Category Archives: Arachnids

Arthropods vs. Cane Toads

Cane toads are toxic because their bodies are loaded with cardiac glycosides, deadly toxins that can stop a predator’s heart. Because the toads are non-native in Australia, the native Australian carnivores aren’t adapted to dealing with them. For some, this is very bad news: freshwater crocodiles, monitor lizards, and pythons have all experienced population declines since the introduction of cane toads in 1985 (Smith and Phillips 2006).

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A cane toad. Photo by Sam Fraser-Smith, licensed under CC BY 4.0.

Not all is lost, however. It turns out that cardiac glycosides are only toxic to a very narrow group of animals: vertebrates. Predatory insects, arachnids, and other invertebrates have no trouble at all with the poison. Furthermore, a study published earlier this year (Cabrera-Guzmán et al. 2015) revealed that many are more than capable of tackling amphibian prey.

Cane toads start their lives as tadpoles, small and innocent, but plenty toxic enough to kill a hungry frog or fish. In Australia, some of their top predators are giant water bugs and water scorpions. Both are insects (not scorpions) that use tube-like mouthparts to inject acid and digestive enzymes into their prey, dissolving them from the inside out. When the tadpole’s innards are sufficiently liquefied, the insects slurp them up like an amphibian milkshake.

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A water scorpion lying in wait for prey. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A young dragonfly's mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

A young dragonfly’s mouthparts. Photo by Siga, licensed under CC BY-SA 3.0.

Dragonfly larvae are also aquatic and predatory, but instead of using acid, they have extendable mouthparts. These are built like the robotic arm on an automatic garbage truck, but less cumbersome and more of a surgical, spring-loaded instrument of death. Most dragonfly larvae eat other insects, like mosquito larvae, but the largest species can easily overpower a small fish or tadpole.

Diving beetles join in the fun. Their mouthparts are less exciting, more like a typical beetle’s with sharp, biting mandibles. What makes them special is speed: their bodies are constructed like those of sea turtles. Like sea turtles, diving beetles are hard-shelled, streamlined, and aquadynamic. Unlike sea turtles, diving beetles use this form to swim after tadpoles that aren’t quite fast enough to escape.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle, waiting for tadpoles to swim by. Photo by N. Sloth, licensed under CC BY-NC 3.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

A diving beetle larva with tadpole prey. Photo by Gilles San Martin, licensed under CC BY-SA 2.0.

Diving beetle larvae are just as fierce, and they too have been observed feeding on cane toad tadpoles. Unlike their parents, larvae are long-bodied, with curved, needle-like jaws which they use to inject digestive enzymes into their prey (like the water bugs).

Any cane toad tadpoles that survive this massacre can metamorphose into toadlets, but until they reach their adult size (4-6 inches) they are still at the mercy of their invertebrate predators.

Experiments and observations in the field (Cabrera-Guzmán et al. 2015) have revealed that crayfish are efficient predators of eggs, tadpoles, toadlets, and even adult toads. Australia is home to 151 species of crayfish, including several of the largest species on earth. The spiny crayfish (Euastacus) in particular, some of which can grow to more than a foot in length, prey not only on toadlets but also on full-sized, adult cane toads.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

A Lamington blue spiny crayfish (Euastacus sulcatus). Photo by Tatters, licensed under CC BY-SA 3.0.

There are, believe it or not, spiders that specialize in running out over the surface of the water to snatch aquatic insects, tadpoles, and small fish. They are the fishing spiders (large ones are sometimes called dock spiders). Experiments have shown that when fishing spiders inhabit a pond, up to 1 in every 4 tadpoles ultimately becomes spider food (Cabrera-Guzmán et al. 2015).

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

A fishing spider, ready for a meal. Photo by Patrick Coin, licensed under CC BY-NC-SA 2.0.

Finally, ants. In the cane toad’s native range of tropical Latin America, meat ants are a major predator. When a toad is attacked, it often stays still, relying on poison for protection. Ants take advantage of this strategy, swarming over the toad’s body and stinging it to death with poisons of their own. Meat ants (Iridomyrmex) and their relatives also live in Australia, and they have been seen dragging the dismembered remains of cane toads back to their nests.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

Meat ants taking down a cicada nymph. Photo by jjron, licensed under GFDL 1.2.

I’m sorry to say giant centipedes did not make the list of cane toad predators in Australia, but I should mention that the Caribbean giant centipede (Scolopendra alternans) has been observed to prey on native cane toads (Carpenter and Gillingham 1984).

Whether bird-eating spiders, bat-snatching centipedes, or tadpole-chasing water bugs, invertebrates that prey on vertebrates are always fascinating. It’s more common than you might think! I’ll conclude by mentioning Epomis, an unusual genus of ground beetles. Both the beetle larvae and adults are specialist amphibian-eaters, and tackle frogs and toads many times their own size.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

Epomis beetles attacking various European amphibians. Photos from Wizen and Gasith (2011), licensed under CC BY 3.0.

I won’t say any more, since Epomis expert Gil Wizen has already written a fantastic blog post about these beetles, complete with videos of predation in action! I encourage you to check it out here.

Cited:

Cabrera-Guzman E., M.R. Crossland, and R. Shine. 2015. Invasive cane toads as prey for native arthropod predators in tropical Australia. Herpetological Monographs 29(1): 28-39.

Carpenter C.C. and J.C. Gillingham. 1984. Giant centipede (Scolopendra alternans) attacks marine toad (Bufo marinus). Caribbean Journal of Science 20: 71-72.

Smith J.G. and B.L. Phillips. 2006. Toxic tucker: the potential impact of cane toads on Australian reptiles. Pacific Conservation Biology 12(1): 40-49.

Wizen G. and A. Gasith. 2011. Predation of amphibians by carabid beetles of the genus Epomis found in the central coastal plain of Israel. ZooKeys 100: 181-191.

Why Scorpion Venom is So Complex

by Joseph DeSisto

Scorpion venom, like many animal venoms, is incredible complex. It is made up of hundreds of different toxins and other proteins, each with a specific function, all mixed together in a lethal cocktail. Why do scorpions need so many different toxins? Last week, scientists at the Chinese Academy of Sciences published the results of their attempt to answer this question (Zhang et al. 2015).

They began by studying a particular class of proteins found in scorpion venom, which work by attacking the sodium ion-channel proteins in their victims.

The stripe-tailed scorpion, Vaejovis spinigerus, ready for action.. Photo by Joseph DeSisto.

The stripe-tailed scorpion, Vaejovis spinigerus, ready for action. Photo by Joseph DeSisto.

Sodium ion-channels help regulate the amount of sodium inside animal cells, which is vital for cells to function properly. In nerve cells, they are even more important: the change in sodium concentration inside and outside the cell is what transmits electric signals.

Toxins that inhibit sodium channels prevent the nervous system from working, which leads to death if the victim is small (like an insect). Scorpions have toxins called sodium-channel toxins to do exactly that. The puzzle is, scorpions have many different genes that produce sodium-channel toxins, each of which has a slightly different structure.

All proteins are essentially strings that are wound, twisted, and tied into a specific structure. The structure of a protein is critical to its function, since proteins need to have certain shapes in order to interact with each other like a lock and key. All sodium-channel toxins have a portion designated as the “interactive region” — the key — which attaches to a series of loops on the prey’s sodium ion-channel (the lock). If the key fits and the connection is successful, the prey’s ion-channel can no longer function.

A mother stripe-tailed scorpion, carrying young. Photo by Joseph DeSisto.

