We live our lives to the background tunes of bird calls — the crow cawing from the branch of a tree, the black kite shrilling from atop a building or the sparrow cheeping on our windowsill.
But what about the call of the insects? They make themselves heard by chirping, hissing, squeaking, sighing, droning, buzzing, humming or shrieking. The songs of some insects are produced at a frequency which is beyond the hearing range of us humans but we know that they are calling if we observe them carefully, from their body movements.
Insects emit calls for various reasons: They may sing ballads to their mates; or call out to assert their territories; keep enemies at bay; or even warn of danger. Each insect species has its own unique way of producing sounds.
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The macho male grasshoppers call out to potential female partners in a chirpy manner by ‘stridulating’, which means rubbing one part of their body against another. They rub a spiny file on their extra-long hind legs against their fore wings, as if they are playing a fiddle.
The katydids and crickets, both of whom could be considered as cousins of grasshoppers, chirp by rubbing their two hind wings together. The katydids are so named because they seem to be calling out ‘katy-did, katy-did’ all day.
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The noisiest of all insects are the cicadas. Walk into any forest, especially at the height of summer, and you will hear them singing — rather a monotonous shrill. But, for the females among them, this noise is music to the ears. The cicadas vibrate a membrane called tymbal inside their abdomens, which act as sound boxes. The females select the best singers to be their partners.
Danger alert
Death’s head hawkmoths (they have a skull-like marking on their wings) are known to force air out of their spiracles (little holes on the sides of their abdomen) to produce squeaks when they feel they are being threatened.
Cockroaches too, are known to hiss in a similar manner.
Some insects, like the death-watch beetles, tap their heads against the walls of their burrow.
The giant cockroaches of tropical America, however, drum on wood with their thighs. And when several of them drum together, the noise is enough to wake up people from their sleep.
Certain species of termites too play the drums. The soldiers among them respond to the slightest of disturbances by hammering their heads against their nest walls. Perhaps, this is to signal to the rest of the colony that danger is around.
Some Arctiidae moths go to the extent of sacrificing for the rest of their species. On sensing the presence of their predators — the bats, they bend and unbend a tough skin on their legs called cuticle to produce clicking sounds. By doing this, they draw the attention of the bats rather than escape from them. On eating them, the bats find out that they are distasteful, not worth eating any more of them. The secret is that the moths feed on the most obnoxious plants. As the adage goes, once bitten twice shy. The bats have learnt their lesson: clicking sounds mean distasteful prey. Keep away from them.
Sound ears
We are all familiar with the buzzing of bees and humming of mosquitoes and flies. These sounds are produced unintentionally, by vibrating their wings very rapidly — as much as 330 to 650 times a second.
Those insects that produce sounds intentionally, naturally need to hear too. They have ears on the most unexpected parts of their anatomy. Grasshoppers for instance, have ears on their abdomen while crickets and katydids hear from their legs.