When you watch spiders for a long time you’re guaranteed to see something weird. In all four giant webs I saw the Guatemalan long-jawed spiders engaged in bungee jumping.
Every once in a while a spider would spontaneously dive from the web on a line of silk. After the drop, sometimes the spider would climb back up leaving the line in place, and sometimes the spider would climb back up rolling that new silk line into a ball of tuft. When it made a ball of tuft, sometimes it would eat the ball, sometimes it would deposit it at the center of its own orb web, and sometimes it would place the tuft on the web and wander off. When they wander off, it’s as if they were creating the tuft not so much for themselves but somehow for the community of spiders living in the web.
Why do these spiders engage in bungee jumping?
On Thursday night, I was out at Arkansas Bend Park, on Lake Travis, and the spiders were still doing this. The few that are left seem to be concentrating together, and they are still making sheets together, even if insubstantial. As I watched, I was able to glean one more piece of this mystery. Sometimes when a spider dropped, it would stop a distance above the ground and assume this striking pose:
Whenever a spider did this, a line of silk would extend from its abdomen horizontally into the breeze. It was allowing the air to pull the silk out of its spinnerets. This silk was sticky, as it would stick to me when the breeze blew it my way. When the silk stuck to something, the spider would usually climb back up the thread back to the place where it had begun its dive. It would roll silk up as it did this, but somehow the horizontal thread would remain, and the spider would climb with it still anchored to that distant place. More often, though, that horizontal line stuck to nothing, and the somehow managed to disappear when the spider climbed again.
The long-jawed spiders frequently made tufts at both Starnes Island and Arkansas Bend Park. At dusk most of the females were busy building orb webs and the males were generally wandering around looking for females. As the males and a few females wandered, they would sometimes drop to the ground and come up again rolling up the silk. They’d stick the tuft to the communal web and wander off, sometimes even vacating the vicinity. What function do these tufts serve, if any? Why do the spiders invest energy and silk into dropping?
I tracked one of these spiders for a while on video tape. This was Thursday November 8th at around 11pm, at Arkansas Bend Park. As you watch the video, see if you can figure out what’s going on, though it’s hard to see the silk threads. The video starts off watching a male on the ground but then switches to tracking a spider that appears to be a female. Notice how this one female exhibits several of the dropping and balling behaviors, including balling up existing web into tufts that she leaves behind. If you can watch this one spider for the entire 9 minute video, consider yourself a certified spider watcher (or a certified something, anyway).
After dark, the females in orb webs would begin dropping too. The female would almost always deposit the tuft on the hub of the web, though on a few occasions I saw her eat the tuft instead. It makes sense that the tuft may serve a function when placed in the hub of the orb web, but I don’t understand why a spider would make a new line of silk and immediately eat it. Many spiders are known to eat their webs when they take them down, allowing them to reuse much of the protein in the next web. But these spiders weren’t always eating old silk. They were often eating silk they had just spun. Perhaps they intended to accomplish something else with the silk and would only ball up the silk if the task was not successful. Or perhaps they’re simply in a habit of making tufts and occasionally do so without meaning to, whereupon they quickly eat the tuft before any other spiders notice (which they probably won’t — their eyes likely do not make images).
As dawn approached and through much of the morning, many more females joined the males in wandering around the web. These females would drop, make tufts, and either leave them or (more rarely) eat them, just as the males had been doing much of the night. Many of the females were also rebuilding their orb webs, perhaps refreshing webs that had tattered over night.
Now after all this talk of tuft-making, I have to make a huge clarification. In my eighty-odd hours observing the spiders at Lake Tawakoni State Park and Wind Point Park, both on Lake Tawakoni, I don’t think I ever saw a spider make a tuft. I’m sure they occasionally did so, but it was not common enough for me to notice it or to get it on tape. I only saw spiders at Starnes Island and Arkansas Bend Park making tufts. Even so, the spiders on Lake Tawakoni dropped just as often.
At Lake Tawakoni State Park, when a spider dropped to the ground, it would soon climb back up the line, but it would not roll the line up. It would climb all the way back up to the web and continue on its way. I have a single video of a female in an orb web dropping a few inches and climbing back up, but in the video I did not see the tuft that was likely made. In all other cases, the spiders were making vertical or diagonal lines and apparently leaving them in place.
Perhaps the most significant observation I have about all this bungee jumping occurred at Lake Tawakoni State Park. One night I periodically observed a particular spot over about two hours, hoping to learn the secrets of how the spiders made their fantastically monstrous web. In this spot, the first spiders to drop would go all the way to the ground. On the ground they wandered around a bit, perhaps anchoring the new line, perhaps simply trying to find the line again. Later spiders would drop to a point just above the ground and anchor their new line to one of the lines that was already anchored on the ground. As time passed, the spiders would drop shorter and shorter distances, apparently stopping when they reached an existing line below. Each time the spider would return to the web above and wander off, often disappearing from sight, leaving its new drop line intact.
This simple dropping behavior of many spiders had made a vertical structure. I had trouble seeing much of the structure at once and so can’t be sure what it looked like, though I suspect it was a random tangle. It seems to me that dropping serves some communal function. Perhaps if the food is abundant, as it was around Lake Tawakoni in the form of midges, the spiders leave the lines in place. If the food is more scarce, as on Lake Travis, perhaps the spiders have stricter criteria about when to leave the line in place, or perhaps leaving tufts on the web helps attract midges or helps the midges to stick.
Bungee jumping may be how the spiders created the dense tangle webs I saw at all the four web locations. The more spiders wandering, the denser the tangle, and the greater the density of spiders the tangle can house. Dr. Ruth Buskirk, an arachnologist at UT Austin, has told me that she has seen other kinds of long-jawed spiders in tangle webs, not necessarily as dense or expansive, and that the tangle likely serve as a retreat that protects the spiders from predators such as wasps. Some of the tangle webs at Lake Tawakoni State Park housed about thirty spiders per cubic foot, and there were certainly wasps scouring the web looking for them.
Even if the bungee jumping somehow explains the tangle webs, I don’t think it explains the sheet webs. I’ll post soon on my observations of how the spiders made the great swathes that wrapped up the trees.