A mother stripe-tailed scorpion, carrying young. Photo by Joseph DeSisto.

Zhang and his colleagues studied the genome of their scorpion, a desert-dwelling East Asian species known as the Chinese golden scorpion (Mesobuthus martensii). They found no less than 29 different genes coding for sodium channel toxins.

There was a time, perhaps hundreds of millions of years ago, when scorpions only had one gene for sodium-channel toxins. Eventually that gene was duplicated, and thereafter the scorpion genome had multiple copies of the same toxin-producing gene. Since then, each copy of the gene has continued to mutate and evolve in its own direction. Now each toxin, despite having the same basic structure, is just a little bit different from the rest.

As it happens, the genes for sodium ion-channels in a scorpion’s prey also exist in multiple copies, each with minor variations. Zhang and colleagues hypothesized that scorpions need so many varieties of toxins because each toxin can only interact with a specific variety of ion-channel. In other words, scorpion venom needs lots of different keys because the prey have so many different locks.

To test this, the scientists examined the different toxin-gene copies to better understand how they had evolved. Sure enough, the “interactive region,” the key, of each toxin had mutated and evolved much more quickly than the “body” of the toxin. This provided strong evidence that natural selection has caused scorpion venom to evolve different types of toxins to keep up with the ever-evolving ion-channels in their prey.

Scorpions are incredible animals for so many reasons. They have been around for more than 400 million years — as long as there have been insects to hunt on land, scorpions have been there to hunt them. They are amazing and diverse in form, lifestyle, and hunting strategy. How fitting that they should be just as amazing on the molecular level.

Cited:

Zhang S., B. Gao, and S. Zhu. Target-driven evolution of scorpion toxins. Nature Scientific Reports 5:14973 doi: 10.1038/srep14973

Are Tarantulas Dangerous? Most Aren’t, A Few Might Be

Of the 900 or so known tarantula species, almost all are harmless (Isbister et al. 2003, Lucas et al. 1994). A bite by any large spider can be painful even if no venom is injected, since the fangs themselves are essentially big needles. Even if venom is injected, however, most tarantula bites result in little more than local pain and swelling. When there are medical problems, the cause is usually shock or an allergic reaction, rather than the action of the venom itself.

The Chilean rosehair tarantula (Grammostola rosea), a hardy and docile pet. Photo from Insects Unlocked, in public domain.

The Chilean rosehair tarantula (Grammostola rosea), a hardy and docile pet. Photo from Insects Unlocked, in public domain.

That said, not all tarantulas are equally venomous. The most common tarantulas sold in pet shops are all pretty benign: the Chilean rose hair, the Mexican redknee, and the pinktoe tarantulas have both mild venom and docile habits. They can be handled gently with almost no risk of being bitten.

Other, more exotic species kept by seasoned tarantula experts include the cobalt blue, the goliath birdeater, and the golden starburst tarantulas. These are beautiful and impressive captives — the goliath birdeater can attain a 12-inch leg span. The cobalt blue and golden starburst are stunningly colorful animals, the latter approximately matching the color of Donald Trump’s hair. These species are also more nervous and willing to bite, and their bites are generally more painful (e.g., Takaoka et al. 2001).

The golden starburst tarantula (Pterinochilus murinus), guarding its silken retreat. Photo by Stefan Walkowski, licensed under CC BY-SA 3.0.

A golden starburst tarantula (Pterinochilus murinus) guarding its silken retreat. Photo by Stefan Walkowski, licensed under CC BY-SA 3.0.

Avid tarantula enthusiasts don’t get most of their spiders from pet shops. Instead they buy tarantulas from other spider-keepers who breed their pets, or from companies that import spiders and other animals from around the world. With international trade, hundreds of species are available for hobbyists collect. Many of these are poorly known, most have not had their venom studied, and a few haven’t even been formally described by scientists.

Where venomous animals are concerned, gaps in scientific knowledge can have serious consequences. A few years ago a Swiss man was bitten by one of his many pet tarantulas — at first, the only symptoms were mild pain, hot flushes and sweating. He brought himself to the hospital 15 hours later, when he began to experience severe muscle cramps and stabbing chest pain. Doctors gave him medication (midazolam and lorazepam) that reduced the symptoms, but muscle cramps did not disappear completely until three weeks after the bite (Fuchs et al. 2014). The tarantula in this case was a regal ornamental tarantula, a magnificent tree-dwelling spider native to India.

A regal ornamental tarantula (Poecilotheria regalis). Photo by Morkelsker, in public domain.

A regal ornamental tarantula (Poecilotheria regalis), with a leg span up to 6 inches. Photo by Morkelsker, in public domain.

There are at least 16 species of ornamental tarantulas, all from tropical forests in India and Sri Lanka. Most of them can be found in the exotic pet trade, and many have become popular with tarantula keepers looking for something a little more exciting. Exciting is certainly what they get: ornamental tarantulas are stunningly beautiful, as well as extremely fast and agile climbers. They are also quick to bite if cornered. Ornamental tarantula venom, while not deadly, is certainly underestimated.

To see if muscle cramps and chest pain were common symptoms of ornamental tarantula bites, Joan Fuchs and colleagues (2014) looked at 26 case reports, most of which were blog entries by seasoned tarantula keepers and breeders. Of the cases, 58% involved muscle cramps, along with other symptoms such as fever and heavy breathing. A few patients even lost consciousness for short periods. All bites were painful, but those that led to muscle cramps were severely so. This led the researchers to believe that, in cases where muscle cramps did not appear, the spider had simply injected much less venom.

The metallic ornamental tarantula (Poecilotheria metallica). Photo by Søren Rafn, licensed under CC BY-SA 3.0.

The metallic ornamental tarantula (Poecilotheria metallica). Photo by Søren Rafn, licensed under CC BY-SA 3.0.

It’s worth remembering that no tarantula bite has ever been fatal. It is a sorry fact, however, that by far the greatest source of knowledge on tarantula bites comes not from scientists, but from spider-keepers who take great pains (literally) to record their symptoms after every bite. This information is shared with other spider-keepers online at websites like Arachnoboards, so other hobbyists know what to expect from each species.

Such informal reports have been done for many species that have yet to be studied closely by scientists, and some that haven’t even been “discovered” (i.e., been given Latin names and formally described). The scientific axiom that “more work is needed” may be a cliché, but regarding tarantula bites and spider venom in general, it is certainly true.

Cited:

Fuchs J., M. von Dechend, R. Mordasini, A. Ceschi, and W. Nentwig. 2014. A verified spider bite and a review of the literature confirm Indian ornamental tree spiders (Poecilotheria species) as underestimated theraphosids of medical importance. Toxicon 77: 73-77.

Isbister G.K., J.E. Seymour, M.R. Gray, and R.J. Raven. 2003. Bites by spiders of the family Theraphosidae in humans and canines. Toxicon 41(4): 519-524.

Lucas S.M., P.I. Da Silva Júnior, R. Bertani, and J.L. Cardoso. 1994. Mygalomorph spider bites: a report on 91 cases in the state of São Paulo, Brazil. Toxicon 32(10): 1211-1215.

Takaoka M., S. Nakajima, H. Sakae, T. Nakamura, Y. Tohma, S. Shiono, and H. Tabuse. 2001. Tarantulas bite: two case reports of finger bites from Haplopelma lividum. The Japanese Journal of Toxicology 14(3): 247-250.

The Wanderer

by Joseph DeSisto

Night, and the Sonoran Desert comes to life. Lizards and mice emerge from their hideaways to eat, fight, and mate, while scorpions and giant centipedes scuttle about, hoping to stumble upon a juicy insect meal. All the while, a female tarantula waits in her burrow.

