In previous newsletters, we have discussed animals such as dolphins and bats that are able to “see” with their hearing, a process known as echolocation, in which the animal can map their environment by sensing sound reflected back to them. There are animals, however, that utilize sound in a slightly different manner – not so much by “hearing” sound waves, but by feeling their vibrations. Read on to learn about audible arachnids.
Most spiders have eight eyes, but their actual sight leaves a lot to be desired. To make up for what they lack in vision, spiders have adapted another method of taking in information from the world around them – they are one of the most vibration-sensitive organisms in the world.
A spider senses vibration through tactile hairs as well as well as tiny slit-shaped receptors located across their exoskeleton, mainly on their legs. The slit receptors are essentially arrays of narrow, parallel openings of different lengths that deform under mechanical pressure. One spider studied in particular, Cupiennius salei, the Central American wandering spider, has over 3,000 of these sensing organs across its body. A compression of a given slit by fewer than 10 nanometers was found to be detectable by the spider; this detection corresponds to a force of only a few millionths of a pound.
But the spiders’ senses aren’t the only thing giving it an edge; many spiders also have another tool in their vibration toolbox: their web.
Spider silk has long intrigued scientists with its unique combination of strength, weight, and flexibility. But a new property has recently been revealed – the ability to transmit information. Studies have shown that spider silk can transmit longitudinal waves (compression waves where the vibration of the medium is in the same direction as the wave propagation) at a wider range of frequencies than almost any other natural or synthetic fiber. Spiders have fine control of the properties of their silk when they spin it, and then pull each thread to the desired tension, much like tuning a string on a musical instrument. Sitting in the center of its web, with each of its eight legs perched on a different radial strand, the spider can sense in all directions. What’s more, because of the silk’s wide range of transmissible frequencies, they can discern between many different vibrational sources: environmental sources like wind or rain, captured prey, a predator, or even a potential mate. The spider itself can also bounce up and down or pluck at individual strands sending out signals to test the composition and structure of its web, locating any potential weak or damaged points. The combination of the silk’s properties and the spider’s sensory capabilities allow them to sense movements as small as 100 nanometers, a distance one-thousandth the width of a typical human hair.
So next time you see your friendly neighborhood spider and want to give it a wave hello, don’t be too offended if they don’t wave back; it may be you’re just not vibing on the same frequency.