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.

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