Magicicada laying eggs (Photo by By Resqgal – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=106029586)

Dear Readers, I subscribe to a Substack by Lev Parikian, who wrote a book called ‘Taking Flight‘ which I love. For those who don’t know, a lot of authors are moving over to Substack – usually you get some free content, and if you like it you can sign up to get additional material, which is a way of supporting writers who, apart from the few who earn megabucks, are mostly living a pretty hand-to-mouth existence. Anyhow, this week I was very taken by Parikian’s piece on the periodical cicadas of the USA, particularly because this year is a bumper year for anyone who loves these intriguing insects.

For those who don’t know, cicadas spend 99 per cent of their lives underground, and then all emerge at once, so that they overwhelm all the many, many predators who feed on them. They then mate, the females cut little slits into twigs to lay their eggs, the larvae feed up and then burrow underground, where they stay for the next 13 or 17 years.

Hmm. Why 13 or 17 years, though? I’d never thought about this, but Parikian has an explanation. Both 13 and 17 are prime numbers (i.e. only divisible by themselves and one). Predators also have life-cycles, so just imagine if the cicadas emerged after, say 12 years – they would encounter all the animals with lifespans of 6, 4, 3 and 2 years, these all being factors of 12. Presumably, over time, evolution has favoured those cicadas who are less predated, and so nature has sorted out this extraordinary pattern. I love it when mathematics and biology converge, as they so often do.

Another hypothesis is that the unusual brood-length is a way of preventing different broods from hybridising with one another. Normally this wouldn’t be such a good thing, as genetic diversity is normally a great way of ensuring resilience, but scientists have hypothesised that keeping the brood length to 13 or 17 years long is so valuable to the species that they don’t want to risk losing it by crossbreeding. My mind is a little blown as to how this would work out in practice, but there are models which support the idea. As with all things in nature, it’s probably a combination of both ideas.

What makes 2024 so special is that, for the first time since 2015, a 13 year brood and a 17 year brood will emerge at the same time. Furthermore, the range of the different ‘broods’ will be extraordinary this year (a ‘brood’ is a group of cicadas that went underground in a particular location at a particular time). And finally, there are seven different species of periodical cicadas (Magicicadas) and this year any cicada-lovers out there should be able to see all seven species as adults, which won’t happen again until 2037. I’m almost sorry that I’m not planning to visit the US this year, though I imagine it’s going to be both noisy and a bit scary with all these critters singing and flying about.

Have a look at this film from the BBC with David Attenborough. Enough of the ‘zombies’ though. These creatures are not undead, but full of vim and vigour.

https://www.youtube.com/watch?v=EWr8fzUz-Yw

Incidentally, here in the UK we have only one species of cicada, which lives quietly in the New Forest. Sadly, it hasn’t been seen for ten years, but as we know, it could be biding its time underground. The charity Buglife has set up the New Forest Cicada project, which includes an app developed to pick up the song of the insect which, unlike the ones in the USA, sings a very high-pitched song which is above the range of normal human hearing. Fingers crossed that this insect (one of the largest in the UK, if it’s still here) will put in an appearance at some point soon. Fortunately the New Forest Cicada isn’t confined to the UK but also lives in other places in Europe and Asia. Sadly, though, it’s considered endangered across its range.

New Forest Cicada (Cicadetta montana) seen in Croatia (Photo by By Fritz Geller-Grimm, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=50838311)