Triangle-weaver spider (Hyptiotes cavatus) trap springing

Welcome! It’s Halloween, a day when things we’d maybe rather not think about come out to play. Which means it’s time for this blog to get spidery.

Today we’re answering an uncomfortable question: what if, when you bumped into a spider’s web, it not only sensed you but sprung a carefully-built trap that actively folds gluey strands around you at supersonic speeds? Before you even know what’s happening you can’t move. But the spider can.

Fair warning: this post has multiple images of spiders—that’s kind of the point. Let’s get a little too close to Hyptiotes cavatus, the triangle-weaver.

A wilder view

Like many of our investigations into animal behaviour, this one starts a long time ago, yet has only recently been fully understood. Our old-timey collaborator is Burt Green Wilder, Professor at Cornell, Civil War surgeon and coiner of the word ‘neuron’. In 1874, he was strolling through the woods near Ithaca, New York—in October, appropriately enough—when he spotted a curious triangular spider web hanging from a hemlock branch. The web’s builder seemed absent, until Wilder noticed a tiny brown blob in one corner, just a few millimetres long.

Being a scientist, Wilder just had to poke the web. To his amazement, it suddenly fell slack, then slowly regained its original taut triangle shape. He realised the spider speck was gradually pulling on the thread in its corner of the web, taking up the line ‘foot over foot’ as he wrote in an 1875 piece for Popular Science Monthly magazine. That’s one of his illustrations in the image at the top of this post, with the spider in its waiting spot.

The arachnid’s attack strategy is simple but ingenious. A flying insect blunders into the web, and struggles a little. In Wilder’s words:

As soon as the violence and repetition of the vibration indicate that an insect is really entangled, the spider awakes from her apparent apathy; she lets go with her hinder feet; the net, released from its tension, flies forward, and at the same time flaps from side to side…as may be imagined, all this is pretty apt to involve the fly beyond the possibility of escape; but, if the spider does not feel certain of this, she creeps backward again, foot over foot, as before, and again springs her net; and this I have seen repeated in quick succession six times before the spider has ventured to make a personal approach.

The italics for ‘six times’ are Wilder’s own—he was evidently impressed with the work ethic of the hungry little creature.

Wilder’s notes were reprinted by many writers in the following decades, and corroborated by new observations, for example this from Ulrich Gerhardt in 1924 (translated from the original German):

Therefore, you can often wait a long time for the spider to move. But if the fly moves only a little, the spider glides on the main thread at lightning speed in a sudden jerk about 2 cm away, a second and third such jump usually follows, and now the fly is completely entangled in the glued meshes of the out of shape net.

Principles of line management

These reports give us the basics, the outline of an unusual hunting plan executed by a minuscule arachnid weighing less than a hundredth of a gram. But the details, like those of a nineteenth century illustration, remained blurry. Speaking of which, here’s a better image of a triangle-weaver in its pre-snap pose, from a 2019 study by Sarah Han at the University of Akron:

Han’s work, which was part of her PhD, finally figured out what the triangle-weaver was up to. And in the process, science encountered the first evidence of an animal building a device to amplify its own power.

The breakthrough came when Han zoomed in and slowed down the moment of trap release. She saw that, contrary to what Wilder and Gerhardt had assumed, the anchor line on which the spider waits is incomplete. The Hyptiotes is actually a living bridge between a coiled back section of line—attached to a hemlock branch, for example—and the unattached rest of the web. The two segments of that line are held in place by little spider muscles in the front and back legs. The web tension truly is, pardon the pun, palpable.

Let’s see it in action, from footage captured for the 2019 study (note this video has no sound). The films are either slowed down to show the moment of release and insect envelopment, or in one colour scene sped up to show multiple web resets and trap releases in a single capture. The final part of the film is a simulation of the forces involved:

Springtime

Many insects can ratchet up energy in their own bodies, allowing for single powerful leaps to escape predators or travel long distances. Elaterid beetles, for example, make a characteristic snapping sound as they unlock the energy stored in a hinge in their thorax (and held in place with an anatomical peg) to suddenly leap, giving the common name ‘click beetles’. But triangle-weavers aren’t storing energy within themsleves. They are merely the holding mechanism. The true potential lies in the way that the spider draws in loops of line via its back legs—that ‘foot over foot’ movement seen by Wilder.

