Male field crickets traditionally attract a mate by “singing” – creating a sound by rhythmically scraping their wings back and forth.

In Hawaii, however, their song attracts a less welcome female: parasitic flies, whose larvae devour and kill the crickets from the inside out.

To survive, some smart field crickets have rapidly evolved to remove the sound-producing structures on their wings, meaning their vigorous “song” no longer endangers them, as it is completely silent.

These mutations were first identified on the island of Kauai and by photographing and recording these insects under lab conditions, scientists have now discovered that the singing continues, even though it appears to be an intensive use of energy for no purpose.

Writing in the journal Biology Letters, researchers from the University of St Andrews and the University of Cambridge reveal how they photographed and recorded the crickets under laboratory conditions to document this silent song.

Currently about 95% of males on Kauai and half the males on the neighbouring island of Oahu display the “flatwing” phenotype, which means they are incapable of producing song by rhythmically opening and closing their forewings.

These crickets have shown markedly higher success at evading the attentions of the parasitic flies. But there’s one major flaw: how can they attract a mate without song?

“It appears that they hang around singing males and intercept females that come in,” said co-author Dr Nathan Bailey of the University of St Andrews. “In effect, they parasitise the songs of the singing males. It’s a bit sneaky.”

But on islands where there are very few singing males left, the populations are enduring, suggesting that non-singing males are finding another way to attract a mate.

The field crickets fascinate evolutionary biologists because a popular hypothesis is that organisms can adapt to stresses such as new pathogens or predators by changing their behaviour. In this case, the rapid adaptation to silent singing is not a behavioural change but a morphological one – the crickets are still going through the motions of “singing”.

Bailey says he and his research group would like to examine whether the crickets now begin to save their energy by rubbing their non-singing wings less. They would also like to observe whether the soundless “singing” – a vestigial trait with no obvious function – could acquire a different function or already serves another undetected function.

“It’s evolution in real time,” said Bailey. “We can observe it happening as it unfolds.”