Do spiders perform 'system identification'?

Question

Structural analysts can identify vibration modes, and often locate defective structural elements, by measuring vibrational responses while applying a driving signal to the strudture under examination. This is referred to as system identification (SI). When you lightly disturb the webs of spiders of particular species (e.g., daddy longlegs) you will see the spider pumping his body to shake the web, sometimes by a reciprocating motion and sometimes by whirling in circular motion. He invariably finds a resonant frequency and his oscillations can become pretty large. Could he be employing SI to detect the presence of, and possibly locate, a prey insect? Web sites I have seen state that the shaking serves either to further entrap the prey, or as a form of camouflage for the spider itself. Could SI also be a reason? Detection/location by SI could be useful at night or if the spider's eyesight isn't very clear.

The daddy longlegs I have in mind is apparently the real thing, a pholcid (2-segment body, makes a sloppy web) living among, or maybe preying on, the pseudo longlegs (harvestmen) that hang out in my bathroom. On further investigation, his circular vibes seem more defensive. Whether the calculated effect is intimidation or confusion I can't tell, but he manages to change from a small arthropod to a probability distribution, a neat trick. He'll move away while continuing to whirl when I get too pushy.
However, I've also seen this behavior in outdoor non-longlegs spiders that weave large well-organized webs, some vibrating perpendicular to the plane of the web. These somehow seemed more deliberate and were more visible, possibly because of smaller vibration amplitudes. I'd like to think it's SI but I could be kidding myself.

Answer 1

While I have always thought of the Pholcus web-shaking as a defensive reaction, I suppose it could be related to some sort of SI technique. Orb-web spinners don't generally do that as far as I know, because the prey almost invariably struggles and the vibrations propagate to the signal line(s) or to the hub, for the spider to use as location indicators. There is at least one animal that is able to avoid being eaten by the spider when it flies into a web - a Soldier Beetle will freeze as soon as it realizes it's stuck, and then very slowly and gradually unstick itself from the web strands until it falls out. It's very neat to watch this happen - I tested this several times with randomly caught Soldier beetles just to make sure it wasn't some fluke. The spider detects the initial hit, but since there's no follow-up shaking, it will file it as 'debris, miscellaneous' and not go to investigate. I haven't noticed non-pholcids shake the web, but if I was looking for that behavior as a prey-snaring or location technique, I would focus on the 'non-sticky' web spinners like the Linyphiids (sheet-web spinners) or Agelenids (funnel-web spinners) or such-like.

Answer 2

Yes, spiders derive a great deal of information from the web's
vibrations. They can tell where a prey organism is located in
the web, distinguish prey movements from inanimate objects
and detect the presence of a male of their own species. Also
the tension of various parts of the web is important to its prey
catching function and is kept properly adjusted. There is a
spider called a daddylonglegs spider by some people, but
this is a confusing term because a real daddylonglegs is not
a spider and doesn't build a web. The daddylonglegs spider
is a pholcid, usually, in the United States or Europe, Pholcus
phalangioides. It is often found in basements or storerooms.

Answer 3

Absolutely, they are able to sense the changes in their webs by vibrations and oscillations. Spiders are masters of their webs as they spend most of their lives on them. I would be astounded if they couldnt do that.

Answer 4

SI was not used in this particular instance. So noted.