What's so Special About the Cicada's in 2024?

You can watch the video that used this script here!

It's a cicada crazy year! Maybe you’ve heard of the cicadas that are out in 2024 and how this is a special occasion not seen in over a hundred years and that won’t be seen for another long time. It seemed so special that I dragged my wife and son on a long camping trip just to see them! And while we had a great time catching bugs, looking at frogs, and being out in nature, it was quite the trip for us. So was it worth it? Is 2024 really special or did I waste a bunch of money on nothing? If it is special, why is it special? And what are these Cicadas anyway? To answer these questions let's start broad and work our way to specifics. 

Cicadas belong to the superfamily Cicadoidea, which is a group within the order Hemiptera. Like other hemipterans they have sucking mouthparts, this mouth is located near the rear of the head. The location of the mouth (known as a rostrum), the short conical antennae, and three tarsal segments puts Cicadoidea within the Auchenorrhyncha suborder. Cicadoidea itself is characterized by a triangular placement of the ocelli, large clear wings held over the abdomen in a tent-like fashion, and a pair of tymbals males use to make sounds that attract females. 

Cicadoidea can be divided into two families; the Tettigarctidae and the Cicadidae. Tettigarctidae or hairy cicadas are strange cicadas! Their heads look like they’ve been caught in some shrink ray and they are very hairy. This combined with the fact that they were once way more common make them seem like the stereotypical caveman of cicadas! This once diverse group has only two species left today in the Australian region. They diverged from the Cicadidae sometime during the late Triassic or early Jurassic, as fossils from both families are found during most of the Jurassic. 

Cicadidae or true cicadas have broad heads with the eyes spaced widely apart, having more developed tymbals and these are only present in males, whereas in the hairy cicadas they are present in both males and females. Because they have more developed tymbals, Cicadidae are some of the loudest insects on the planet. The loudest is the shrill thorntree cicada (brevisana brevis) which reaches 107 decibels which is about the same pitch as a power saw or power mower and can cause hearing loss after prolonged exposure from a short distance!

Cicada ovipositor

Cicadas spend most of their lives underground attached to the roots of trees, slowly feeding off the sap. When they are ready, the alien looking juveniles emerge and slowly dig their way out, climb a vertical structure, and go through a process known as ecdysis. Ecdysis is when an arthropod sheds its old exoskeleton and is sometimes somewhat incorrectly referred to as molting. After ecdysis the newly emerged adult will need to harden its soft exoskeleton through a process called tanning. This can take several hours but after it is finished the insect is now ready to reproduce.  After mating the female cicada will use a structure called an ovipositor to dig into branches of the trees and lay eggs. To reduce wear and tear on the ovipositor, cicadas reinforce it by adding metals they accumulated as nymphs! 14 different metals have been found in cicada ovipositors and include Iron, Magnesium, Copper and Zinc! After some time inside the branches of the trees the eggs will hatch and the nymphs fall to the ground which they burrow into and where they will spend the next several years.

Someday we’ll have to do a full video just on Cicadidae because of how diverse and amazing they are! For now let's continue making our way to the stars of 2024.

Within Cicadidae we can divide all the species into 3 ecological groups. These groups are not fully monophyletic, meaning that they might not be as closely related to each other as they are to members of other groups, but they do share similar behaviors and that makes talking about them a lot easier. 

The first group is the annual cicadas. As the name suggests these cicadas emerge every year. This does not mean their life cycle is a single year though. They can spend anywhere from 1-5 years underground but their population is staggered so that some individuals are coming out every year.

The second group is the proto-periodical cicadas. These cicadas have a similar life span as the annuals but instead of staggering their emergence they have all synchronized their emergence to rain fall. They only emerge after a specific amount of rain has fallen in their area since their last emergence. For example one group measures the amount of rain for up to 6 years waiting for an accumulated rainfall of 1,181 mm of rain over that entire period! They are literally living rain gauges! This group has only been recently studied so much of this understanding might change and the species that are currently considered proto-periodical are bound to change.

