Cicadas, especially the well-known periodical species, spend the vast majority of their lives—from a few years up to seventeen years—as wingless juveniles called nymphs, burrowed into the earth. This prolonged subterranean existence is a complex strategy driven by nutritional necessity, physical safety, and an evolutionary advantage. This strategy culminates in their dramatic, synchronized appearances above ground.
The Necessity of Slow Growth and Xylem Feeding
The primary reason for the cicada nymph’s slow development is its highly specialized diet of xylem sap. Nymphs use specialized, piercing mouthparts to tap into the root xylem vessels of host trees and shrubs, drinking the sap. This fluid, which transports water and nutrients, is mostly water with very low concentrations of proteins and carbohydrates, making it nutritionally dilute.
The low caloric density of the xylem sap means the nymphs must consume enormous volumes over a long time to accumulate enough energy for their entire lifespan, including final metamorphosis and reproduction. To overcome the incomplete nutrient mix, cicadas rely on symbiotic bacteria housed within their bodies to synthesize essential amino acids. This slow-feeding, resource-intensive growth process necessitates the long nymphal stage, which can last anywhere from two to seventeen years depending on the species.
Safety from Predators and Environmental Extremes
Living beneath the soil surface serves as a powerful shield against the numerous threats of the above-ground world. The underground environment insulates the slow-moving, soft-bodied nymphs from the vast majority of surface predators, such as birds, reptiles, and small mammals. Enough nymphs survive the subterranean stage to ensure the population’s future, despite threats from predators like moles.
The soil also provides a stable buffer against extreme weather conditions that could be lethal to a surface-dwelling insect. Buried a foot or two down, the nymphs are protected from freezing winter temperatures, drought, and wildfires. This stable environment allows them to feed and grow continuously, regardless of the seasonal fluctuations occurring above them.
Synchronization Strategy of Periodical Cicadas
For the 13-year and 17-year periodical cicadas, the long underground period is a central component of an extraordinary evolutionary strategy known as predator satiation. These species emerge in overwhelming numbers simultaneously, sometimes reaching over a million individuals per acre. This sheer volume ensures that local predators—including birds, raccoons, and squirrels—can eat their fill, but only a small percentage of the total cicada population is consumed.
The choice of a prime number life cycle, specifically 13 or 17 years, is an adaptation to minimize the overlap with the shorter, cyclical populations of their predators. Since a prime number is only divisible by one and itself, the cicada’s emergence year rarely coincides with the peak population year of a predator that might have a two-year, three-year, or five-year cycle. This mathematical avoidance strategy maximizes the time between synchronized mass feedings.
How Cicadas Know When to Emerge
After years of development underground, the final emergence is precisely timed by environmental cues. The most important trigger is the soil temperature reaching a specific threshold, signaling the beginning of spring and sustained warmth. Periodical cicadas begin their ascent when the soil temperature at a depth of about eight inches reaches approximately 64 degrees Fahrenheit.
Nymphs also track seasonal changes in the chemical composition of the xylem sap they consume, which fluctuates with the host tree’s yearly growth cycle. These internal and external signals coordinate the entire brood to tunnel to the surface almost simultaneously. The mass emergence often occurs at dusk, providing the flightless nymphs protection from daytime predators as they crawl up vertical surfaces to molt into their adult form.