A Texas blonde tarantula, perched eagerly at the edge of her burrow. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

A blonde tarantula from Arizona, eagerly perched at the entrance to her burrow. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

She has no need for sight or smell. She only has to feel — lines of silk trace the ground around her burrow, and she keeps her feet on these silk lines to feel every vibration.  A pair of lizards chase each other, dangerously close to the burrow, and the spider flinches in predatory excitement, but bides her time. A few seconds later, a june beetle, weary from a night of flying, lands by the entrance and takes a few ill-fated steps. Crunch!

A Texas blonde tarantula with june beetle prey. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

A blonde tarantula with june beetle prey. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

Tarantulas are found all over the tropics and subtropics, from rainforests to mountaintops to deserts. The blonde tarantulas of North America’s deserts, in the genus Aphonopelma, are some of the toughest and longest-lived arachnids on earth. Females reaching maturity after a decade, and can live another several years after that. The longest-lived specimen known to science survived for more than 17 years (Ibler et al. 2013).

The female blonde tarantula spends nearly her entire life underground, in a short vertical burrow. This burrow, and the patch of earth around it, is her entire world — for ten years or more she ambushes and feeds on beetles, scorpions, and other unlucky passers-by. As a result, females are seldom seen except by those curious enough to wander through the desert flipping large rock slabs and inspecting the bases of bushes.

A female blonde tarantula. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

A female blonde tarantula. Photo by Michael Wifall, licensed under CC BY-SA 2.0.

During my trip to Arizona this summer, I only found one. I provoked her with a blade of grass, to see if she would leave her burrow. Even though her home was all but destroyed by my rock- flipping, this spider adamantly refused to leave. She held her ground, furiously biting and lashing out with her front legs.

Male blonde tarantulas are much more easily seen, and during my trip to Arizona I found several crossing roads at dusk. For the first one, I stopped and left my car to get a closer look, eager to see what this amazing creature was all about.

A male Arizona blonde tarantula, likely Aphonopelma chalcodes. Photo by Joseph DeSisto.

A male Arizona blonde tarantula, likely Aphonopelma chalcodes. Photo by Joseph DeSisto.

Tarantulas are big — this one had a leg-span approaching five inches. It was easy to coax him into a jar, since without a burrow to defend, blonde tarantulas are quite docile animals. Had I been a little braver, I probably could have picked him up without being bitten. If I had failed, the bite would have been painful, but no more dangerous than a bee sting.

Compared to the female tarantula, the male is leaner, with a smaller body and longer, skinnier legs. The sexes are also different in another respect: experiments have shown that when at rest, male blonde tarantulas can get by on significantly less oxygen than their mates (Shillington 2005).

There’s a good reason for that. At around ten years of age, while females remain in their burrows, males reach sexual maturity and begin the “wandering phase” of their lives. When he is ready to mate, a male blonde tarantula emerges from his burrow and strides off into the desert in search of a female.

A wandering male blonde tarantula from Texas. Photo by Dallas Krentzel, licensed under CC BY 2.0.

A wandering male blonde tarantula from Texas. Photo by Dallas Krentzel, licensed under CC BY 2.0.

The desert is a big and lonely place, and female tarantulas may be few and far between. Here a male’s athletic prowess comes in handy: males that can walk the longest and fastest without tiring are the most likely to find a not-yet-mated female. This is important, since a female who has already mated might prefer to eat him rather than entertain a second suitor.

During his walk-about, a male blonde tarantula faces many hazards, from spider-eating birds and  wasps to the desiccating sun and wind. Nothing but death can dissuade a male from his journey. Texas tarantulas with radio transmitters have revealed that while some can get by traveling only a short distance, others may journey more than two miles over a period of several weeks (Stoltey and Shillington 2009).

A male tarantula from New Mexico. Photo by Robert Sivinski, licensed under CC BY-NC 3.0.

A male tarantula from New Mexico. Photo by Robert Sivinski, licensed under CC BY-NC 3.0.

My road-crossing specimen didn’t take to captivity very well. I provided him with plenty of room, soil and a place to hide, but all he could do was pace, back and forth across his cage. He did not eat or rest — he only walked. Finally I took pity and released him, and watched my eight-legged friend stumble back into the desert.

If a male blonde tarantula fails to find a mate, he will simply walk until he dies. If he does manage to reach a female’s lair, and he is lucky, she will mate with him. After, she might eat him, or she might not. It depends on how she’s feeling. The outcome hardly matters to the male. If she eats him, his body will be a final, nutritious gift to his offspring, yet to develop inside her. If she allows him to live, he will simply return to the surface, stretch his hairy legs, and keep walking.

Cited:

Ibler B., P. Michalik, and K. Fischer. 2013. Factors affecting lifespan in bird-eating spiders (Arachnida: Mygalomorphae, Theraphosidae) — a multi-species approach. Zoologischer Anzeiger – A Journal of Comparative Zoology 253(2): 126-136.

Shillington C. 2005. Inter-sexual differences in resting metabolic rates in the Texas tarantula, Aphonopelma anax. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 142(4): 439-445.

Stoltey T. and C. Shillington. Metabolic rates and movements of the male tarantula Aphonopelma anax during the mating season. Canadian Journal of Zoology 87: 1210-1220.

Recluse Spider Venom: How it Works

by Joseph DeSisto

Poisons and venoms often contain hundreds of different chemicals, each with a special role. When venom is dangerous to humans, it is useful to know which of the molecules involved is causing harm. In the case of the blue-ringed octopus, tetrodotoxin is the culprit, paralyzing your nervous system and keeping you from breathing. Cobras and sea snakes accomplish roughly the same thing with a whole suite of neurotoxins. Vipers, on the other hand, use hemotoxins to destroy blood vessels and force clots to develop, so oxygen can’t reach the cells that need it.

A brown recluse (Loxosceles reclusa). Photo by the Smithsonian Institution Insect Zoo, licensed under CC BY-NC 2.0.

A brown recluse (Loxosceles reclusa). Photo by the Smithsonian Institution Insect Zoo, licensed under CC BY-NC 2.0.

Brown recluse venom is far less dramatic. As in all spiders, the primary purpose of venom is to kill and digest insects or other prey, but recluse spiders and their relatives (members of the family Sicariidae) have a protein that does something special. That protein is called sphingomyelinase D – we’ll call it SMD. Exactly what SMD does when injected into insects isn’t clear, but when it enters a person via a spider bite, the protein may cause the tissue around the bite to die and form a necrotic lesion. In extremely rare cases, SMD can be carried by the bloodstream to other parts of the body, causing illness or even death.

Recluses and their cousins, the sand spiders (genus Sicarius), belong to the family Sicariidae or six-eyed spiders, which includes all the 132 spiders that use SMD in their venom (Binford and Wells 2003). No other animals are known to produce SMD, but there are bacteria that make it, and it just so happens that all of these bacteria are known to cause infections in humans (Binford et al. 2005).

A brown recluse spider with a penny. Photo by the Smithsonian Institute Insect Zoo, licensed under CC BY-NC 2.0.

A brown recluse spider with a penny. Photo by the Smithsonian Institute Insect Zoo, licensed under CC BY-NC 2.0.

One of them is Clostridium perfringens. Chlostridium is a genus of bacteria with around 100 species, all but five of which are completely harmless. See the layer of dust at the back of your desk? Run your finger through it – write your name, or make a smiley face. Now look at the dust that’s gathered on your fingertip. Chances are you’ve just collected some Clostridium, which is also found in soil, water, plants, and the digestive systems of animals. Clostridium spores are virtually indestructible, so the bacteria can contaminate just about any surface. Since the vast majority of species are harmless, that isn’t much of a problem.

