The triangulate cobweb spider (Steatoda triangulosa) can capture animals that are up to 50 times its own weight, lifting them up off the ground into its web using five different kinds of silk. This would be like you or me hoisting as much as 4 tons off the ground using only secretions from our bodies. It’s mind-boggling (and truthfully kinda gross) to picture how we might accomplish that.
As for the spider, we learn much more about how this is done, thanks to a description by Gabriele Greco and Nicola M. Pugno in the February issue of the Journal of The Royal Society Interface.
Here’s a better look at our new friend:
Steatoda triangulosa enjoys a meal
Greco and Pugno studied the architecture of the web and the technique the spider uses by planting some big cockroaches on the ground and observing. Here’s a lovely example that they filmed:
You can see there’s a big supporting web above, and there seems to be some stuff dangling from it. You might also notice that the spider keeps going back and forth to accomplish the hoist.
If you or I were to attempt to get a piano into our second-floor apartment, we’d probably get rope and some kind of pulley. But that’s not what the spider is doing. Let’s look at the diagram that Greco and Pugno give us:
The architecture of the triangulate cobweb spider’s web
The silk with label (b) is the supporting silk, and it’s very strong and tough. The label (d-e) refers to anchor points, which are made at the top and bottom by a sort of cementlike silk. The silk at (f) is meant to reinforce junctions. But the (c)-type silk is perhaps the most interesting. It’s very elastic, like a bungee cord, and near the ground it’s covered with blobs of glue. Here’s a closeup of that (c)-type silk near the ground:
Elastic silk with glue droplets
So an unsuspecting lizard, mouse, or cockroach walks by and gets stuck on some of the dangling threads. They detach from the ground fairly easily when they’re struck, and because they’re like bungee cords, they cause a twang to shoot up to the spider, giving the signal that it’s time to get to work.
The spider then starts weaving more bungee-like silk and pulls it down to the prey, attaching it with some anchorage silk. I found a guy on the Internet who was willing to demonstrate this technique, which was very nice of him:
Bungee cord training
If this guy wanted to pull a huge cart, he wouldn’t get too far hooking up that lone bungee cord to it. But if he kept on bringing more cords and kept hooking them to the cart, eventually the tension in the bungee cords would start pulling the cart. That’s more like what the spider does.
The (c)-type elastic silk has to snap back really well, like a bungee cord. There are other elastic spider silks that stretch like this but don’t recover as well, such as capture silk that’s set out for big insects to fly into. Capture silk needs to give but then not recoil back too hard, or else the flying insect would get launched back across the room, which wouldn’t be very useful, other than maybe as entertainment, to the spider.
The basis of these elastic silks, which are made of protein, seems to be little “nanosprings” in its structure. A typical repeating unit in a spider silk is glycine-proline-glycine-glycine-tyrosine (abbreviated GPGGY), and we can draw one unit of this with the Tulane peptide drawer. This is just the straight-line view:
GPGGY linear structure
In reality, when a bunch of these units are strung together, they’ll form a springlike structure. Proline is really good at introducing kinks into a protein, so the sharp turns you see in the figure below are at the prolines. (Not to be confused with pralines, which are much tastier.) The tyrosines are the ones with the little rings dangling off the chain.
A series of GPGGY units strung together and modeled in 3-D
These nanosprings are even more effective in certain configurations. If a protein has a network of these, it can actually pull back harder and harder the more you lengthen it by pulling. One model as a simple example of how this can happen looks like this. If you pull on the right side, it gets more difficult as you go, because once you extend the lone spring, then you’re pulling on two springs and then four:
One spring arrangement that would explain an exponential force-length curve
Real spider silk has arrangements that are more complicated than this, but you get the idea. These elastic silks can break or lose their springiness if you stretch them too far, but the spider somehow knows not to do that.
The spider has some other tricks, too: it will also wrap the prey in yet another type of silk to help restrict its motion, and it will ultimately inject the prey with venom to paralyze it.
So you have this astonishing little machine that secretes all the right things at all the right times, which still amazes author Gabriele Greco, who says:
The spider is a perfect factory of silk. It produces multifunctional materials in less than milliseconds — at least five different materials — each one optimized for that property, so it's crazy.
I’ve got some furniture I’ve been meaning to move upstairs, so if any spiders are out there reading, I pay competitive rates. Let’s save the venom thing for another job, though, OK?