This sequence from a follow-up paper by Han in 2021 shows that movement broken down into frames, as the spider moves back and up to the right (the back legs are coloured for emphasis):

The speckled coils of the anchor strand building up underneath the spider are the key to the trap. You can see them more clearly in the earlier image by Han, and the coils might be either tight or more loose, as she captures here:

Once the spider loosens its back grip on the tense anchor line, it hurtles forward at astonishing speed. It can end up travelling at over two metres per second, hundreds of bodylengths in the blink of eight eyes. The maximum recorded acceleration, almost 800 metres per second squared, is 20 times more than that of the Saturn V rocket that took Apollo 11 to the moon. For the sake of Halloween let’s imagine a human-sized (2m tall) spider performing the same trick. It would spring its trap from more than a football field away before you could even start to wonder what sticky mess you’d blundered into. It’s seriously quick.

The Hyptiotes web also slows the spider down again as it unravels, albeit with some rocking motions as the spider regains its balance. Where the web folds against itself it easily separates for the next launch, but where it contacts prey it remains stuck fast. Ultimately the arachnid moves in for the traditional wrap-and-drain procedure, leaving a bound husk of whatever unlucky bug it snared. Back in Ithaca, Wilder watched this part too, documenting how the spider flings its web around a fly like a blanket, then:

turns it over and over as a ball, hanging the while by her front legs, and, with the hinder pair used now alternately, drawing out from the expanded spinners broad sheets of silk which, relatively to the power of the fly, are like steel bands upon a man.

And there you have it. A spider that makes a catapult/crossbow in which its own body is a central component. Just like for those human weapons, Hyptiotes offloads the power needed for its sudden attack onto its own creation, which means it doesn't need to evolve that same power within itself. Perhaps the greatest remaining mystery is how it is able to hold that tension for many hours while waiting for prey to stumble by. Both Wilder and Han marvel at that feat, more than 140 years apart, without having a ready answer. A true Halloween mystery.

(One quick aside: Han and her colleagues came up with this chart showing how much of the trap’s power comes from externally stored energy, fed bit by bit into the web by the spider. It nicely illustrates just how effective the trap is, while inventing a scale that seems suited to more than just spider power: the ‘web jerk magnitude’. Feel free to adopt that scale for your daily online interactions.)

Finally, because of the nature of this blog we need to ask whether or not this is an example of animal tool use. The spider is actively holding and manipulating the web, and propelling it in a specific direction at great speed, all of which matches traditional views of how tools work. Yet the other two corners of the web are held fast, and the trap is never a discrete object unlike, say, the net of the ogre-faced spider, or the sticky bolas swung by those eponymous arachnids. Hyptiotes instead acts more like someone who wields a large machine, a net-loaded turret gun or trebuchet perhaps, where the design, control and launch are all self-directed, but the environment assists by giving a solid grounding.

We therefore have a good candidate for a concept we’ve recently covered here. The trap is a fixel: an immobile or semi-mobile object that performs a tool-like function, but is not physically separated from the world. A scratching post, not a hand-held backscratcher. And a weapon that you should be grateful belongs to an attacker no bigger than a grain of rice, should you walk face-first into its trap this fine Halloween.

Sources: Wilder, B.G. (1875) The Triangle Spider. Popular Science Monthly 6:641-655. || Gerhardt, U. (1924) Weitere Studien über die Biologie der Spinnen. Ebenda Jg. 90, Abt. A, 85. || Han, S.I. et al. (2019) External power amplification drives prey capture in a spider web. Proceedings of the National Academy of Sciences, USA 116:12060-12065. || Han, S.I. et al. (2021) Permanent deformation of triangle weaver silk enables ultrafast tangle‑free release of spider webs. The Science of Nature 108:60.

Main image credit: Wilder (1875) || Second image credit: Han et al. (2019) || Third and fourth image credits: Han et al. (2021) || Video credit: Han et al. (2019); https://www.youtube.com/watch?v=Xpb_14sWFYU