Five, however, can cause serious illness in humans. The best-known is Chlostridium botulinum, which makes a poison called botulinotoxin and causes, as you might have guessed, botulism. C. perfringens, meanwhile, can cause food poisoning and gangrene using, among other proteins, SMD. Here’s where the plot thickens: Brazilian researchers discovered in 2002 that perfringens often lives in the venom glands of recluse spiders, manufacturing SMD alongside the spider’s own arsenal of venom-producing cells (Monteiro et al. 2002).

The same scientists then tested the strength of the spiders’ venom on rabbits, using spiders with and without bacterial “infections.” As expected, spider bites led to bigger lesions when bacterial colonies were present in the spider’s fangs, suggesting these bacteria might actually help the spiders by making their venom stronger. In return, the bacteria get a relatively safe place to multiply, within their host’s venom glands.

A sand spider (Sicarius) from the Namib Desert in southwestern Africa. Photo by Jon Richfield, licensed under CC BY-SA 3.0.

A sand spider (Sicarius) from the Namib Desert in southwestern Africa. Photo by Jon Richfield, licensed under CC BY-SA 3.0.

Recluse spiders aren’t unique in using bacteria to help manufacture toxins. Pufferfish, for example, wouldn’t be poisonous if it weren’t for tetrodotoxin-producing bacteria that live within their skins and livers. What’s different about the recluse-bacteria relationship is that neither party truly depends on the other – perfringens bacteria can easily find shelter elsewhere, and recluse spiders can make plenty of SMD without the microbial help, thank you very much.

The gene that allows for SMD in both spiders and bacteria reveals that life was not always so. Instead, more than 150 million years ago, an enterprising spider stole the bacteria’s SMD-making genes and inserted them into its own DNA toolkit (Binford et al. 2005). That spider did very well — all of today’s six-eyed spiders, the SMD-producing recluses and sand spiders, are descended from that individual.

Bacteria aside, all recluses are not equally dangerous to humans. The North American brown recluse (Loxosceles reclusa) is relatively tame: bites are very rare, to the point that 80% of alleged recluse bites aren’t actually recluse bites (Swanson and Vetter 2005). A review of “suspected” brown recluse bites revealed that around a third of bite victims developed lesions, 14% fell ill, and none died or suffered serious complications (Wright et al. 1997). The United States is also home to five other recluse species, none of which are known to be harmful to people.

The Arizona recluse spider (Loxosceles arizonica). Photo by Sean McCann, used with permission.

The Arizona recluse spider (Loxosceles arizonica). Photo by Sean McCann, used with permission.

Travel to western South America and the story changes. The Chilean recluse (Loxosceles laeta) is the most dangerous of the recluse species, partly because it likes to live in and around buildings. In Spanish it goes by the name araña de rincón, which means “corner spider,” after this spider’s habit of finding shelter in the secluded, dusty corners of old homes. Even though Chilean recluses are not especially aggressive, their preferred habitats makes encounters with humans very likely, and a few bites each year are inevitable.

Bites from Chilean recluses are also more toxic than those of their northern cousins. A survey of bite cases found that 84% of people developed lesions, 15% fell ill, and 3.7% died (Schenone et al. 1989). A 3.7% death rate sounds pretty scary, but this was in 1989 – medical care has improved since then, and spider antivenom today is much more widely available (Lucas 2015).

The eyes and fangs of a Chilean recluse -- note the six eyes, a characteristic of sicariid spiders. Photo by Ken Walker, licensed under CC BY 3.0 AU.

The eyes and fangs of a Chilean recluse — note the six eyes, an important feature of sicariid spiders. Photo by Ken Walker, licensed under CC BY 3.0 AU.

Sand spiders also possess SMD and can cause lesions in humans. Yet despite sand spider venom being far more toxic than most recluse venoms (Van Aswegen et al. 1997), no human deaths have ever been reported. Why? Because sand spiders, unlike recluses, don’t tend to live in places where human encounters are likely. Instead they inhabit remote desert regions of South America and Africa. Even though sand spiders have some of the most powerful venoms of any spiders, loaded with SMD, but bites are extremely rare and the worst cases have only resulted in lesions similar to those caused by recluse bites (Lopes et al. 2013)

Millions of years of desert living have hardened the sand spiders – some species can live longer than a decade. Opportunities to catch prey in the desert are rare, so strong venom might help reduce the spiders’ error rate. Sand spiders are also experts at camouflage, often covering their bodies with sand to disguise their bodies in a barren landscape.

The Brazilian sand spider (Sicarius ornatus), camouflaging itself with sand. Photo from Lopes et al. (2013), licensed under CC BY 4.0.

The Brazilian sand spider (Sicarius ornatus), camouflaging itself with sand. Photo from Lopes et al. (2013), licensed under CC BY 4.0.

So what’s the difference? Why are some recluses and sand spiders more toxic than others? Since among all the proteins in recluse and sand spider venom, SMD is the one that causes harm to humans, it makes sense that spiders with higher levels of SMD in their venom are more dangerous. Specifically, we would expect the sand spiders and the Chilean recluse to have more SMD in their venom than the other, less dangerous recluse spiders. But how to test our theory?

The first step: extracting venom from recluses and sand spiders, which is easier than you might think. Scientists at the University of Arizona (Binford and Wells 2003) did this by knocking out spiders with carbon dioxide, then shocking their fangs with tiny electrodes. The minute electric shock caused the unconscious spiders to release all their venom into tiny vials, which could then be stored in a -80̊ C freezer. The same technique works for extracting venom from all kinds of animals, from rattlesnakes to scorpions to honey bees.

Next comes measuring the SMD levels in venom from each species — in this experiment, ten recluses and two sand spiders. The results were surprising in that the sand spider and Chilean recluse venoms had moderate concentrations of SMD — no greater or smaller than those of the other spiders. Instead, these three differed from the others in another regard.

A brown recluse from Kansas. From Saupe et al. (2011), licensed under CC BY 4.0.

A brown recluse from Kansas. From Saupe et al. (2011), licensed under CC BY 4.0.

They had more venom by, on average, nearly seven times. Usually, large spiders have more venom than smaller ones, but all the spiders in this experiment are roughly the same size. Why the sand spiders and the Chilean recluse should have so much more venom than their relatives is unknown for practical purposes, it doesn’t really matter. What matters is that, even if SMD concentrations are about equal, sand spiders and Chilean recluses still have seven times more SMD than any of the other recluses.

Venoms and poisons are bewilderingly complicated. They’re also amazing, and locked within each molecule are incredible opportunities to understand the natural world, improve medical care, and even save lives. When the diversity of life on earth meets the diversity of biochemistry, it’s clear that studying these amazing substances will keep scientists occupied for as long as there are spiders, hiding in corners and striding across the sand.

Thanks are owed to Sean McCann, who gave me permission to use his photograph of an Arizona recluse (Loxosceles arizona), rarely seen in the United States. You can check out more of his photography at his website, http://ibycter.com/.

Catherine Scott, a PhD student and arachnologist at the University of Toronto, maintains a blog devoted to spider biology. She wrote a fantastic article on identifying brown recluses, which you can read here. You can also follow her on twitter (@Cataranea) and inquire about spiders you think might be brown recluses.

