What triggers cicadas to make noises in unison, then go silent periodically?
Guy Cox
Sydney, Australia
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Male cicadas sing to attract mates, and the Australian greengrocer is reputedly the loudest of them all. The problem is that this also attracts birds, for whom a cicada would be a tasty meal. The insects have therefore evolved to all sing together. Any passing birds are confused and don’t know where to go, but a female cicada can still identify a nearby male. As for why they stop, I guess it is when a sufficient number are otherwise engaged! Once again, it is bad news to be the last singer.
Mike Follows
Sutton Coldfield, West Midlands, UK
The males of some species synchronise their singing when there are several of them in a given area, though they don’t start or stop simultaneously. Each species has its own song and singing together might be a strategy to attract more females from a bigger area, as the louder sound travels further. If they didn’t sing together, it might be more difficult for females to discern the song signalling their species.
There are more than 3000 species of cicada and they spend most of their lives underground as nymphs, emerging for a few weeks of adult life.
The males of most species use an organ in their abdomen called a tymbal to sing. This includes a pair of ribbed membranes that they flex rapidly, with the resulting vibrations producing sound. Both sexes have tympana, which serve as ears. To avoid deafening themselves, the males switch off their ears while singing. They emit some of the loudest sounds of any insect.
Singing in unison is unlikely to deter predatory birds, as some have suggested. When a cicada detects a predator nearby, it sings more quietly or stops altogether in the hope the hunter is attracted to a neighbouring cicada.
The cicada belongs to a division of insects called the Neoptera, which includes most of the winged insects. Neoptera also includes other “singing” insects like the cricket. Crickets are interesting because of Amos Dolbear’s , which he devised in 1897 to describe how they can be used as thermometers. A shortcut method is to count the number of chirps you hear in 8 seconds and then add 5 to get a temperature in degrees Celsius.
[Ed’s note: This year will see two broods of periodical cicadas, Broods XIII and XIX, emerge in the US. This is the first time the two broods, which emerge every 17 and 13 years respectively, have coincided in 221 years.]
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