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Songs of love that only they can hear

Sanjida O'Connell finds many insects are on the same romantic wavelengt h
Summer holidays and exotic places conjure up the heavy hum of cicadas. But in fact many cicadas, crickets, grasshoppers and other insects sing silently. Airborne sound takes a lot of energy to produce; it requires transferring vibrational energy from a solid object to a gas at audible frequencies, and is an extremely inefficient process. For small insects, being quiet can be an advantage.

Singing is a way of attracting the opposite sex, but the problem is that eavesdroppers can tune in. Other males can lie in wait, then nip in and mate with the female you have spent hours serenading. It is also an auditory Belisha beacon for predators.

Silent singers make inaudible noises in three ways. Some fruit flies flick their wings to produce complex patterns of pulsed air that are decoded by their partners' antennae. The low frequency signals are in the 100-500Hz range. They are intense and highly directional, but travel for less than 10mm. Then there are the "tymbal singers", such as plant hoppers and spittlebugs. Males and females vibrate a pair of organs, the tymbals, located at the base of the abdomen. Energy is concentrated below 1kHz and transmitted to the ground via the legs or mouthparts. Green lacewings and tremulating katydids jerk or circle their abdomens at 30-120Hz, shaking the substrate at the same time. Sensitive receptors in their legs are tuned to the frequency range characteristic of their species.

One small fly uses this method to track down females. Virgin females hatch in widely scattered reed galls that they rarely leave. Moreover, the males do not fly; they walk. To avoid climbing every reed stem, the male remains at the bottom and vibrates it. He may have to do this for hundreds of stems for most of his life, but if he comes across a female, she will send a signal down the stem from her eyrie.

Because these silent songs are meant for one pair of ears only, they have become the exclusive province of insect romance. The privacy of virtual silence means the risk of detection by predators is low, and so courtship singing has evolved in females whowould otherwise be too sensible to broadcast their whereabouts. This has led to a decline in male fights and prolonged duetting between couples, rather than loud and indiscriminate advertising by males for any passing female.

Another effect is to increase the overall number of insect species. Usually, a new species will evolve if a genetic mutation accidentally confers some kind of benefit to the individuals within the species. Here new species arise in a completely arbitraryway by both males and females choosing each other on the basis of novel songs. Professor Charles Henry, of the Department of Ecology and Evolutionary Biology, University of Connecticut, says this could happen if songs were under relatively simple genetic control. A single mutation of the gene for singing could produce a new song "unacceptable'' to the rest of the species. He says "a single serendipitous success in mating by an individual bearing the mutation'' would be all it would take to introduce the new gene into the population. Unlike many of these so-called sibling species, the silent singers, who really differ only in their songs, can all coexist together. At least five species of lacewings have all been found in the same bush in North America.There were no hybrid species present and the lacewings flummoxed all attempts to create one in the lab.

Xie Ye, a Chinese poet whose work was recently read during the National Poetry Festival, takes poetic licence when she writes "insects make the earth sing''. Either that or she knew about the transmission of low frequency tremulation.