GED Practice Test 13

Thirteen and Seventeen

¹In certain years, in parts of the eastern United States, the ground seems to come alive. Over a few nights in late spring, millions of insects called periodical cicadas crawl out of small holes in the soil, climb the nearest tree trunk or fence post, and split open their skins to emerge as winged adults. For the next four to six weeks, the males fill the air with a loud, buzzing chorus to attract mates. Then the adults die, and the woods fall quiet again for 13 or 17 years.

²Those long silences are the most remarkable part of the cicada's life. After mating, a female lays her eggs in slits she cuts in small twigs. When the young hatch, they drop to the ground and burrow down to the roots of trees, where they spend more than a decade drinking fluid from the roots. Underground, they grow slowly through several stages. Then, in the spring of their 13th or 17th year, when the soil about eight inches down warms to roughly 64 degrees Fahrenheit, nearly all members of a group emerge within days of one another. Scientists call each group that emerges on the same schedule in a region a brood.

³Why would an insect evolve such a strange schedule? One leading explanation is known as predator satiation. Birds, squirrels, snakes, and even pet dogs eagerly eat cicadas, which cannot sting or bite and are clumsy fliers. But when cicadas appear in the millions at once, predators eat until they are full and still leave most of the insects alive. A cicada that emerged a year early or late, on the other hand, would face hungry predators nearly alone. Field counts support this idea: researchers have found that predators consume a large share of cicadas where emergences are sparse but only a small share where emergences are dense.

⁴The numbers 13 and 17 may also matter. Both are prime numbers, meaning they can be divided evenly only by 1 and themselves. Some researchers have suggested that prime-numbered cycles help cicadas avoid predators or parasites whose populations rise and fall on shorter cycles. A predator with a 4-year cycle, for example, would peak in the same year as a 12-year cicada brood at every emergence, but it would peak with a 13-year brood only once every 52 years. This hypothesis is harder to test, and scientists still debate it; no predator that tracks cicadas on a regular multi-year cycle has been clearly identified.

⁵Periodical cicadas do little lasting harm. Egg-laying can kill the tips of branches, and orchard owners sometimes cover young trees with netting during an emergence. Mature trees, however, generally recover. When the adults die, their bodies decompose on the forest floor and release nutrients into the soil, a brief burst of fertilizer from insects that spent most of their lives out of sight.