Cicadas: An insect horde

In spring in eastern North America, swarms of brown bugs emerge from the soil in their billions after an astonishingly-long stint underground.

The bugs are periodical cicadas and within the space of a few weeks they will shed their skin to become adults, sing to attract a mate, breed and die.

Their behavior has long fascinated observers. 

In the late 1800s, entomologist C.L. Marlatt found there were distinct cicada broods with either 17 or 13-year lifecycles. He assigned the various broods Roman numerals as identifiers.

Each brood emerges en masse (sometimes in their billions) in specific years. The simultaneous arrival of all insects in a brood is thought to be a tactic that helps them avoid predators. There's safety in numbers.

The animals develop at different rates, but those who grow faster tend to wait for the others to catch up. And when the soil temperature hits 64 degrees Fahrenheit (18 degrees Celsius), they make their appearance.

In May and June 2017, Brood VI emerged across US states such as Georgia and North and South Carolina, and are expected again in 2034, according to Cicada Mania, a project "dedicated to cicadas, the most amazing insects in the world." 

Cicada watchers enthusiastically document and notify the website when they see or hear the animals - the cicada mating opera is unmistakable and loud.

Why they spend so long underground developing and feeding on fluid from tree roots remains something of a mystery; as does their ability to know when exactly to emerge together. But scientists hypothesize a molecular clock, tied to the yearly cycle of the trees they feed upon, plays a role.

When they do finally emerge and mate, the male dies. The female lays her eggs in slits in tree bark and then succumbs to her own mortality. Once hatched, the nymphs - as immature cicada are called - drop to the ground and burrow down until it's their time to materialize.