Brood X Cicadas, Pt 1

There is a lot of “buzz” this spring about periodical cicadas, using scary terms like “billions” and “invasion.”  But these are insects that have always occurred here, at least for hundreds of thousands of years, and are one of our special natural phenomenon. 

Each individual cicada is older than nearly all children alive in Philadelphia right now – yes, an insect that is 17 years old! And their appearance — red eyes and orange and black bodies – are so cool, and make some of us think about another season — Halloween! 

Entomologists like me are so excited to see these insects. But there are lots of misconceptions about these cicadas, and that might be giving you some anxiety. I hope this post will ease some of that and help you appreciate and celebrate these insects if you see them this spring. 

The emergence holes and cast nymphal skins of the periodical cicada, taken in Kansas in 2015

Why should I care about cicadas? 

Cicadas are much beloved around the world, including in China, Japan, New Zealand and France, where they have inspired cultural stories and art. Among the 3,000 species worldwide, there are some as colorful as butterflies, some as cryptic as leaves, and some that advertise their toxicity with spooky colors. They are some of the largest insects that we see during the year. Their body structures have been studied and adapted to solve some of our engineering problems in material science.

They have inspired Pokemon creatures and other fun games and toys, and all kinds of art. They provide food for birds, mammals, other insects and even us. They provide the background songs to our summer days, including from those with interesting common names such as dog-day, scissor-grinder, lyric, dusk-singing and cactus dodger cicadas. And we are so lucky in this region to have one of the most special group of cicadas in the world, the periodical cicadas

I hear or see cicadas every year in the Philadelphia area. Why are these periodical cicadas so special? 

Several cicada species  occur in the Philadelphia region. Most are ones we see and hear as adults every summer, and this group is called annual cicadas.  But then there is a special group of three species that only emerge every 17 years, and these are called periodical cicadas. Annual and periodical type of cicadas are easy to distinguish. 

Annual cicada adults first appear around the beginning of July; periodical cicada adults are seen from mid-May to mid-June. So the adult activity season doesn’t overlap between the two groups. 

Annual cicada adults in this region are often colored in green to brown tones with dark eyes; periodical cicadas are in orange and black tones, with red eyes. 

What periodical cicadas do we see in our region? 

There are two main groups of periodical cicadas, and they are defined by how long it is between emergences of adults. One group is the 13-year periodical cicadas, but they don’t occur in our region. The group that occurs here is the 17-year cicadas, with the adults emerging every 17 years.

Within the 17-year group, there are three different species (or kinds). They are all placed in the same genus group with a wonderful scientific name, Magicicada. So within the genus Magicicada, we have the species Magicicada septendecimM. cassini, and M. septendecula. (In Latin, both septendecim and septendecula mean 17 years). Each of these species has its own slightly different color pattern in the body, and the males of the three species have quite different songs. All three species can emerge together in one place or there may be only one or two species present.

This year they keep calling it Brood X. What does that mean? 

Although the three 17-year cicada species occur across the eastern half of North America (from Kansas to the Atlantic Ocean) the different populations of the species don’t have the adults emerging in the same 17-year cycle. Each population has a separate 17-year cycle, and that is what is termed a brood. There are 13 different 17-year broods, or populations, with distinctly different emergence times (different years and locations). And to make it a bit more complicated – different broods can overlap in a region! For example, in the Philadelphia region we have three brood cycles of the 17-year cicada: 

Brood X – this year’s brood in 2021 (last was 2004) 

Brood XIV – next emergence In 2025 (last was 2008) 

Brood II – next emergence in 2030 (last in 2013) 

Apparently different broods can occur in the same woods, but they are separated by 4-year emergence cycles, as we see above.

Specimens in the Entomology Collection

Why do they stay underground for so many years? 

This is a great scientific question. Annual cicadas take 3-5 years for the nymphs to reach maturity, so every year we see annual cicada adults, although they are of different ages. So already we know that these large insects take a long time to develop. Somehow the periodical cicada lineage evolved to have a much longer life cycle and to synchronize their emergence. One main reason is likely that when the entire population emerges all in one year, it swamps any impact from predators (there is safety in numbers). Those few adults that emerge in odd years are often “sitting ducks” for bird predators. So selection for predator protection may have reinforced a synchronized life cycle.

Studies have shown that the final nymphal stage may take 4-5 years to reach maturity, and those that develop fastest will “wait” out the years underground to emerge with the rest of the population (patiently waiting for them to catch up). In addition, the periodical cicadas seem to have a penchant for 4-year cycles – either 9, 13, 17 or even 21 years, with the 13-year cicadas likely evolving from 17-year ancestors. Today we see for 17-year broods, that some early emergence occurs at 13 years, and some late at 21 years, although the vast majority of the population comes out at the 17-year mark.  But how do they mark the time? We don’t know yet. Perhaps a budding scientist will solve the riddle of how this lifespan evolved.

What do they do underground? 

Cicadas have sucking mouthparts basically built like a soda straw. This structure  is long, and you can see it extending between the bases of the legs when the cicada isn’t feeding. The nymphs feed on the nutrient-poor fluids in tree roots called xylem and require bacterial endosymbionts to provide some essential amino acids and vitamins (think of it as their own probiotic mixture!). This diluted food source may be why these insects are so slow growing. 

Closeup of the straw-like mouthpart extending between the base of the legs

What triggers them, after just the right amount of years, to decide to show up above ground? 

As the timing of the emergence gets close, the nymphs will move close to the soil surface, and build exit holes, often with mud turrets surround the holes. They track soil temperature and the photoperiod of light (how long the day and night are) until it is the right time to emerge. Starting in the evening, nymphs crawl up any vertical vegetation to support them, and the exoskeleton splits, and the adult emerges and expands its wings and hardens its body exoskeleton. By morning, they will move up into the trees, leaving the nymphal exoskeleton (like a husk) behind. 

Some individuals in a brood will emerge one or four years before or after the cycle, but it is a small number compared to the main population. It is not clear how they count the years in their lifecycle, and apparently some count wrong and miss the party! 

Resources: 

iNaturalist identification app – https://www.inaturalist.org/home  

Cicada information page from Chris Simon – https://cicadas.uconn.edu/  

Magicicada app – http://cicadasafari.org/  

Hear Periodical Cicada researcher Chris Simon, PhD, of University of Connecticut speak about the latest  knowledge about these insects – https://www.youtube.com/watch?v=GyurQlxe80U  start at 14  min mark.   

Cicada Mania site, all kinds of information on cicadas – https://www.cicadamania.com/  


Text, images and video by Jon Gelhaus, PhD, Curator of Entomology at the Academy of Natural Sciences of Drexel University and professor, Biodiversity, Earth and Environmental Sciences

To read more posts in the cicada series, visit

Brood X Cicadas, Pt 2

Is Philly Being Snubbed Again?

How to Celebrate 17-year Cicadas — Eat One!


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