We're all familiar with the serenades that crickets sing and cicadas chirrup, but have you ever heard a caterpillar whistle? Jayne Yack from Carleton University, Canada, has – but only when it's under attack. ‘I am fascinated with the idea that animals have many unusual forms of communication, so when we see caterpillars that are communicating with some of their predators with ultrasound, that interests me,’ Yack explains. Surveying caterpillars from a wide range of Bombycoidea species to discover what sounds they make when feeling threatened, Yack's student, Veronica Bura, discovered that walnut sphinx caterpillars make an unusual squeak. Curious to find out how the caterpillars produce this sound, the duo decided to analyse the larvae's behaviour (p. 30).
Trapping adult female moths and collecting the eggs, Yack and Bura waited for the eggs to hatch and reared the larvae to the 4th and 5th instar on tree cuttings. Then the duo gently squeezed the caterpillars with blunt tweezers to see how they reacted. Sure enough the caterpillars made the strange squeak and when Yack and Bura analysed the squeak's frequency spectrum, they found that it spanned frequencies ranging from those audible to birds and humans up to ultrasound. But where were the sounds coming from?
Watching the squeaking caterpillars under a microscope, it was clear that the larvae were not opening their mouths and clicking their mandibles to make the sounds. However, they were pulling their heads back, compressing the body cavity. Wondering if the larvae were forcing air out of their spiracles to produce the sound, Yack suggested that Bura gently apply latex to cover all eight pairs of the caterpillar's abdominal spiracles and then uncover each pair systematically while disturbing the larva. ‘Sound production did not occur until Veronica got to the 8th abdominal spiracle, but then they made sounds like before,’ remembers Yack. And when the duo monitored air flow from the spiracles using lens paper and a laser vibrometer as the caterpillar squeaked, they only recorded vibrations coming from the 8th spiracles. So the larvae squeak by blowing air out of the 8th spiracles to produce trains of whistles lasted anything up to 4 s.
But why do the caterpillars whistle when it would seem to be a perfect way to advertise your presence to predators? Yack and Bura decided to find out by asking some birds. Teaming up with Vanya Rohwer and Paul Martin at Queen's University, Canada, who had captive yellow warblers – a native predator of the caterpillars – Bura and Yack put a caterpillar on a twig in a cage with a yellow warbler and patiently filmed the encounter. Amazingly, when the bird attacked the caterpillar, the caterpillar whistled and the bird dived for cover. The caterpillars were so successful at frightening off their predators that the birds did not eat a single one.
‘We were very surprised; we didn't think the birds would react like that. They were scared,’ says Yack. ‘If you put yourself in the bird's place you have limited foraging time, you're hunting around the foliage and you find something that is cryptic, you attack it and then it goes “wee” – I can guarantee you'd abandon it and start looking for something else,’ she says. So walnut sphinx caterpillars whistle at their predators to startle them away before resuming the quiet life, tucked away in the foliage.