for her to keep her distance in threatening situations than get close enough to use them (Photo: S. McCann)

Only when the spiders were being pinched between two fingers (with the mouthparts already positioned right up against their ‘attackers’) did biting start to become a more common, last-resort tactic: 60% of the spiders bit the fingers as a result of being squeezed for an extended period of time, delivering on average 2.7 bites each. Pinching also resulted in silk-flicking by about half of the spiders, and a few played dead.

This is all great information, but when the spiders did bite the gelatin fingers, there was no way of knowing how much venom they injected, if any at all (sometimes venomous animals deliver ‘dry’ bites). The next question the researchers wanted to answer was, do the spiders control whether and how much venom they inject when biting? In particular, they wanted to know if the amount of venom injected would vary depending on the type of threat (in this case either pinching a leg with forceps, or grasping the abdomen with gloved fingers).

For this experiment they came up with a clever method to collect the venom: a small vial with a thin membrane over the opening was presented as a target for the spiders to bite. If a spider did bite, her fangs would pierce the membrane (the number of holes would indicate how many times) and any venom she expelled would be collected in the vial so the volume could subsequently be measured.

It turned out that more than half of all bites were dry (no venom was detected in the vials). The black widows delivered more bites per target when they were pinched on a leg than on the abdomen, but more venom was released with each bite when the abdomen was pinched. Being grasped by the body is a high-risk situation for a black widow because her abdomen is unarmored and vulnerable; a strong squeeze or puncture can be deadly. Pinching a single leg, on the other hand, represents a non-life threatening attack. Spiders can autotomize (drop) their limbs and survive without significant ill effects.

The team also found evidence that the spiders delivered more venom per bite when repeated threats were spaced 5 minutes apart than 5 seconds apart. Attacks after the longer intervals might have been interpreted as coming from new assailants, each requiring a larger dose of venom than a second or third bite to the same persistent attacker.

The results all indicate that black widows have fine control over how much venom they inject when biting. First, they can decide whether or not to use venom at all. Some spiders gave dry bites, then wet bites, as well as vice versa, demonstrating that dry bites were not simply a result of running out of venom. Furthermore, they can vary the amount of venom they inject during individual bites and in response to different kinds of threats.

Both silk and venom are metabolically expensive to manufacture, so it makes sense that spiders would be selective about when and how much of these resources to deploy in defense. This study suggests that they are able to assess risks and adjust their responses accordingly, only dipping into their reserves of silk and venom as the threat level escalates towards a life-or-death situation.

What does this all mean for humans? Grabbing and pinching spiders is generally not a good idea – they might get injured and could bite defensively. This is just good sense and didn’t require a scientific study to confirm, but the new data suggest that even if a black widow does bite, she’s not necessarily going to inject any venom. It’s also important to note that in the experiments where bites did occur, the spiders always had a ‘finger’ or target placed in direct contact with their mouthparts.

An unaggressive female black widow takes a stroll across my hand. Although I never grab spiders to pick them up, coaxing them onto my hands and letting them wander around on their own steam has never been a problem. (Photo: S. McCann)

The most exciting thing this study tells us is that spiders can make decisions about how to respond to threats (which sometimes include humans) – further evidence of their incredible sophistication. Perhaps more importantly for the arachnophobic, it suggests that black widows would much rather conserve their valuable venom for use in dispatching their next meal than waste it on a human who is of no interest as prey!

Spiders in general are amazing creatures worthy of our admiration and respect. I hope that this new information about black widows might convince some that there is more about them to be fascinated by than to fear!

References and related reading:

Nelsen, D. R., Kelln, W., & Hayes, W. K. (2014). Poke but don’t pinch: risk assessment and venom metering in the western black widow spider, Latrodectus hesperusAnimal Behaviour89, 107-114. http://dx.doi.org/10.1016/j.anbehav.2013.12.019

Vetter, R. S. (1980). Defensive behavior of the black widow spider Latrodectus hesperus (Araneae: Theridiidae). Behavioral Ecology and Sociobiology7(3), 187-193. doi:10.1007/BF00299363

W. Cranshaw (2014). Western widow fact sheet: http://www.ext.colostate.edu/pubs/insect/05605.html