As the old joke goes, you can prove that a spider’s ears are on its legs, because if you pull all its limbs off, it doesn’t come when you call its name.
Actually, spiders don’t have ears as such, but they do indeed hear with their legs, by means of a smattering of specialised fine hairs that are stimulated by airborne vibrations and connected to nerve cells at their base.
However, at least one species augments these capabilities by constructing an enormous ‘eardrum’ 10,000 times bigger than itself, sitting right in the middle of it and picking up the vibrations through its feet.
In fact, the orb webs spun by the bridge spider (Larinioides sclopetarius) – a UK species so-called because it is most commonly encountered on human-made structures near water – are more sensitive to acoustic vibrations than any known conventional eardrum.
And because the web is made by the spider and is external to its body, the spider is able to adjust and regenerate its external “eardrum” as it needs to.
A study published in The Proceedings of the National Academy of Sciences on the bridge spider (also known as the grey cross spider) in March 2022 describes how “hearing in the orb-weaving spider is functionally outsourced to its extended phenotype”, i.e. to its constructed web. The authors of the paper say the sensory ecology of spiders – “one of the oldest land animals” – should be re-examined.
Main image: Larinioides sclopetarius. © schizoform via Flickr (shared under CC BY 2.0)