Cited:

Binford G.J., M.H.J. Cordes, and M.A. Wells. 2005. Sphingomyelinase D from venoms of Loxosceles spiders: evolutionary insights from cDNA sequences and gene structure. Toxicon 45: 547-560.

Binford G.J. and M.A. Wells. 2003. The phylogenetic distribution of sphingomyelinase D activity in venoms of Haplogyne spiders. Comparative Biochemistry and Physiology Part B 135: 25-33.

Lopes P.H., R. Bertani, R.M. Gonalves-de-Andrade, R.H. Nagahama, C.W. van den Berg, and D.V. Tambourgi. 2013. Venom of the Brazilian spider Sicarius ornatus (Araneae, Sicariidae) contains active sphingomyelinase D: potential for toxicity after envenomation. PLoS Neglected Tropical Diseases 7(8): e2394. doi: 10.1371/journal.pntd.0002394

Lucas S.M. 2015. The history of venomous spider identification, venom extraction methods and antivenom production: a long journey at the Butantan Institute, São Paulo, Brazil. Journal of Venomous Animals and Toxins Including Tropical Diseases 21: 21.

Monteiro C.L.B., R. Rubel, L.L. Cogo, O.C. Mangili, W. Gremski, and S.S. Veiga. 2002. Isolation and identification of Clostridium perfringens in the venom and fangs of Loxosceles intermedia (brown spider): enhancement of the dermonecrotic lesion in loxoscelism. Toxicon 40: 409-418.

Saupe E.E., M. Papes, P.A. Selden, and R.S. Vetter. 2011. Tracking a medically important spider: climate change, ecological niche modeling, and the brown recluse (Loxosceles reclusa). PLoS ONE 6(3): e17731. doi: 10.1371/journal.pone.0017731

Schenone H., T. Saavedra, A. Rojas, and F. Villarroel. 1989. Loxoscelism in Chile: epidemiological, clinical, and experimental studies. Revista do Instituto de Medicina Tropical de São Paulo 31(6): 403-415.

Swanson D.L. and R.S. Vetter. 2005. Bites of brown recluse spiders and suspected necrotic arachnidism. The New England Journal of Medicine 352: 700-707.

Van Aswegen G., J.M. van Rooyen, D.G. van der Nest, F.J. Veldman, T.H. de Villiers, and G. Oberholzer. 1997. Venom of a six-eyed crab spider, Sicarius testaceus (Purcell, 1908) causes necrotic and haemorrhagic lesions in the rabbit. Toxicon 35(7): 1149-1152.

Wright S.W., K.D. Wrenn, L. Murray, and D. Seger. 1997. Clinical presentation and outcome of brown recluse spider bite. Annals of Emergency Medicine 30(1): 28-32.

Poison-Laced Silk

by Joseph DeSisto

The bivouac spider (Parawixia bistriata) is ordinary in appearance, brown with a sagging and trapezoidal abdomen, but with one of the strangest behaviors of any spider. They are social, working together to build large webs between rainforest trees, but that isn’t the wierdest thing — other spiders are social, too. What’s really strange is that bivouac spiders are social during the daytime, but at night, retire from society to form individual webs (Wenseleers et al. 2013). They viciously defend these webs against other spiders, even their former collaborators.

Regardless of their odd social lives, bivouac spiders are otherwise typical orb-weavers, constructing spiral-shaped webs with many spokes, nested between branches and trunks. When a fly or moth lands on the web, the spider rushes out to quickly wrap its prey in silk before injecting a lethal dose of venom.

A related species of bivouac spider, Parawixia audax. Photo by Nicolas Olejnik, licensed under CC BY-NC 3.0.

A related species of bivouac spider, Parawixia audax. Photo by Nicolas Olejnik, licensed under CC BY-NC 3.0.

Spider venom contains hundreds of different toxic chemicals, ranging from simple molecules with just a few atoms to complex proteins. There are also more than 40,000 species of spider, each with its own unique combination of chemicals, and some with toxins found nowhere else in the animal kingdom. As a result, chemists who study spider venom often make unusual and surprising discoveries.

Bivouac spider venom was the first, and remains the only, spider venom known to contain a unique class of compounds called tetrahydro-β-carbolines (Cesar et al. 2005) — we’re going to call them THβCs because, frankly, I’m pretty sure you skipped over that word, and I don’t blame you. Complex chemicals often have long names, but that isn’t the fault of chemists. The diversity of chemicals in nature is simply so great that having nice, easy names for all of them is a laughable impossibility. In this respect, organic chemistry and biodiversity have a lot in common.

Anyway, back to venom. Finding THβCs was exciting because these molecules aren’t only found in spider venoms — they’re also found in a number of plants used as medicine by indigenous peoples from Asia to Africa to South America. Seed extracts from Syrian rue (Peganum harmala), a tiny flowering plant, have been used for hundreds of years in northern China to combat both malaria and throat cancer (Cao et al. 2007). Today, scientists are studying the THβCs found in the same plant, but in a laboratory setting — it turns out they really do have anti-malaria and anti-cancer properties.

Syrian rue for sale in a Kasakhstan market. Photo by Yuri Danilevsky, licensed under CC BY-SA 3.0.

Syrian rue for sale in a Kasakhstan market. Photo by Yuri Danilevsky, licensed under CC BY-SA 3.0.

THβCs are also powerful insecticides, and probably evolved as a way for plants to defend themselves against leaf-eating insects. These are, however, a diverse group of molecules and their effects can be variable. In the Amazon, THβC-laced plants are used as recreational drugs, causing hallucinations (Cao et al. 2007). The particular variety found in bivouac spider venom, dubbed PwTX-I, causes nervous convulsions in rats (Cesar-Tognoli et al. 2011). More important to the spiders, it kills insects instantly.

The presence of THβCs in bivouac spider venom is a beautiful example of convergent evolution: the same strategy evolving to solve the same problem, in multiple organisms. Both Syrian rue and bivouac spiders needed a way to kill insects (albeit for different reasons), and each evolved the ability to manufacture THβCs to do the job.

I said earlier that bivouac spiders are the only spiders known to use THβCs in their venom — technically, that’s true. There is, however, another orb-weaving spider that uses THβCs to dispatch its prey. That spider is the giant golden orb-weaver or “banana spider” (Nephila clavipes), a songbird-sized behemoth found throughout the tropical Americas. And yes, their silk really does shimmer gold in the right lighting.

The giant golden orb-weaver, with prey in her shimmering web. Photo by Victor Patel, licensed under CC BY-SA 2.5.

The giant golden orb-weaver, with prey in her shimmering web. Photo by Victor Patel, licensed under CC BY-SA 2.5.

These are big spiders that build big webs, with a silken spiral more than three feet across. The gold shimmer comes from a yellow, reflective substance called xanthurenic acid, which the spider weaves into its silk. In 2005, however, the same Brazilian scientists discovered these spiders were also lacing their webs with THβCs — not the same kind as the bivouac spiders, but a new molecule.

When a insect, such as a moth, hits a spider web, it immediately becomes stuck in a tangle of sticky threads. These threads are covered in oil droplets, which stick to and cover the insect. As the prey wrestles with the snare, it only becomes more hopelessly tangled. Normally, this is the point where an orb-weaver drops down and bites its prey, injecting venom, but a golden orb-weaver plans ahead. The oil droplets on its web are filled with THβCs, which seep into the insect’s body as it struggles.

