Their variety is quite amazing
In many countries, the feeling of summer is related to the sound the cicadas make. Some call it “noise”, others describe it as “singing”. The entomologists refer to it as clicking sounds created by tymbal membranes on each side of the abdomen that are drawn together by muscles a few hundred times per second in a way rather similar to when one clicks the slightly raised lid of a tin inward and lets it bounce back to its original position. What’s interesting is that all the different cicada species anywhere in the world use the same method to produce their sounds. Crickets chirp (or sing) by rubbing one wing against the other and, once again, crickets all over the world use the same technique. Grasshoppers, no matter where use their hindlegs to ‘scratch’ their wings and create their distinct sounds in this way. It seems that once an ideal way to make sound has evolved it becomes the standard method and won’t change. With beetles it is different. Their sounds are softer, but produced in many more different ways.
Many years ago when still a PhD-student at the Australian National University’s Neurobiology Department, I became interested in sound-producing beetles. Some, like the longicorn beetles, have a structure with a washboard-like surface on the dorsal side of the mesonotum, called the file. To create their rasping sounds, they move their head back and forth, which causes a hard part of the pronotal cuticle to rub across the file. The sounds are quite audible in the larger species and faint, but present, in the smaller species as well. Insect sounds that are the result of two body parts being rubbed against each other are termed “stridulations” and although all cricket species of the world use the same method and all the grasshoppers of the world (although different from that of the crickets) also use one and the same method (see above), stridulating beetles are different. Amongst them, one can find at least 10 non-identical ways. Adding sound-emissions made by some beetle species as part of other activities, e.g. the explosive sound in the Hot and Scaldingbombardier beetle or the ticking sound of Anobium when it bangs its head against the wood, or feeding sounds, the variety of sound emission methods in beetles is amazing.
On the basis of this variety, can we conclude that in evolutionary terms beetles have not yet found and perfected the ‘ideal’ method to make sounds or does it mean that sound production methods are linked to the different environments and the different life styles that the various species of beetles are adapted to? I studied the sound emission apparatus and the sounds created in beetles of three families: Cerambycidae (the longicorn beetles), Passalidae (bess beetles), and Scarabaeidas (rhinoceros beetles). Although stridulation is involved in all of them, the body parts they use to create their sounds vary.
Whether the sounds confer any message to conspecific individuals or are simply warning or distress sounds to fend off predators is still not entirely clear, but what is known is that in the bess beetles, who communicate with their also sound-producing larvae, 14 different acoustic signals have been recognized. A warning sound is more likely the reason for the ‘rasps’ of the longicorn beetles and distress may be something the rhinoceros beetles expresses with sound production. The big questions still to solve are what structure or organs beetles possess to hear with, which sound frequencies and intensities they can detect, and what meaning or purpose their sounds may have.
As a trombone player, I tested my sound-producing species in this way: whenever I produced a short approx. 80-phon loud frequency of tone “f”, I hit the beetle’s antenna with a pencil. After about 30 such punishments, beetles that had ignored the trombone tone before the training now responded with an antennal reaction. But what did the beetles react to: was it sound pressure or displacement (=movement of air molecules)? Sadly, I never continued my work with the beetles.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2020.
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