Annual and Periodic Cicadas
The 17-year periodic cicadas emerged this past spring, but annual cicadas are present every year.
While standing outside talking on the phone the other day, I had to momentarily pause the conversation – the cicada singing overhead was just too loud for me to hear the person on the other end! Periodic cicadas emerged earlier this spring in many places in the Eastern U.S., but now the dog day Cicadas are here, announcing the later days of summer with their usual noisy confidence. Cicadas are present every year, just about everywhere, with nearly 200 kinds of annual cicadas found in North America alone.
All cicadas are insects that undergo what’s known as hemimetaboly, or incomplete metamorphosis. When you think of metamorphosis, you probably think of butterflies and their cycle of egg, larva (caterpillar), pupa (chrysalis), and adult. Cicadas skip the pupal stage, and their larval stage, when they’re known as nymphs, is spent mostly underground. So most people only see cicadas in their adult form, or find their exuvia, the shed skin they leave behind when they molt to that form. (How’s that for a lot of big science words!)
Cicadas lay their eggs on the tips of tree branches. The hatched larvae fall to the ground, where they burrow beneath the soil and suck the sap from roots of trees. The difference between periodic cicadas and annual cicadas is the amount of time they spend feeding as nymphs below ground. Annual cicadas spend about 2 years eating underground, and adults emerge every year for mating. Periodic cicadas eat and grow for either 13 or 17 years, depending on the species. All periodic cicadas emerge at more or less the same time in those years, usually in late spring. This behavior, known as predator saturation, is actually form of defense against the many creatures that consider cicadas a tasty snack. When so many cicadas emerge all at once, each individual cicada is less likely to be gobbled up by a hungry bird or raccoon.
Cicada songs are performed by males to find mates. Their unbelievably loud buzzing (up to 120 decibels, about the same as a chainsaw) is produced by a structure on their exoskeleton called a tymbal. Essentially, they click the tymbal back and forth at increasingly fast speeds. Their abdomens are nearly hollow, serving only to amplify that sound so much it can sometimes be heard a mile away. Male cicadas actually have the ability to disable their “ears” (called tympana) so they don’t cause themselves to go deaf!
Cicadas are harmless to humans, even if their loud singing is sometimes a nuisance. They don’t generally do any lasting damage to trees, although some tip browning and wilting may occur when eggs are laid. Their abundance, whether annually or periodically, make them an important part of the food chain. Bird watchers especially should welcome cicadas, as many birds include them as an important part of their diets as they prepare for fall migration.