In combination with another toxin (specifically an organometallic 1-(diazenylaryl) ethanol: see Marques et al. 2004), THβCs make short work of the trapped insect. If the spider is lucky, death sets in before she even has to move. This might cruel, but trapping prey is risky business. Insects in spider webs flail about, trying desperately to free themselves, and some have nasty weapons of their own. By killing its prey from a distance, without ever having to lift a leg, a golden orb-weaver can avoid risking injury to itself, increasing the chances that it will live to hunt another day.

A golden orb-weaver in Jamaica. Photo by Charles Sharp, licensed under CC BY-SA 4.0.

A golden orb-weaver in Jamaica. Photo by Charles Sharp, licensed under CC BY-SA 4.0.

This is the second in a series of articles exploring how animals use chemical weapons to capture prey and defend themselves. Instead of focusing on a particular animal, each article will focus on a particular chemical, and how it is used by a variety of creatures. The first article in the series explores tetrodotoxin and the newts, snakes, fish, caddisflies, sea slugs, and other animals that use it. To read that article, click here.

You might remember that in the second paragraph I mentioned that some spiders, including Parawixia, are social. It just so happens that I wrote an article on social spiders several weeks ago — you can read that by clicking here.

Cited:

Cao R., W. Pang, Z. Wang, and A. Xu. 2007. β-carboline alkaloids: biochemical and pharmacological functions. Current Medicinal Chemistry 14: 479-500.

Cesar-Tognoli L.M.M., S.D. Salamoni, A.A. Tavares, C.F. Elias, J.C. Da Costa, J.C. Bittencourt, and M.S. Palma. 2011. Effects of spider venom toxin PwTX-I (6-hydroxytrypargine) on the central nervous system of rats. Toxins 3(2): 142-162.

Marques M.R., M.A. Mendes, C.F. Tormena, B.M. Souza, S.P. Ribiero, R. Rittner, and M.S. Palma. 2004. Structure determination of an organometallic 1-(diazenylaryl)ethanol: a novel toxin subclass from the web of the spider Nephila clavipes. Chemistry and Biodiversity 1: 830-838.

Marques M.R., M.A. Mendes, C.F. Tormena, B.M. Souza, L.M.M. Cesar, R. Rittner, and M.S. Palma. 2005. Structure determination of a tetrahydro-β-carboline of arthropod origin: a novel alkaloid-toxin subclass from the web of spider Nephila clavipesChemistry and Biodiversity 2: 525-534.

Wenseleers T., J.P. Bacon, D.A. Alves, M.J. Couvillon, M. Karcher, F.S. Nascimento, P. Noguiera-Neto, M. Ribiero, E.J.H. Robinson, A. Tofilski, and F.L.W. Ratneiks. 2013. Bourgeois behavior and freeloading in the colonial orb web spider Parawixia bistriata (Araneae, Araneidae). The American Naturalist 182(1): 120-129.

Things that Sting

by Joseph DeSisto

I love scorpions. They’re fast, predatory, venomous … really, everything you could want in animal. So in Arizona, while I was supposed to be collecting caterpillars, I occasionally took a “scorpion break,” flipping rocks and digging in the sand.

We spent a day collecting along Montezuma Pass, a mountain road that winds through Coronado National Forest. Towards the base of the mountain, where pine forest gives way to grassy scrub, I struck gold. Scorpions were everywhere.

The stripe-tailed scorpion, Vaejovis spinigerus, ready for action.. Photo by Joseph DeSisto.

The striped devil scorpion, ready for action.. Photo by Joseph DeSisto.

By far the most common was the striped devil scorpion (Vaejovis spinigerus), which is found in dry, lowland habitats through much of Arizona and New Mexico. These are big scorpions, approaching 3 inches in length, with a sting that is painful but not medically threatening (except in the case of an extreme allergic reaction).

The biggest scorpions were the mothers, carrying babies on their backs. Although striped devil scorpions can reproduce by mating, if the pickings are slim, a female can also produce young asexually, without mating.

A mother stripe-tailed scorpion, carrying young. Photo by Joseph DeSisto.

A mother striped devil scorpion, carrying young. Photo by Joseph DeSisto.

So I wandered through the sun-baked grass, flipping rocks and scrutinizing the ground. I was so focused on my scorpions that I stumbled into one of the strangest things I had ever seen. In the middle of the field was a circular clearing, about six feet across, where no plants grew. There was only sand and, on closer inspection, ants.

A harvester ant "arena." Photo by Joseph DeSisto.

A harvester ant “sand garden.” Photo by Joseph DeSisto.

The ants in question were harvester ants (genus Pogonomyrmex). Harvester ants are scavengers that subsist mostly on seeds. I watched as teams of ants brought back seeds and the occasional dead insect from the surrounding grassland, stuffing them into small entrances that led to a vast network of underground tunnels and storage chambers.

I’d heard that harvester ants were defensive and had powerful stings, but these ones seemed comfortable with me strolling across their sand garden, and even kneeling to get a closer look. To see if their reputation was justified, I grabbed one and pressed its abdomen against the soft skin of my wrist.

A team of harvester ants carting a stink bug back to their nest. Photo by Joseph DeSisto.

A team of harvester ants carting a dead stink bug back to their nest. Photo by Joseph DeSisto.

Sure enough, it stung me and administered a healthy dose of venom. The result was a sharp pain, like being stuck with a pin, and this pain grew over the next 30 minutes or so. Ultimately, however, I was disappointed — it didn’t hurt that much. Were these ants, which seemed to have such a well protected territory, aggressive at all?

Perhaps the ants simply weren’t frightened by me. I decided to give them a real threat, and see how they reacted. So I flipped a few rocks, grabbed the first scorpion I could find, and dropped it into the center of the sand garden.

The response was immediate and severe. The moment the scorpion hit the ground, a party of three ants grabbed onto it with their large jaws and began to sting. As they did so, they released a pheromone, a chemical alarm signal that brought dozens more ants to their aid. Within seconds, the scorpi0n was surrounded.

A scorpion after being killed by an army of harvester ants. Photo by Joseph DeSisto.

A scorpion after being killed by an army of harvester ants. Photo by Joseph DeSisto.

A few minutes passed, and the ants piled on. The scorpion quickly expired, as sting after sting injected deadly venom. Still, the ants held guard over the invader for several hours, as if to make sure it didn’t come back to life.

The same scorpion, long after death, still guarded by a team of ants. Photo by Joseph DeSisto.

The same scorpion, long after death, still guarded by a team of ants. Photo by Joseph DeSisto.

Harvester ants clearly don’t like scorpions, which makes sense. It’s likely that, if caught alone, a single ant would be easy prey for a 3-inch-long scorpion. But a scorpion, no matter how large, would be unwise to enter a harvester ant colony’s sand garden. Given how quickly and violently the ants responded, I thought that perhaps I too should give them a wide berth. Striped devil scorpions might be large, intimidating, and ready to sting, but they are by far not the fiercest or most venomous animals of Montezuma Pass.

The Green Lynx: Friend or Foe?

by Joseph DeSisto

I’m currently in Arizona, and don’t have much time for writing, but I’m just seeing too many cool things not to show you one of them. There are some amazing arachnids in the Southwest: so far I’ve seen tarantulas, scorpions, sun spiders, and black widows, all of which will appear on this site in some fashion. As a teaser, here’s a green lynx spider, common in much of the southern United States:

The green lynx spider (Peucetia viridans), with a yummy burnet moth. Lynx spiders are extremely versatile predators -- this moth is highly toxic, but the spider doesn't seem to mind. Photo by Joseph DeSisto.

The green lynx spider (Peucetia viridans), with a yummy burnet moth. Lynx spiders are extremely versatile predators. Photo by Joseph DeSisto.

