Cicadas are large, noisy insects known for their sudden, massive appearances. While they spend most of their lives beneath the surface, they are not in a state of true hibernation. Their subterranean existence is a long developmental stage that sets the stage for their brief, loud emergence.
Dispelling the Hibernation Myth
The belief that cicadas hibernate for years misunderstands insect biology. True hibernation is a metabolic process in mammals, involving deep dormancy with reduced body temperature and heart rate to conserve energy. Cicadas, however, are actively developing for many years as wingless nymphs in the soil.
The nymphs are awake and feeding throughout most of their time underground. Like many other insects, they may enter diapause, a programmed developmental arrest, typically in response to dropping temperatures. This pause helps them survive the winter, but their time below ground is a period of slow activity rather than true dormancy.
Life Underground: The Nymphal Habitat
Cicadas live in the soil, primarily within the root zone of deciduous trees. After hatching from eggs laid in tree branches, the nymphs drop to the ground and immediately begin to burrow. They typically live between 6 and 24 inches deep, though depth varies depending on the soil composition.
The nymphs use their specialized, shovel-like forelegs to dig tunnels and construct small chambers around tree roots. They spend their years feeding on the watery fluid, known as xylem sap, which they extract from the roots using needle-like, piercing-sucking mouthparts. Xylem sap is low in nutrients, which is why the nymphs must feed continuously and grow so slowly.
As they grow, the nymphs shed their hard outer shell, or exoskeleton, in a process called molting, typically five times before they are ready to emerge. They require space underground, necessitating the maintenance and expansion of their subterranean tunnels and chambers. Their presence also benefits the soil by creating aeration tunnels that allow water and air to penetrate the ground. This underground habitat is dark, humid, and stable, protecting the developing insect from predators and weather extremes.
When and Why Cicadas Emerge
The timing of emergence depends on the type of cicada, falling into two main categories: annual and periodical. Annual cicadas, which are seen every summer, still spend multiple years underground, but their generations overlap so that some mature every year. Periodical cicadas, belonging to the genus Magicicada, are the groups that emerge in massive, synchronized numbers every 13 or 17 years.
The final trigger for the periodical cicadas to exit their underground home is a precise environmental cue: soil temperature. Nymphs wait until the ground temperature, measured at about eight inches deep, reaches a consistent 64 degrees Fahrenheit. This temperature threshold signals that the weather is warm enough for their metamorphosis and subsequent mating.
Once the temperature is reached, the nymphs construct exit tunnels, sometimes building mud “turrets” above the surface, before crawling out. They then climb onto the nearest vertical surface, such as a tree trunk or plant stem, for their final molt into the winged adult form. This precisely timed mass emergence is a survival strategy that overwhelms predators, ensuring enough individuals survive to mate and restart the cycle.