The last group is the group we are here for, the periodical cicadas! This group is monophyletic, a single genus with the coolest name, Magicicada. The magicicadas have synchronized their emergence so that the entire population in an area comes out at the same time. This overwhelms predators and provides the best chance for individuals to mate before being eaten. Their life cycle spans a period of 17 or 13 years depending on the group. Notice that 17 and 13 are prime numbers, it is thought that this makes it harder for other animals to sync up their life events with these cicadas. Now there are individuals in every population that will come out earlier or later than they are supposed to. Usually they are off by a year but sometimes groups will emerge 4 years early if they are one of the 17 year species or 4 years late if they are from the 13 year species. It is thought that this could be how the different species evolved from being 17 year cicadas into 13 year cicadas

When the cicadas come out there are tons of them! They slowly dig their way up out from underground where they spent the last 17 years feeding on the roots of trees. Just outside our little KOA cabin we saw their holes, literally thousands of them! And of course we saw the sheds (called exuviae) that were left clinging to every structure in sight. As adults they only live 2-3 weeks and their bodies cover the ground, decomposing and giving a much needed boost of nutrients to the trees they fed on as nymphs. The amount of insects in such a short time provides a large boost in the ecosystems. Many animals thrive on the abundance of food. 

Cicada killer wasp

Parasites of cicadas in particular do really well during the boom. The large cicada killer wasps (Sphecius) will catch cicadas mid flight and use their venom to paralyze them. Later their larvae will eat the cicadas alive within the wasps burrows (you know typical wasp behavior). The leaf gall mite or "itch mite" (Pyemotes herfsi) will feed off the eggs and become so numerous during this time that people will complain about hives and bites from the itch mites for a long time. The coolest parasite is a fungus, Massospora cicadina. This fungus specializes only on the periodical cicadas. This fungus invades the abdomen and slowly replaces it, destroying the reproductive, digestive, and most of the respiratory systems. It keeps the cicada alive for as long as possible and even with the reproductive system gone the fungus gives the cicada a sex drive making them try to mate with as many cicadas as possible passing its spores to new cicadas. This sex drive is ramped up and will even make males pretend to be females to try and mate with the males. The white fungus on the back end of the abdomen has given the infected cicadas the nick-name salt shaker cicadas.

Now I had heard about this fungus and so I was super excited when we found several cicadas that had been infected by it! They were flying around looking for mates but for at least this guy, he’s just a thorax! Isn’t this so wild?! You felt a little bad for them but man was it cool!

For several years scientists tried to track the different populations by designating them by the year of their emergence, but this got a little confusing to track a single population. So in 1907 entomologist Charles Marlatt proposed a new system that borrowed certain aspects from several different proposed systems. Starting in 1893 Marlatt assigned each year a roman numeral (I-XVII) for 17 years for the 17 year species and each group was referred to as a brood. There are 3 species of 17 year cicadas and all usually appear in every brood with a few exceptions. Also starting in 1893 he assigned the subsequent roman numerals to each year for 13 years (XVIII-XXX). With 4 species of 13 year cicada with all 4 usually appearing in every brood here as well. Making in all 30 different broods or populations of cicadas with 7 species total. 

Marlatt's brood system

Now you may have heard that 2024 is special because it has two Broods emerging at the same time but there are a couple of things you may have noticed about the Brood explanation that may put holes in this year's specialness. First, with this system every year somewhere in the US there should be some periodical cicadas emerging. Second, every time a 13 year cicada is emerging it is emerging with a 17 year cicada but on a rotation, so that the same two broods rarely emerge at the same time. 

While it is true that every year there should be periodical cicadas according to Marlatt, we don’t see that. That’s because 4 of the 17 year broods and 9 of the 13 year broods never existed. When Marlatt made his cicada broods many of them were theoretical and found later but these 13 broods were never found, and so as far as we can tell never existed. In addition Brood XI was last seen in 1954 and is now considered extinct. And Brood XXI went extinct before the Brood system was proposed and was last seen in 1870. In fact, Marlatt mentioned within his paper several broods in decline, one of which was Brood XI. Despite missing 15 broods we maintain Marlatts classification as it is efficient and convenient. 

Marlatt's brood system with broods that were never found marked with blue arrows and broods that are extinct marked with red.

It is also important to remember that because the broods are based on populations that come out at the same time they also share the same distribution and separate broods tend to not overlap geographically. Some of the distribution of these broods can be very small. With the 15 broods missing and with small geographic ranges of other broods it sometimes feels like long periods of time between emergences, especially if you stay in a single part of the country. This way whenever they come out around very populated areas the media tends to pick it up fast. 

