Interested in how caterpillars use acoustics to communicate, Jayne Yack of Carleton University, Canada, noticed that the literature was littered with anecdotal reports of clicking caterpillars. But it wasn't until she was approached by George Boettner from the University of Massachusetts, who was rearing a species of clicking silk moth (Antheraea polyphemus)caterpillar, that Yack decided to explore further why and how caterpillars click (p. 993).
Yack and student Sarah Brown started out by pinching caterpillars' heads with forceps to induce clicking, while filming their movements and recording the clicks. They found that caterpillars click when they close their mandibles, which are covered with serrated, tooth-like ridges. Yack suspects that these toothed edges scraping over each other produce the distinctive clicks. After clicking, the caterpillars often regurgitated a dark brown liquid. `The clicking is a warning signal, telling a would-be predator that the caterpillar is about to regurgitate a bitter liquid' says Yack. This response wasn't unique to the silk moth caterpillars: many other species silkmoths and hawkmoths tested clicked too.
When the team analysed the clicks in more detail, they found they lasted around 25 ms each, and were quite loud close up, between 58–79 dB. `They were very short and don't carry long distances, ideally suited to a short-distance warning' says Yack, they `wouldn't attract other predators'.
To further test their idea that clicking was a warning signal for regurgitation, the team monitored caterpillars' defensive behaviour after they had pinched them once, twice or five times. They found that more pinches caused more caterpillars to click, and to click for longer. More pinches increased regurgitation as well. `Producing a chemical defence is costly' says Yack, `so you'd give a warning first'. However if the pinches keep coming, the caterpillars will regurgitate as a last line of defence.
Having shown that caterpillars clicked and regurgitated when pinched with forceps, the team wanted to test whether they would behave in the same way when pecked by a predator, in this case a domestic chick. Chicks peck more forcefully than pinching with forceps and this affected the caterpillars'response. `The caterpillars clicked more often' explains Yack, `they also regurgitated more'. Many of the caterpillars regurgitated at the same time as clicking, and all of them survived the attacks. Although, researchers will need to do more experiments to determine how the caterpillars' defences are affecting the chicks, Yack explains.
Finally, to determine how palatable regurgitant was to invertebrates they soaked meal worm segments in the brown liquid and measured how long it took ants to eat or reject the worms. The ants took much longer to accept the food covered in regurgitant over uncoated food. When mice were given a choice between standard food and food soaked in regurgitant, they preferred the untainted food, showing that the regurgitant is unpalatable to vertebrates,too. `If the regurgitant was lethal and completely disgusting, it's likely that the caterpillars would advertise this by using a long distance visual cue, like bright colouration' says Yack, who is planning to study the regurgitant's unpalatable qualities in more detail. These caterpillars are probably relying on cryptic colouring to keep them hidden, clicking and regurgitating only when they are discovered.