Last updated February 28, 2018 at 10:33 am
When is a caterpillar like a kettle? It sounds like a Christmas cracker joke, but it isn’t, especially in the case of the Nessus sphinx hawkmoth.
The moth species (Amphion floridensis) is common in the eastern regions of the United States and Canada, and because they are active during the day the adults are a familiar sight in fields and suburbs alike.
The caterpillars, however, are less visible, at least to humans, being a dull brown colour and fond of eating the leaves of grape vines and cayenne pepper bushes.
Many species of bird, however, see them plainly enough and regard them as a potential tasty snack.
When birds zoom in for the kill, though, they discover that the larva of the Nessus sphinx hawkmoth has a very unusual defence mechanism: it screams. More specifically, it emits a loud whistling sort of noise for 370 milliseconds, followed by several short, repeated 23 millisecond squeaks.
It is by no mean the only type of insect capable of producing sound – think of grasshoppers and crickets, for instance. However, most species do so by rubbing or vibrating bits of their hard exoskeletons. This particular caterpillar has evolved a markedly different strategy.
Now, a team of biologists led by Conrado Rosi-Denadai from Carelton University in Ottawa, Canada, has worked out exactly what that is.
To all intents and purposes, the Nessus sphinx hawkmoth has learned how to whistle.
In principle, it works exactly like a boiling kettle, or a folk musician playing an old whisky jug.
Using a combination of microscopy, close-up videos and numerical modelling, Rosi-Denadai and colleagues discovered that the caterpillar does indeed make the sound using its mouth – or, more precisely, its oral cavity.
Helmholtz resonator effect
The animal pulls air in and out through an orifice between two chambers in its foregut – the crop and the oesophagus. As the air is sucked inwards, it forms vortices, like tiny whirlwinds.
This in turn sets up what’s known as a Helmholtz resonator effect – the kind of “throbbing” noise that arises when air under pressure is forced to pass through a narrow opening. To complete the effect, the throbbing is amplified by the space inside the caterpillar’s oesophagus chamber.
The scientists established that the initial long scream was made by air being sucked into the system, and the shorter squeaks came about as the chamber deflated, forcing it out again.
The research is published in the Journal of Experimental Biology, and concludes on one of the weirdest sentences ever committed to print: “Our results provide evidence … showing that caterpillars employ mechanisms similar to rocket engines to produce sounds.”