The spider and the tuning-fork

Having started to look at identification of spiders, I've been meaning to buy a tuning-fork for some time, and last week I finally remembered to go into a music shop and get one. 'What has a tuning-fork got to do with spiders?' you may ask. Well, for quite some time, it's been known that spiders detect vibrations from their prey, such as insects caught in their webs and from the exact strands that are vibrating know where on the web their potential prey is located. They don't respond to slower taps on their webs as these are far too low a frequency and might be a larger predator hunting them instead. So, if you get a vibrating tuning-fork and touch an occupied spider web, there's a fair chance that the spider will emerge, lured out by the fake meal. Just watch...

I haven't identified the spider, but it looks like one of the several similar species of Tegenaria that inhabit such places - houses, outbuildings, and (as in this case), garden sheds. You can see that it emerges when the vibrating fork touches the web, and immediately attacks the 'prey' (suggesting that vision is less important at this point) - when the fork is removed, the spider returns to its funnel-shaped retreat. This is a good example of this 'trick', but things are rarely as simple as they seem, so what's going on from a biological perspective?

It was way back in the late 19th century when the 'influence of a tuning-fork on the garden spider' was first (as far I am aware) scientifically reported (Boys, 1880). Since then, a considerable amount of research has been undertaken. For example, Parry (1965) investigated the signal that a prey insect generates, noting that previous work had tended to focus on artificial signals similar to the sinusoidal wave generated by a tuning-fork. Tretzel (1961) had already found that prey made higher frequency sounds with a greater range of intensities than young spiders (which were therefore lower-pitched and more even) but was uncertain whether spiders used frequency to differentiate between prey and their own young, though they clearly can differentiate between them in some way; similarly at this time there was a lack of evidence about how spiders differentiated between prey and inanimate objects hitting the web. Taking a more straightforwardly visual approach, Savory (1952) had observed responses of Tegenaria to tuning-forks and found that:

• The spider first runs to the fork and try to grasp and bite it (as seen in the video).
• If the fork is used repeatedly, the attack reponse ceases as if the spider has learned there is no prey ( I found that the spider in my video only responded twice - the third time it stayed in its retreat).
• However, if a fork with a different vibration frequency is used, the attack reponse starts again (if it were a prey insect, there would be a complex range of frequencies of vibration).

Conveniently given the subject of my video, Parry also looked at Tegenaria and found segments of sound that he termed 'fast transients' - essentially parts of the prey insect (legs, antennae) 'plucking' web threads like a guitar string, either by dragging across them or by becoming detached and allowing the thread to snap back into position.

As the spider grips a bundle of threads in the claws of its front legs, these 'plucks', along with high-frequency vibrations are easily transmitted to it from the web. Little more could be concluded but Walcott (1969) investigated the structure of the sensitive receptors on the 'feet' of all eight legs of the house spider Achaearanea tepidariorum. He found a lyriform ('lyre-shaped') sense receptor organ with 10 receptor units, each sharply tuned to particular frequencies between 60 and 1400 Hz, although the sensitivity of each  unit depended on the tension of the slits forming the organ (think of it as a 'harp' or 'guitar'-like arrangement). However, it was responsive to air-borne vibrations, not web-borne ones, presumably because the web of this species was a poor transmitter. This arrangement also implies that the 'learning' response that prevents repeated attacks on a fake prey stimulus (if the same frequency of fork is used) is linked to one or a few units being effectively switched off for a period). Later research by Barth & Geethabali (1982) investigated the function of the lyriform organ in more detail and concluded that the units/slits were not tuned to particular frequencies but that there was some difference in the precise response curves that could allow frequency discrimination; they also concluded that vibration sensing was only one function of the lyriform organ and that it might also be involved in proprioception (i.e. the spider's own sense of where its limbs are, just as you know where your arms and legs are when you have your eyes shut). This contradicts Walcott (1969) but this may of course be due to genuine differences between spider species.

Now, I could go on - and indeed much has been written on this subject, but it turns out that the range of responses is highly variable. Different species of spider focus on different frequencies (or ranges of frequency with differing response intensities); the extent to which visual cues are used varies (e.g. in the ogre-faced spiders Deinopsis, the simple eyes are large in order to hunt at night, while some species have reduced eyes), as does the response e.g. the 'attack' seen in the video is common, but a 440 Hz fork (as used above) has been reported as causing the orb-weaver Eriophora sagana to fall from its web in a predator-avoidance response rather than treating the vibrations as prey (e.g. Nakata 2008). This work has indicated that airborne vibrations are used by E. sagana to detect insect predators, and that predation can lead to changes in web structure - thus there is a complex interaction between prey availability (in the sense of web structure being related to prey capture) and predation risk, with vibration detection being a mechanism that at least partly informs this balance.

The upshot of this? Well, first of all there is clearly more to be understood, even with a simple experimental tool such as a tuning-fork. I certainly intend to look at the responses of other spiders such as orb-weavers and jumping spiders. Secondly, I might have to buy a wider range of tuning-forks. Thirdly, if you've tried this, please do leave a comment to say what happened.

References

Barth, F.G. & Geethabali (1982). Spider vibration receptors: Threshold curves of individual slits in the metatarsal lyriform organ. Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 148(2): 175-185.

Boys, C.V. (1880). The influence of a tuning-fork on the garden spider. Nature 23: 149-150.
Nakata, K. (2008). Spiders use airborne cues to respond to flying insect predators by building orb-webs with fewer silk threads and larger silk decorations. Ethology 114: 686-692.
Parry, D.A. (1965). The signal generated by an insect in a spider's web. Journal of Experimental Biology 43: 185-192.
Savory, T.H. (1952). The Spider's Web. Warne, London.
Tretzel, E. (1961). Biologie, Okologie und Brutpflege von Coelotes terrestris (Wider) (Araneae, Agelenidae). Teil II : Brutpflege. Z. Morph. Okol. Tiere 50: 375-542.
Walcott, C. (1969). A spider's vibration receptor: its anatomy and physiology. Amer. Zool. 9(1): 133-144.