And while the emergence of 2 broods at the same time is not unusual (in fact, it's more unusual for the 13 year broods to emerge by themselves), it is a rare occurrence that the same two broods (one of the 17 and one of the 13) will emerge at the same time, about 220 years. 

SO IS 2024 SPECIAL OR NOT?! The answer is yes! One of the broods that came out is Brood XIX which has the largest distribution cutting through much of the Eastern US and several large cities. This Brood also contains all 4 species of the 13 year cicadas. The other brood, Brood XIII has the most individual cicadas that come out at the same time and it contains all 3 species of 17 year cicadas. The last thing that makes 2024 special is that the two broods almost overlap!  In central Illinois they get really close and in some places they are expected to be mixed together. To make sure we were within one brood's territory we drove from one brood to the other in about 2 hours. This ensured we were able to find all 7 species without too much mix-up (or at least we hope!). All of these factors combined made entomologists very excited and that trickled into the news. Whenever insects make it into the news in a good way it's great!

Now let's talk about the 7 species. How are they distinguished from each other? Of the 7 species there are 3 pairs that are pretty much indistinguishable but luckily the ones that are similar to each other are in different broods so knowing what brood is coming out is essential to identification.

I made a simple key that you can find below. Remember that there is individual variation and this key is based primarily on color (which is never a great trait to look at). Another great way to identify them is through the song but you run into the same problem with the different groups or pairs where you just need to know what brood they are.

Overall for me it was worth every penny to go see this great boom! I’ve never experienced so many insects out in one place before. In some parks they covered the trees, bushes and benches. And while I left Illinois with a little more hearing loss it was definitely worth it!

Now you know all about the 2024 cicadas, the broods, and periodical cicadas!

Key

1a No orange on sternites….2 (Cassini group)

1b Orange stripes on sternites….3

2a 17 years…..M. cassini

2b 13 years…M. tredecassini

3a Orange stripes narrow….4 (Decula group)

3b Orange stripes bold….5 (Deciem group)

4a Usually less orange striping (barely any) 17 years…. M. septendecula

4b Usually more orange striping, 13 years…. M. tredecula

5a Abdomen mostly orange….M. tredecim

5b Striping apparent and thick….6

6a 17 years…..M. septendecim

6b 13 years…M. neotredecim

Poem

Tree and shrub and grasses tall

They share a common bloom.

In shades of coal and redrock,

With wings of glass and filigree.

A shrieking rhythmic thrum,

Hundreds. Thousands. Hundred thousands.

The empty shells of past lives,

Cling to wall and tree.

The fallen heap upon the ground,

Their lives’ a victory.

This bloom, this mulch, this chorus.

This magic plague they call Cicada.

-Rebecca Jensen

References

• Chatfield‐Taylor, W., & Cole, J. A. (2017). Living rain gauges: cumulative precipitation explains the emergence schedules of California protoperiodical cicadas.

• Chatfield-Taylor, W. (2020). Predator avoidance leads to separate emergence cycles in the protoperiodical Okanagana magnifica Davis, 1919 (Hemiptera: Cicadidae). The Pan-Pacific Entomologist, 96(3), 135-141.

• Cicada Wikipedia Retrieved September, 2024, from https://en.wikipedia.org/wiki/Cicada

• Genus Magicicada - Periodical Cicadas. (2024, May). Bugguide. Retrieved September, 2024, from https://bugguide.net/node/view/6970

• Lehnert, M. S., Reiter, K. E., Smith, G. A., & Kritsky, G. (2019). An augmented wood-penetrating structure: Cicada ovipositors enhanced with metals and other inorganic elements. Scientific reports, 9(1), 19731.

  Marlatt, C. L. (1907). The periodical cicada (No. 71). US Department of Agriculture, Bureau of Entomology.

• Marshall, D. C., & Cooley, J. R. (2000). Reproductive character displacement and speciation in periodical cicadas, with description of a new species, 13‐year Magicicada neotredecim. Evolution, 54(4), 1313-1325.

• Petti, J. M., (1997) Chapter 24: Loudest. Book of Insect Records. University of Florida.