The prey above is a burnet moth in the family Zygaenidae. Its colors are a warning sign that this moth is extremely toxic, loaded with hydrogen cyanide, but this spider doesn’t seem to mind.

Most spiders fall into one of two categories based on hunting strategy: ambush predators (including web-builders) and wandering hunters. Lynx spiders are both. The one above waited patiently before grabbing a passing moth. Tomorrow, though, it might just as easily stride over the foliage, searching for less active preys such as caterpillars.

Aside from being an efficient hunter, green lynx spiders are very adaptable and do well in gardens and farms. Because they can be extremely abundant, biologists have studied their ecology to determine whether they might be useful to farmers. Whitcomb et al. (1963) recorded them as important predators of several important crop pests, notably the corn earworm, cotton leafworm, and cabbage looper, all three both as adult moths and as caterpillars.

The corn earworm, an important pest caterpillar in American corn fields, and prey for lynx spiders. Photo by Cydny Sims Parr, licensed under CC BY-SA 2.0.

The corn earworm (Helicoverpa zea), an important pest caterpillar in American corn fields, and prey for lynx spiders. Photo by Cydny Sims Parr, licensed under CC BY-SA 2.0.

The trouble is, lynx spiders don’t really care if the insects they eat are pests, and some of their favorite prey are beneficial insects. They are more than happy to eat wasps, which themselves are important caterpillar predators. The honey bee, an important pollinator, is one of their favorites (Whitcomb et al. 1966).

To determine just how useful lynx spiders actually were, Randall (1982) conducted a survey of prey items captured by lynx spiders throughout Florida. He then rated the spiders’ victims by importance to farmers: an insect cadaver could be either extremely harmful (e.g., cotton leaf worm), extremely beneficial (e.g., honey bee) or anywhere in between.

The green lynx spider. Photo by Joseph DeSisto.

The green lynx spider, Peucetia viridans. Photo by Joseph DeSisto.

A total of 66 prey items were recorded, and in the end, data pointed to these spiders eating as many beneficial insects as harmful ones. So the green lynx spider might not be especially important to farmers. Or, more accurately, the green lynx is neither beneficial nor harmful. It is simply a spider trying to make a living however it can, irrespective of human interests.

Cited:

Randall J.B. 1982. Prey records of the green lynx spider, Peucetia viridans (Hentz) (Araneae, Oxyopidae). Journal of Arachnology 10: 19-22.

Whitcomb W.H., H. Exline, and R.C. Hunter. 1963. Spiders of the Arkansas cotton field. Annals of the Entomological Society of America 56(5): 653-660.

Whitcomb W.H., M. Hite, and R. Eason. 1966. Life history of the green lynx spider, Peucetia viridans (Araneida: Oxyopidae). Journal of the Kansas Entomological Society 39: 259-267.

Can Spiders Have Personalities?

by Joseph DeSisto

Should you ever find yourself in the Kalahari Desert in southern Africa, keep an eye out for small spiders with plump, velvety bodies, black-banded legs, and a thick white stripe running down the abdomen. These spiders are likely to be Stegodyphus dumicola, the social velvet spider.

Now capture as many as you can and put them in a jar. If these were ordinary spiders, the jar would immediately become the stage for a bloodbath: most spiders are fiercely cannibalistic. But velvet spiders are not ordinary spiders. Put them in a jar, and they will begin to work together, filling the jar with silk. All of this happens with virtually no disagreement, bullying, or typical spider humorlessness.

The Kalahari Desert, dominated by the small shrubs on which Stegodyphus spiders like to build their webs. Photo by Harald Süpfle, licensed under CC BY-SA 2.5.

The Kalahari Desert, with the shrubs and small trees on which Stegodyphus spiders like to build their webs. Photo by Harald Süpfle, licensed under CC BY-SA 2.5.

Recently I had the chance to talk with Colin Wright, a PhD student studying social spiders at the University of Pittsburgh. As it happens, he has been to the Kalahari and seen firsthand what happens when these spiders team up. He told me, “These spiders are highly tolerant of one another. In fact, they are so tolerant that they will even get along with other closely related Stegodyphus spiders being in their colonies.”

Outside the jar, social spiders work together to construct large webs. These webs contain shelters in which the spiders can hide, and sheet-like portions used to capture insects. Although each spider alone is small and unlikely to succeed in tackling dangerous prey, together these spiders can take down animals much larger than themselves. I asked Wright what the record was for largest prey in a social spider’s web:

“Believe it or not, we have found the carcasses of mice in some of [dumicola‘s] capture webs. We have yet to see any active feeding on small mammals, but I don’t doubt for a moment that, in larger colonies, they will devour a mouse. We have fed these things giant locusts, while it’s a struggle, they don’t have that much difficulty taking them down.”

A variety of Stegodyphus species, with varying degrees of social aptitude. S. dumicola is shown in G and H. Figure 3 from Miller et al. (2012).

A variety of Stegodyphus species, with varying degrees of social aptitude. S. dumicola is shown in G and H. Figure 3 from Miller et al. (2012), licensed under CC BY 3.0.

Working together, a team of social spiders can be as formidable as any tarantula, but not all spiders participate equally in prey capture. If a moth tumbles into the web, no problem, but when a locust or wasp gets stuck, only a brave spider wants to take the first bite. Here’s where things get complicated — some spiders are more cautious spiders than others, and when a colony is dominated by cautious spiders, life becomes much more difficult.

In any society, two major factors affecting success are a) the number of individuals and b) the personalities of those individuals, whether they be spiders, bees, or humans. But which is more important? You might think that since spiders aren’t especially bright, numbers matter more than something as abstract as personality, but Keiser and Pruitt (2014) would disagree. They did just what I told you to do on your hypothetical Kalahari vacation, only instead of putting spiders in jars at random, they tested each spider to determine whether it was “bold” or “shy.”

When reading peer-reviewed papers, it’s tempting to skim the “Methods” section, since often the methods are barely comprehensible to a non-expert. This paper, however, was worth a closer read. The authors tested spider bravery — or in their words, conducted “individual personality assays” — by puffing air on them using  “an infant ear-cleaning bulb.” Arachnologists like to have fun.

A colony of social velvet spiders, Stegodyphus dumicola, in South Africa. The silk fortress in the center is a shelter, into which the spiders can retreat if disturbed. Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

A colony of social velvet spiders, Stegodyphus dumicola, in South Africa. The silk fortress in the center is a shelter, into which the spiders can retreat if disturbed. Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

When so accosted, the spiders stopped moving, and the time it took for them to start moving again was used to decide whether the spider was bold, shy, or in-between. Keiser and Pruitt then assembled these spiders into colonies, either with only bold individuals, only shy, only in-between, or a mix. They also made some colonies larger than others, to see whether colony size was more important than personality.

Next, Keiser and Pruitt wanted to see how quickly the spiders in each colony responded to a prey item. Naturally everything had to be standardized, so instead of using live insects they simulated a prey item in each web using “a battery-powered handheld vibratory device.” As expected, colonies with more bold spiders responded the fastest to the simulated prey, since bolder spiders have fewer qualms about leading the attack.

Personality had a much bigger role than colony size in deciding which colonies responded the fastest to prey, so much so that “colonies of just 10 bold spiders would attack prey with as many attackers as colonies of 110 ‘average’ spiders” (Keiser and Pruitt 2014). Even more revealing, a separate experiment showed that if you add just one bold spider to a colony of timid spiders, the whole colony becomes more aggressive (Pruitt and Keiser 2014).

