“And today we’re talking about a periodic pest. But more of them later.”
All across the eastern U.S., a tenacious bug makes a long-awaited debut. Teeming just below the ground beneath your feet, millions of cicadas will wait years to emerge into the wild blue yonder. But why do they wait so long? And how do they know how long to wait? It’s all a game of numbers as this bug ensures its survival with the awesome power of math here in Life Death and Taxonomy.
Description
- Cicadas are a relatively large family of insects with large heads, large eyes that are set apart widely.
- Cicada’s spend most of their lives as nymphs, which are brown wingless insects with oval shaped bodies.
- When they reach the adult stage they have wings. These wings are delicate with a thin membrane supported by chitinous branches.
- The Southeast Asian cicada is the largest and most colorful with black bodies that sporting yellow, red, and blue stripes. We all know how much carlos loves blue animals.
- Cicadas live all over the world including the Americans,
Measure Up
Welcome to the beloved Measure Up segment. The official listener’s favorite part of the show! The part of the show when we present the animal’s size and dimension in relatable terms through a quiz that’s fun for the whole family. It’s also the part of the show that’s introduced by you when you send in audio of yourself saying, singing, or chittering the words measure up into ldtaxonomy at gmail dot com. We don’t have a new measure up intro this week! That means we get to hear from an animal, and Carlos has to guess what it is. Gopher sounds!
- Finch
- Gopher
- Baby gator
- Or a squeaky shopping cart wheel
Length
- The Southeast Asian cicada is the largest species between 4.7 and 5.7 cm (1.9 – 2.2 in).
- Let’s call it 2 inches.
- How many cicadas go into the length of the longest motorcycle…
- Hint: Bharat Sinh Parmar holds the record for the longest motorcycle. He was also a claimant for the longest crutches and the longest spade. To gain the title he had to ride the bike 100 m unassisted.
- Answer: 517.5 cicadas. The bike was 26.29 m (86’3’’)
Decibels
- The loudest cicada is the African cicada that reaches 106.7 decibels. African cicada calls are equal to the combined sound of how many near by whispers?
- Hint: 0 dBA marks the threshold of audible sound to a human being and 140 dBA is threshold of physical pain and hearing damage. Cicadas are louder than a motorcycle, that’s 30 feet away, a nightclub, and a construction site. They’re a little less than a chainsaw.
- Answer: 3.5. A whisper that’s 3 to 5 feet away is 30 decibels.
Fast Facts
- Cicadas are a food source for bats, wasps, mantises, spiders, birds, fish, amphibians, and basically anything that’s willing to eat a big bug.
- Cicadas are clumsy and sluggish in their adult stage so they many turn to camouflage, playing dead, flashing bright colors to stun and escape, and mimicking toxic animals.
- Though these large flying insects are intimidating to people that are squeamish with bugs, they are relatively harmless to humans.
- They don’t bite or sting.
- They feed on plant sap, though they may land on you mistaking you for a plant.
- However, sustained blasts from a male cicada’s call can damage human hearing.
Major Fact: A 17-Year Slumber
- I know we went over the Giant Colorful Cicada, but the species with the most study on its life cycle is Magicicada septendecula, which is endemic to the U.S. So that’s the one we’re mostly going to talk about here
- Most insects have a pretty short life span – some flies, like Mayflies, only live for 24 hours. Their entire lives amount to hatching, feeding, mating, and dying – all within a day.
- This is most likely due to the fact that insects are rarely equipped to handle winter as adults, so their lifespan is usually shorter than a year, with larvae hatching in the spring and dying before winter
- There are exceptions, though, with some insects. For example, some termite queens can live for ten to fifteen years just laying eggs all day.
- But the cicada even has the queen beat
- After mating, a female cicada will lay her eggs in a slit she cuts into the bark of a tree branch.
- Those eggs will hatch into nymphs that drop from the branch and burrow into the ground with their strong forelimbs.
- They’ll spend up to 17 years burrowing around, feeding on xylem sap (sap from the roots of plants).
- If they’re in a swamp or mud, they’ll create burrows above ground in little mud towers
- At the end of the instar, the nymph will carve an exit tunnel and climb up onto the tree they hatched from (or one nearby). There, they’ll molt their exoskeleton, leaving behind an exuviae, or discarded shell. The shell splits at the back and the adult cicada climbs out, wings and all.
- I used to find these exuviaes all over the place at my grandparent’s house in Indiana. We called them “locust shells”. I’d pick them off the tree trunks and hang them on my shirt.
- Some variants have a 13-year cycle while others go up to 17. These two groups, otherwise known as the periodic cicadas, are broken down into 15 broods. In each brood, all the cicadas emerge at the same time every 13 or 17 years, which is chaotic.
- Especially since they have such a loud chirp.
- Put a few million of them in a square mile and you have a deafening love ballad on your hands
- For example, Brood X is located in the eastern US. Every 17 years, all of the nymphs in Brood X will emerge, mate, and die within two weeks. The most recent emergence for Brood X was 2004, so we can expect it to emerge next summer.
- But that’s just one brood and each brood emerges in a different year. There are 15 broods across the country, and their regions often overlap. So many areas see 17 cicadas every year, but just different broods.
- There are 12 broods that are on the 17-year cycle and 3 broods on the 13-year cycle
- Once they emerge, they are eaten en masse by pretty much everything that eats insects. But there are so many that predators barely make a scratch on the population.
- This is called “predator satiation” – a reproductive adaptation where an animal will spawn large volumes of members at the same time to ensure that they won’t be predated to extinction. (krill, sardines, and even fruit bearing trees)
- But the real major fact is in the numbers
- If you notice, 13 and 17 have something in common – they’re prime numbers
- Predators populations have cycles too, which means they peak at certain times. In fact, predators often adapt their life cycles to coincide with the cycle of a prey species so that they can take advantage of spawning times.
- So when there are more rabbits around, there are also more foxes
- So if cicadas had a 12-year cycle, then any predator with a 2, 3, 4, or 6-year cycle could adapt to coincide with the cicadas emerging every 12 years, drastically increasing the number of predators that are around when they emerge and increasing the chances of being hunted to extinction.
- But because their life cycles are prime numbers, meaning they can’t be divided by any number other than one and themselves, it reduces the chances of a specialist predator adapting its life cycle to take advantage of the cicadas’ emergence.
- So if a predator had a population cycle of 5 years would only coincide with a particular brood of 17-year cicadas every 85 years.
- Therefore, predators can’t develop to depend on a certain cicada brood and must instead adapt to the life cycle of a different prey species, likely an even number of years.
- But these primes don’t just protect them from predators, they protect them from each other
- If two different broods of cicadas interbreed, they can throw off the entire life cycle of both broods since their cycles are determined by their genes.
- In 2015, Brood 9 [17-year brood called the Kansan brood] and Brood 23 (13-year brood called the lower Mississippi river valley brood) emerged at the same time and their regions were close to each other, but fortunately they don’t overlap. Because their cycles are prime numbers, this only happens every 221 years.
- So broods 9 and 23 won’t emerge at the same time again until 2236.
- But how do the cicadas know when to emerge?
- They obviously can’t count, but it’s kind of survival of the fittest. If a cicada emerges too early, it’s probably not going to find a mate before it gets eaten. So eventually, all of the cicadas with loner genes are picked off, and only the ones with 13 or 17-year lifecycles are left.
Ending: So burrow good, stay with the pack, and for goodness sake, don’t let your periodic life cycle coincide with the population cycle of a specialist predator like the cicada here in LDT.
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