The ancient Romans called these last days of summer the “dog days” in honor of the rising of the dog star, Sirius, with the morning sun. They thought that the heat and often unbearable humidity of the late summer were due to the combined powers of these two stars bearing down on the Earth. It was said to be a time of madness when wine soured and both man and beast hovered on the edges of despair and rage.
It’s not that bad, really, although the dog days do mean that summer is coming to an end. These dog days are also the time for the annual cicadas (called the “dog day cicadas”) to rise up into the trees and begin to sing.
The dog day cicadas have life cycles that range from two to five years in length. A given area, though, will have cohorts that reach their adult stages in the late summer of any given year. So, as we approach any given August, we will be greeted by the nearly continuous singing of the “annual” cicadas.
These cicadas begin their lives as eggs deposited in clusters under the bark of small tree branches and twigs. In six to seven weeks the eggs hatch into tiny nymphs which drop to the ground and burrow into the soil. They will live in the soil, feeding primarily on the sap from tree roots (especially oaks, ashes, and maples) for the next two or more years. They grow and undergo molts and metamorphic changes until they are at last ready to molt into their adult forms. In the late summer they crawl up out of the soil and climb back up the trunks of the same trees that housed their eggs and whose roots have nourished them for so long. On the trunks and branches of these trees the cicadas carry out their last molt and are transformed into adults. The dry exoskeletons of their pre-adult stages can often be found empty but still clinging to the rough surface of the tree bark!
Male cicadas climb further up the tree and begin to sing. They have thin, exoskeleton membranes (called “tymbals”) on the sides of their abdomens that they can pull inwardly and then release to make a loud “click.” The males’ bodies are also quite hollow and act as amplifying, resonance chambers for the generated sounds. The purpose of the song is, of course, to attract females for mating. The mated females will then lay their cluster of eggs under the bark of a twig or branch of the tree and start the life cycle all over again. Interestingly, the females have very solid, “meaty” bodies. They require more metabolic energy and more elaborate internal organs for the production of their eggs. One consequence of these morphological gender differences is that females are the preferred food for most cicada predators (including birds, squirrels, raccoons, and even people (many cultures include annual cicadas as a popular, seasonal food!).
The soils under our trees are quite rich with developing cicada nymphs and each year a significant number of them mature and emerge. It is thought that the species reduces its overall losses to predation by concentrating its adult emergence into a very narrow time window. Their numbers overwhelm potential predators and then they suddenly disappear. This transient existence also keeps predators from specializing on the cicada adults.
Other cicada species (called the “periodic cicadas”) have taken this idea of transient, predator satiation even further by extending their soil dwelling, nymphal stages out to thirteen or even seventeen years! These periodic “locusts” are so rarely abundant and when out are in such incredible numbers that predatory species are not only overwhelmed (and satiated) but also are stymied from evolving any specialized feeding strategies.