Mathematical, Mysterious Cicadas

The photo above is a picture I took last year in Belize of a molted cicada skin. This is the husk the insect leaves behind as it finally emerges as an adult insect after its long life cycle. Despite the fact that I constantly heard cicadas while living in Belize for 13 years- I’ve only ever seen two adults. The insects are notoriously good at hiding themselves. More often, I’d come across these shed husks on the side of tree trunks.

The noise, however, is ubiquitous. Cicadas create the loud clicking noise using a drum-like structure on the side of their abdomen known as a tymbal. This structure vibrates rapidly and can cause clicks so loud they have been recorded in excess of 120 decibels- about as loud as most rock concerts. The noise can be heard for miles. This serves one purpose- to attract females.

After mating, the female carves out slits in the bark of trees then lays eggs in them. When the eggs hatch, the nymphs drop the the ground where they burrow into the soil, where they stay for years.

Different cicada species have differing lengths when it comes to how many years they spend under the ground. This is perhaps the most mysterious part of the cicada’s life cycle. The number of years between the cicada nymph burrowing into the soil and its emergence as an adult is often (but not always) that of a prime number. 2,3,5,7,11,13 and 17 year life cycles have been recorded- all prime numbers.

It is not known why the breeding cycle often revolves around prime numbers. One theory is that it is harder for a natural predator to predict the cicada’s emergence through such irregular timing. The cicadas also all emerge at the same time. This is also thought to be a evolutionary advantage against predators, as thousands of cicadas emerging at once makes it harder for predators to eat them all.

Manchester Museum has quite a large collection of cicadas, most of which were collected by Herbert Stevens, a tea planter who lived in India. I’ve finished documenting the entire collection, along with the Fulgoridae (lantern bugs) Stevens also collected.

Tosena, a cicada genus from Southeast Asia:

Other specimens of note were those collected by Allan Brindle, who was Keeper of Entomology at The Manchester Museum from 1961-1982. In 1942, Brindle was called up to the Lancashire Fusiliers, where he was quickly transferred to the Intelligence section due to his ability to speak many languages. He was posted in India, where he collected many specimens of insects during his service there, including some of the Museum’s cicadas.

A few weeks ago, The Manchester Museum held a Big Saturday event called “Bug Art”. Art students from Stockport College had been working closely with the entomological collections to create insect based artwork. Two of the students focused on cicadas. One of them created a template to “make your own origami cicada.” It was a big hit and I had to give it a try!

As well as the art, we curators had insect specimens out on display. The cicadas were actually a big draw, with many people wanting to know more about what they were, where they were found and the mysterious complexity of their life cycle. I enjoyed myself and was pleased that visitors enjoyed learning about these beautiful insects too.

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About Gina Allnatt

I have just finished a year long traineeship as a Biology Curator at Manchester Museum. I am currently a research and curation volunteer in the Entomology and Botany departments. -Gina Allnatt
This entry was posted in Entomology, In the field, learning and tagged . Bookmark the permalink.

3 Responses to Mathematical, Mysterious Cicadas

  1. I really love your cicada pics! Those shells are really cool…

  2. Genevieve says:

    Are 9 and 15 really year they come out or do they actually only emerge during years that are prime numbers?

    • Oops! Genevieve you are absolutely right- 9 and 15 are not Prime numbers. I had cut and pasted that string of numbers from a website. Just goes to show I should be more careful when researching!

      Thanks!

      Gina

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