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Introductio n Almost every year on a warm spring evening somewhere in the eastern United States, billions of periodical cicadas begin to emerge from the ground for the first time in 17 years. After shedding their exoskeleton, for a few weeks adult cicadas swarm, sing, reproduce, and die. It will be 17 years before they are seen again. Periodical cicadas are one of the most unique creatures on Earth. In this magazine and the video documentary Return of the Cicadas, you’ll get an amazing glimpse of the lives of these fascinating insects, through stunning close-up video and time-lapse photography. Return of the Cicadas focuses on Brood X, which emerged throughout the Midwest in 2004, and will again in 2021. Like a cheap 1950s science fiction movie, the invasion of billions of insects taps primeval human emotions and arouses curiosity about the natural world. Brood X also offers a dramatic and visually compelling lesson about ecology, animal behavior, biodiversity, forests, insects, and the influence of humans on nature. No insects attract public attention like cicadas. What are Cicadas? What makes these insects so interesting is their unique life cycle and strategy for survival. There are seven species of periodical cicadas, and they only occur in the eastern half of the United States. There are many other types of cicadas around the world, but they are not periodical. Periodical cicadas (scientific name Magicicada) are classified in the Homoptera, a large group of insects that suck sap from plants. Aphids (or plant lice) are another common type of Homoptera. Homoptera are just one of many groups, or orders, that make up the insects, the largest and most diverse form of life on Earth. Insects, in turn, are one particular type of arthropod, which include all animals with exoskeletons, segmented legs and segmented bodies. All insects specifically have bodies divided into three parts (head, thorax, and abdomen), and three pairs of legs. Annual cicada. Picture by Tom Ingling. Compound eye (and antenna) of the periodical cicada. Picture by Rudy Turner. Life Cycle A unique feature of periodical cicadas is their long lifespan and unusual life cycle. While most insects live only a few days, weeks, or months, periodical cicadas live for either 13 years or 17 years. This is what distinguishes them from non-periodical cicadas. We think that 13-year cicadas descended from 17-year cicadas when their life cycle shortened by four years. Newly hatched cicada nymph. Picture by Rudy Turner. Broods The broods are given Roman Numerals (like the Super Bowl) to indicate what year they emerge. Because there are both 17-year and 13-year periodical cicadas, there are potentially 30 different broods. However, there are actually fewer than 30 broods because some have gone extinct, or never existed. For example, the 17-year periodical cicadas Brood X (ten) emerged in 2004 and will emerge again 17 years later in 2021. Brood XI (2005) and Brood XII (2006) do not exist, as far as we know. Brood XIV is scheduled to emerge in 2008 and last emerged in 1991. And Brood I emerged in 1995 and will again in 2012. So every 17 years, the brood numbers start over. We don’t really understand why the generation time of periodical cicadas is so long (nearly the same as for humans), or why they are prime numbers (1, 2, 3, 5, 7, 11, 13, 17, 19, etc.). Almost the entire lifespan of the periodical cicada is spent underground. Only the last month occurs above ground, when we first really notice them. This is equivalent to a human with an 80-year lifespan spending 79 years and 7 months underground. In different parts of the country, different broods of periodical cicadas emerge in different years. Map of the distribution of Brood X. Thirteen-year periodical cicadas are numbered from XVIII (18) up to XXX (30). Brood XXIII emerged in 2002 and will emerge again in 2015. You can see what broods are emerging in a given year, and where they will emerge, on our interactive Web site www.indiana. edu/~preserve/research/CicadasPres/ slide14.html. Map of the distribution of 17year (blue dots), and 13-year (red dots) periodical cicadas. Multiple es cicada speci Another unique feature of periodical cicadas is that a brood consists of multiple species, not just one. Further, all of the species are emerging on exactly the same cycle! The 17-year cicadas include the species Magicicada septendecim, M. cassini, and M. septendecula. During 2004 and Brood X, all three species could be found buzzing around together. There are four species of 13-year periodical cicadas (Magicicada tredecim, M. neotredecim, M. tredecassini, and M. tredecula). Each 17-year species has a similar 13-year species. For example, M. cassini and M. tredecassini are very similar and may actually be the same species on a different life cycle. Why should all three (or four) species in a brood emerge together instead of each emerging in a separate year? One idea is predator satiation. If many cicadas emerge at the same time, they may be less likely to be eaten than if only a few emerge. The different species of cicadas do better if they all emerge together than if they emerge independently. Imagine if you are swimming alone in a big pool that also happens to have a hungry great white shark. Compare that to swimming in the same pool with the same shark but 100 other people. Where are you most likely to escape being eaten? We learned that periodical cicadas are members of the Homoptera, or sap-sucking insects. During their long life-cycle underground, the juvenile cicada nymphs attach to the roots of trees and shrubs and suck xylem fluids from the roots using a straw-like mouthpart. Xylem