The sight of cicada shells clinging to the bark of a tree is a common yet fascinating occurrence during the warmer months. These empty exoskeletons, left behind after a dramatic metamorphosis, represent a remarkable biological event. Often found in clusters, they signify that the area is a prime location for these insects to complete their life cycle. Understanding what these shells are and why they appear is the first step in appreciating the complex world of cicadas.
The Biology of the Cicada Shell
A cicada shell, technically known as an exuvia, is the discarded outer covering of the insect. This process, called ecdysis, is necessary for the cicada to grow. The nymph, which lives underground for years, emerges, splits its old skin, and expands its wings and body to reach adulthood. The shell is a perfect replica of the insect, from the delicate wings to the intricate patterns on the back. It is usually light brown or tan, blending with the tree bark to provide camouflage against predators.
The Emergence Process
The emergence is a vulnerable time for the cicada. After climbing to a suitable height, often the trunk of a tree, the nymph grips the surface and begins to inflate its body with hemolymph. This pressure causes the old exoskeleton to rupture down the back. The insect must then carefully extract its legs and wings from the old skin, a process that can take several hours. The newly formed adult is pale and soft, requiring time to let its wings dry and harden before it can fly and sing.
Why Trees Are Central to Their Life Cycle
Trees are fundamental to the survival of cicadas for two primary reasons: they serve as the anchor for emergence and as a source of food. The nymphs live underground, feeding on the sap from tree roots. This dependency on tree roots is why large populations are found in wooded areas and urban landscapes with mature trees. Once they emerge, the adults remain in the trees, using the branches for resting, mating, and laying eggs.
Selecting the Perfect Spot
Not every tree trunk is suitable for a cicada emergence. The insect seeks out a vertical surface that provides stability and easy access for climbing. The texture of the bark is also a factor; a rough surface offers better grip for the nymph as it pulls itself free. You will often find the highest concentration of shells on the trunk, just a few feet off the ground, where the insect felt safest during this critical transformation.
Ecological Significance and Misconceptions
While the appearance of shells might seem like an invasion, they are a natural part of the ecosystem. The emergence provides a protein-rich food source for birds, mammals, and insects. The discarded exuvia themselves are consumed by scavengers, completing a small but important part of the nutrient cycle. A common misconception is that the shells indicate an active infestation of harmful insects, but the cicada nymphs underground do not damage the tree, and the adults do not feed on leaves.
The Sound of Summer
The shells you see on the tree are often accompanied by the loud, rhythmic buzzing of the male cicadas. This sound is produced by specialized organs called tymbals, located on the sides of the abdomen. Males create this noise to attract females for mating. The collective sound of a cicada chorus is one of the defining auditory signatures of summer, a stark contrast to the visual evidence of their discarded skins found on the tree trunks below.