Stegodyphus dumicola, at the edge of the web. Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

Stegodyphus dumicola, at the edge of the web. Photo by Bernard Dupont, licensed under CC BY-SA 2.0.

The bolder spiders tend to be the largest (Wright et al. 2015), and because spiders don’t take turns eating, smaller or slower individuals sometimes have to skip meals. Colonies don’t always harvest enough insects to keep everyone plump and happy, but if a web isn’t very productive, spiders always have the option of leaving to start their own colonies. When things get tough, a social spider can make a balloon out of silk and float away on a draft (Schneider et al. 2001).

We can learn a lot about the evolution of social behavior by studying social spiders. There’s still a lot left to learn, but one thing is clear: the next Spiderman movie must feature a superhero who can amass a spidery army to take down even the biggest, baddest villains, and then balloon his way out of tense social situations.

A big thank you is owed to Colin Wright for answering some of my questions about Stegodyphus dumicola. Wright is a student in Dr. Jonathan Pruitt’s lab at the University of Pittsburgh: you can learn more about that lab’s work here.

Several months ago I wrote an article called “Spiders Only Love Once” about another Stegodyphus species, the lined velvet spider (S. lineata). Lined velvet spiders don’t form colonies, but they do have amazing lives, which you can learn about here.

Cited:

Keiser C.N. and J.N. Pruitt. 2014. Personality composition is more important than group size in determining collective foraging behaviour in the wild. Proceedings of the Royal Society B 281(1796), doi: 10.1098/rspb.2014.1424

Miller J.A., C.E. Griswold, N. Scharff, M. Řezáč, T. Szűts, and M. Marhabaie. 2012. The velvet spiders: an atlas of the Eresidae (Arachnida, Araneae). ZooKeys 195: 1-144.

Pruitt J.N. and C.N. Keiser. 2014. The personality types of key catalytic individuals shape colonies’ collective behaviour and success. Animal Behavior 93: 87-95.

Schneider J.M., J. Roos, Y. Lubin, and J.R. Henschel. 2001. Dispersal of Stegodyphus dumicola (Araneae, Eresidae): They do balloon after all! The Journal of Arachnology 29: 114-116.

Wright C.M., C.N. Keiser, and J.N. Pruitt. 2015. Personality and morphology shape task participation, collective foraging and escape behavior in the social spider Stegodyphus dumicola. Animal Behavior 105: 47-54.

A Tetragnathid Spider Walks into a Bar …

by Joseph DeSisto

… and the bartender asks, “Why the long fangs?”

A Tetragnatha species from Vermont. You might have to look closely to see the fangs, they are quite inconspicuous. Photo by Tom Murray.

A Tetragnatha species from Vermont. You might have to look closely to see the fangs, they are quite inconspicuous. Photo by Tom Murray, used with permission.

But seriously. Long-jawed orb-weaver spiders, family Tetragnathidae, sport some of the longest, scariest, most ridiculous-looking fangs in the spider world. Why?

As their name suggests, long-jawed orb-weavers build orb-webs, which are spiral-shaped webs with radiating spokes. Walk along the edge of a pond when the weather warms up and you may see horizontal orb-webs covering the vegetation just above the water. These webs are built by tetragnathid spiders hoping to catch insects that spend their larval stages in the water before undergoing metamorphosis and flying off. Many mosquitos end their lives early in this way, without even completing their first flight.

The web of Tetragnatha laboriosa. Can you find the spider? Photo by Alex Wild, in public domain.

The web of Tetragnatha laboriosa. Can you find the spider? Photo by Alex Wild, in public domain via Insects Unlocked.

Since long-jawed orb-weavers are predatory, and their fangs are used to inject prey with venom, we might expect that long fangs are adaptations for dispatching a particular kind of prey. This makes sense — other spiders with large fangs often specialize in dangerous prey. For example, there is an ant-mimicking jumping spider (Myrmarachne plataleoides) from Southeast Asia with fangs even longer than those of tetragnathids. Myrmarachne uses its fangs to capture — you guessed it — stinging ants.

A male weaver-ant-mimicking jumping spider, showing off his freakishly long fangs.

A male weaver-ant-mimicking jumping spider, showing off his freakishly long fangs. “Myrmarachne plataleoides male at Kadavoor” © 2010 Jeevan Jose, Kerala, India is used here under a Creative Commons Attribution-ShareAlike 4.0 International License.

But look a little closer, and it turns out that the long fangs of Myrmarachne aren’t crucial for ant-killing. How do we know this? Because the females don’t have them. They have fangs of course, but only normal-sized ones, and these work for taking out venomous ants just fine. The females may even be better predators, since they are more accurate ant mimics.

The males are lousy hunters. Their ridiculous fangs mean they aren’t very good at mimicking ants, and what’s more, their fangs don’t even have ducts for injecting venom. Instead, they use their fangs as swords in male-to-male combat (Pollard 2009).

The female Myrmarachne plataleoides, same species as the male above. She has more ordinary-sized fangs, but is a better ant mimic and so a more effective than the male, who instead uses his fangs as swords against other male spiders. Photo by Sean Hoyland, in public domain.

The female Myrmarachne plataleoides, same species as the male above. She has more ordinary-sized fangs, but is a better ant mimic. Photo by Sean Hoyland, in public domain.

Tetragnathids, living in their orb-webs, don’t have to worry too much about ants. In fact their prey are mostly defenseless flies and other short-lived insects emerging from the water. Like Myrmarachne, their enlarged fangs have a reproductive function, but instead of males using them to fight off other males, both males and females use them during courtship.

Here’s how it works (see Eberhard and Huber 1998). When a male long-jawed orb-weaver finds a female’s web, he approaches cautiously — if the female is receptive, there is a chance she will not eat him. He starts by signalling his presence with precisely timed “twangs” of her web, strumming the orb’s spokes like guitar strings. If she approves, she allows him to continue tapping at her web and her body with his legs.

The orchard spider (Leucauge venusta), a common tetragnathid in North and Central America. Photo by Andrea Westmoreland, licensed under CC BY-SA 2.0.

The orchard spider (Leucauge venusta), a common tetragnathid in North and Central America. Photo by Andrea Westmoreland, licensed under CC BY-SA 2.0.

Now the fangs come in. The female orb-weaver opens hers wide, the male moves forward, and their fangs interlock. At the base of each fang is a ridge or tooth that helps keep the couple together, so that the male spider can only leave if the female allows him to do so.

When the male has finished transferring his sperm, the female releases him from her grip and he can leave. Post-mating cannibalism in tetragnathids is rare, despite being common in other families of orb-weaving spiders (i.e., araneids).

Tetragnatha laboriosa, the spider that weaved the web shown earlier. Photo by Alex Wild, in public domain.

Tetragnatha laboriosa, the spider that weaved the web shown earlier. Photo by Alex Wild, in public domain via Insects Unlocked.

Two of the photos used in this article are from Insects Unlocked, a project run by the infamous macro-photographer Alex Wild. The goal of this project is to generate high-quality photographs of insects and other invertebrates for the public domain, available to everyone, everywhere, for free. If you enjoy this blog and others that depend on freely available insect photography, please consider donating to Insects Unlocked.

Cited:

Eberhard, W.G. and B.A. Huber. 1998. Courtship, copulation, and sperm transfer in Leucauge mariana (Araneae, Tetragnathidae) with implications for higher classification. Journal of Arachnology 26(3): 342-368.

Pollard, S.D. 2009. Consequences of sexual selection on feeding in male jumping spiders (Araneae: Salticidae). Journal of Zoology 234(2): 203-208.