fluid is basically the water trees take up from the soil along with dilute nutrients dissolved in the soil water. The poor food value of xylem might be one reason that it takes them 17 years to mature. During their underground phase, periodical cicadas construct burrows or tunnels that allow them to move around underground fairly easily. These tunnels are well constructed and remain watertight during floods for example. When their 17-year life cycle begins to draw to a close, they build tunnels upwards to the soil surface so they can quickly emerge from the ground when conditions are right. Fe e d i n g Emergence The mass emergence of periodical cicadas from the ground is one of the most dramatic biological events that anyone will ever see. In late spring of the year they are scheduled to emerge, often following a warm rain, something triggers the periodical cicadas to abandon their underground homes and emerge into a completely different world aboveground. This trigger is related to soil temperature. When the soil temperature reaches 64 degrees Fahrenheit, it is time to go. Periodical cicadas know to emerge when the soil reaches 64°, but how do they know if it is the right year? If you emerge a year too early or a year too late, you have missed the big party and are likely to get eaten! We don’t know definitely how periodical cicadas tell time, but it appears that they are counting the years and know to emerge in the seventeenth year when the soil is 64°. They are probably not counting years directly, but rather the seasonal growth cycles of their host trees. And some do get mixed up and come out a year early or late. But the great majority count correctly and emerge from the ground right on schedule! The first nights, the emergence is slow. But then more and more cicadas begin to emerge day after day until the ground is literally erupting with cicada nymphs. Attack of the cicadas As the day turns to dusk, the cicadas begin to come out in ever-increasing numbers. The twilight emergence probably is also a way to avoid predators that are most active during the daylight hours. The density of cicadas can be staggering. We have counted over 200 nymphs emerging per square meter of ground in the forest. This is equal to about five million per acre. In certain suburban or park-like areas, the numbers can be even higher. And periodical cicadas are not tiny insects like gnats or mosquitoes. They are the size of small shrimp. That is a lot of cicada biomass and a lot of potential food for other animals. During periodical cicada emergences the ground begins to look like Swiss cheese. The bodies surface. It is an invasion, and it will happen again in 17 years. The process of eclosion, or emerging from their exoskeleton, is a very risky time for periodical cicadas. Once the nymphs emerge from the ground, they search for and climb up any vertical surface. This is often a tree trunk but can also be fence posts, telephone poles or even the sides of houses. Life in the suburbs Before European colonization of North America, most of the eastern landscape was forest. Periodical cicadas are forest insects, but they have adapted well to the suburbanization of our landscape. Housing developments, college campuses, parks, golf courses, and cemeteries support very high densities of cicadas, probably far higher than in the past. Scattered, well-maintained trees and shrubs —such as those found in developments and parks—are ideal cicada habitats, especially if they are close to forest habitats and migrant cicadas. However, all landscape changes are not good for cicadas. Forest clearing and agriculture can lead to the local extinction of periodical cicada broods. For example, Brood X once occurred throughout Indiana but is now gone from most of the north-central part of the state. This area is now vast fields of corn and soybean with few trees in sight. The landscaping in new developments takes time to mature. If there are reasonably large trees and shrubs present when a brood emerges, they may be colonized by periodical cicadas, especially if there are nearby areas with dense populations. During 2004 and Brood X in Bloomington, Indiana, developments built in the 1960s had huge numbers of periodical cicadas while developments built in the 1970s and 80s had fewer. Developments built in the 1990s and 2000s had virtually none at all. Historic college campuses, which have been stable for a long time, are especially good habitats for periodical cicadas. When they find a likely spot, the cicada nymph grabs a secure hold and then begins to pulsate in a rhythmic fashion. The back of the exoskeleton then splits open and the undeveloped white adult stage climbs out. The periodical cicada then flips upside down and will hang motionless overnight until its wings expand and its body hardens and turns black. If all goes well, a fully developed and healthy periodical cicada is ready for its first flight the next morning. Unfortunately it does not always go well. Many cicadas become infected by a pathogenic fungus while they are emerging from the ground. The sheer numbers of cicadas leads to competition for an unoccupied perch with no disturbance while their body develops. Many cicadas get knocked off their perches by other cicadas, and often the crowding results in malformed and misshapen wings that will never fly. Further, while the nymphs are crawling over the ground and climbing up trees, and when the immature adults are handing upside down, they are sitting ducks for predators. Eclosion Mating Predation The numbers of periodical cicada nymphs and adults during an emergence is staggering. Catching them is easy and almost every animal around will take advantage and