Dear Readers, you might remember that I’m following along with the 72 microseasons in Nature’s Calendar, and I am already behind because the last day to look out for ‘dew-drenched cobwebs’ was actually yesterday. But! I have always loved spider’s webs, and in this chapter, Kiera Chapman introduces us to a number of new concepts and words that I thought I’d share with you all. You all know that I love a new word, and Chapman has a corker – ‘Biotremology’. This is the study of how animals use vibration in order to gather information about their worlds, and spiders are experts at this. Most spiders (with the exception of jumping spiders) have very poor eyesight, and so they rely on vibrations that they detect both directly and via their webs in order to know when prey has arrived, if a possible mate is in the vicinity and if an approaching wingbeat indicates an predator or a meal.
First up, Chapman explains that spiders have three anatomical features that help them to detect and act upon different kinds of vibration.
Firstly, spiders have lots of hairs that connect to nerves that detect touch directly – this is why they often have such hairy legs, but as humans we should recognise this phenomenon too – we can detect pressure which doesn’t actually touch our skin, but which comes into contact with a body hair.
Secondly, spiders have a special kind of hair called a trichobothrium (another great new word) which is very, very fine, and can detect air movement at a distance, so that an incoming fly is detected by ‘feeling’ the air movement generated by the buzzing of wings. I’ve watched spiders rush to cut a bumblebee free from their web, and also gallop along to wrap up a fly for later consumption, and I’m now thinking that the different kinds of vibration possibly inform the spider’s decision.
Thirdly, the spiders have holes in their skin called ‘slit sensilla’, and these are extremely sensitive, enabling the arachnid to sense vibrations along the silk strands of their webs (the slit sensilla are especially numerous in the legs of web-spinning spiders). This is useful for a variety of reasons – if a spider is hiding away in the corner of a window frame, it enables them to sense the arrival of dinner without being conspicuous. Furthermore, a male spider often plucks the web of a female like a guitar string at a particular speed and frequency so that she knows that he has amorous intentions and isn’t edible.
So, the spider doesn’t just rely on its own body to sense the world around it – its web is an extension of its senses, enabling it to position itself in space and to know what’s happening even when it can’t see. They can be fooled though: if you are feeling particularly mean, you can take an electric toothbrush and gently touch it on a web. In some cases, the spider will rush out with a look of expectation on its face (well, not that they are very expressive but it’s how I would look if I was a spider and thought that my lunch had been delivered).
Incidentally, though spiders are extremely vibration-sensitive, they are not the most vibration-sensitive. That honour goes to the humble cockroach. But that’s a story for another time.