By Veronika Meduna Veronika.Meduna@radionz.co.nz
‘Webs are an extension of their sensory system. They can tell all sorts of things about their world from the vibrations in their web.' _Anne Wignall
Spider courtship can be a deadly affair. As the smaller male enters the female’s web, he risks being mistaken for prey and eaten on the spot. Somehow, he has to convince her to delay her usual predatory response.
Unlike jumping spiders, whose vision is extraordinary, web-building spiders don’t see all that well. Instead, they rely on vibrations to make sense of their world and to communicate.
Anne Wignall, an evolutionary biologist at Massey University, has tuned into the courtship chatter of male Argiope spiders and found that they produce a vibratory shudder, perhaps the equivalent of a cautious knock on the door, as they enter the web of the object of their desires. The larger and usually aggressive female stops and listens out for more.
'We can pretty safely say that their main sense is vibration. All web-building spiders tend to have quite poor vision and they hang out in webs most of their time. Silk is a really excellent substrate for transmitting vibrations. Vibrations are physical movements of a substrate and it just passes easily through the web.'
Female spiders “read” the vibrations through slit sensors on their exoskeleton, mostly on their legs, and sensory hairs across their body. Their detection of minute oscillations is so accurate that they can even tell the weather.
“They can tell how windy it is because wind will produce this low-frequency noise in the web. They can detect prey struggling in the web, often how big the prey is or what type it might be, and they can listen to the courtship songs of males as they go into the web."
To decipher the spider talk, Ann uses a machine called a laser vibrometer, which sends out a laser beam that is reflected off a thread of silk. As the silk vibrates, the laser light changes and this signal is then converted into a sound file that she can listen to.
A fly hitting a web sounds like a massive impact – a loud high-amplitude bang. “Once the spiders detect that in the web you often see them tense up and become really alert. They are listening for more information.”
The initial impact is like a signal, she says, while the later vibrations once the insect is caught give the spider more information about where in the web it was caught and how big it is. Unlike the messy, noisy vibrations of struggling prey, spider courtship signals are very deliberate and rhythmic movements. To a female spider, these repeated shudder vibrations deliver an instant message to say “don’t attack me, I’m not food, just wait for more information”.
Some predators use this finely tuned signalling to their advantage. There are some spiders and even one insect, the assassin bug, that have learned to pluck the silk to generate vibrations that mimic the small and tired movements of prey. Once the female pounces, expecting a meal, she becomes prey instead.
Anne Wignall says she now wants to hone in on particular aspects of the information the females are listening to, such as the males’ mating thread dance. “If things are going well, he’ll cut out a section of the female’s web, using his mouth to cut the silk threads, and he’ll build a single thread across that gap, called a mating thread. He’ll then hang upside from it and begin plucking it with his legs.”
To explore the finer details of vibration-based courtship communication, she uses an electromagnetic shaker – an instrument otherwise used by engineers to test bridges – to play back signals to females at different frequencies and amplitude, in the hope of deciphering the best pick-up lines.
Spider webs are thought to have evolved once insects started to fly. “But once spiders started building webs, the males had the pressure on them to go into the females’ predatory trap, which is a dangerous place to be. That affected how they communicate with the females … to identify themselves without being attacked.”
Experiments show that if you play back courtship vibrations of the wrong species of male into a female’s web, she will still respond appropriately and delay her predatory response.
'That’s quite amazing in terms of courtship signals because often we think of courtship as being this great division between species because it makes sure that you don’t end up mating with the wrong species. So this is interesting in terms of the evolution of communication in spiders.'