gorge on cicadas. Birds, snakes, squirrels, raccoons, fish, dogs and cats all start eating cicadas. Even animals like field mice that normally eat plants and seeds will start eating cicadas. Quite a few people also eat cicadas. Newly eclosed adults fried in a little butter are quite tasty, if you like eating insects. A common complaint during Brood X was that people’s pet dogs and cats had eaten so many cicadas they would not eat their own food. And fishermen know not to bother during an emergence because the fish are all stuffed with cicadas. But there are so many periodical cicadas, they can’t all be eaten. Many, if not most, escape, find a mate and reproduce. The mass emergence of periodical cicadas totally overwhelms the capacity of predators to eat them all. The huge bolus of food can have a big effect on wildlife populations. For example, the year after Brood X there were record numbers of wild turkeys hatched in southern Indiana because the adult turkeys were so healthy and well-fed. Another dramatic feature of periodical cicada emergences is the overwhelming volume and incessant singing. The sounds of thousands or millions of periodical cicadas singing simultaneously have a strange science fiction quality. There is a constant ebb and flow of buzzing that starts at dawn and continues all day. Only male cicadas sing. The songs are produced by a pair of organs called tymbals, which are found on either side of their abdomen. Each tymbal consists of a ribbed membrane that stretches like a drumhead across an opening in the abdomen, which is hollow and acts as a resonating chamber to amplify the sound. The sound can be very loud, with background noise levels during Brood X exceeding OSHA standards for the workplace. Another type of cicada from Australia produces the loudest sound by an insect ever recorded. The males gather in sunlit trees to sing. Singing takes a lot of energy so the bright sun raises their collective body temperature and energy level. Thousands of singing males in the same tree form a chorusing center. Female cicadas, which do not sing, are attracted by the sound and go in search of a mate. They seem to find most attractive the individual males with the loudest and clearest songs. In many other animals, like birds and frogs, females will also choose mates based on their singing ability. Once the female identifies a possible mate, she will flick her wings to show interest. The male responds with another type of call to acknowledge the female. Then a series of back-and-forth wing flicks and male calls help to confirm that the female has made the right choice. Then the cicadas couple rear to rear and the first step of reproduction occurs. A tymbal (noisemaking organ) of a periodical cicada. Picture by Rudy Turner. Egg laying s t h g u o h t l a n Fi Fertilized females lay their eggs in pencilsized branches of trees. They have a special organ called an ovipositor that can cut into hard wood and lay the eggs inside the branch. Each female can lay several hundred eggs in batches of 10-20 into individual slits on branches. After 6-8 weeks the eggs hatch and the young nymphs emerge. They then drop to the ground and burrow, not to be seen again for 17 years. The decision of what tree to lay her eggs in is an important one for a female cicada because the belowground phase of the life cycle will likely be spent feeding on that same tree. Some tree species like apples, maples, and sycamores seem to Female cicada laying eggs. be highly preferred while others like sumac, sassafras, and walnut are mostly avoided. It may be that trees with oils and resins are poor hosts for eggs and nymphs. Female cicadas prefer to lay eggs in smaller, younger trees in sunny locations. They are trying to predict which trees have the best change of survival for the next 17 years. But a windstorm or chainsaw can kill a tree, and eventually all of the cicada nymphs attached to its roots. The damage to small branches from egg-laying can cause them to die or snap off. Trees with heavy egglaying appear sick with hundreds or thousands of dead branches. Small trees can be killed outright or severely deformed. Many people purchase nets to drape over prized specimens and protect them from cicadas. Ovipositor of female periodical cicada. Picture by Rudy Turner. Scarring on twigs from egg laying (left). Periodical cicadas are unique, but illustrate many more common biological interactions and relationships. Unlike many pests, periodical cicada emergences are completely predictable. For us, the year 2021 is circled on the calendar. That is when Brood X will next emerge in the Midwest. But you don’t have to wait until then to see periodical cicadas. Different broods are emerging almost every May and June somewhere. Many people make travel plans specifically to go to an area at the time when periodical cicadas are emerging. It is worth the trip. And if you can’t experience the wonders of a periodical cicada emergence in person, this video documentary will have to suffice. But if you are lucky enough to experience an outbreak in person, this documentary and publication provide a lot of background information for a better understanding and appreciation of what you are experiencing. Return of the Cicadas was filmed by Samuel Orr, produced by Samuel Orr and Keith Clay, presented by WFYI of Indianapolis, and funded by the National Science Foundation’s Communicating Research to Public Audiences (CRPA) program with a grant to Keith Clay and the Indiana University Research and Teaching Preserve. You can learn more about cicadas at www.indiana.edu/~preserve/research/cicada/index.html and you can contact us at [email protected]. © 2008 Indiana University and Keith Clay