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.

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