Jane Stevens, an entomologist at the St. Louis Zoo, offers the following answer.
Image: Roy Troutman
The 13-year and 17-year cicadas, known as periodical cicadas, are both large-bodied insects with orange-veined wings. And this summer, the raucous buzzing heard around St. Louis comes from both varieties--the first such occurrence in 221 years.
Despite their fearsome appearance, with bulging, bright red eyes, these members of the Homoptera order of insects (other members being leafhoppers and aphids) are harmless to animal life and all trees but young saplings. The daytime noisemakers are indefatigable in calling for a mate. The din from large broods readily overwhelms competition from even the noisiest lawnmower.
Periodical cicadas occur in three pairs of species--each pair comprised of a 17-year and a 13-year species. (There are other cicadas, such as dog-day cicadas, which have only a four-year life cycle.) The periodical varieties look, sound and behave alike. The only characteristic distinguishing them from one another is the length of time they spend in the ground during the nymphal stages--either 13 or 17 years.
After cicada eggs hatch, the antlike nymphs quickly drop from the trees and burrow five to 46 centimeters (two to 18 inches) underground in search of tree roots to feed upon. After 13 or 17 years, a natural "clock," which remains a mystery to scientists, indicates that it is time to surface; the nymphs leave the ground, abandon their brown and brittle exoskeletons and ascend to the treetops to begin their constant buzzing calls. Mating occurs, eggs are laid and the cycle begins again.
Image: Roy Troutman
Scientists identify groups of cicada species that emerge at the same time and in the same place by an assigned brood number. Brood maps show where specific groups of cicadas will emerge, and brood lists estimate the years during which they will appear.
Periodical cicadas have several distinguishable calls: calls to attract males to make the all-male chorus larger and louder, which in turn attracts more of the silent but interested females; courting calls; and the alarm call, that loud buzzing you hear when you pick one up.
After mating, adult female cicadas use their blade like ovipositor to make elongated openings in new growth sections of tree branches. A female usually lays 20 to 30 eggs in each opening, and there can be several egg "nests" per branch. During her brief adult life stage, each female lays approximately 600 eggs. The eggs take six to eight weeks to mature--after which the nymphs drop to the ground and immediately begin their descent into an underground habitat. Their lengthy nymphal stage is unparalleled within the animal kingdom and continues to draw the interest of scientists.
For an insect aficionado, it's been a great summer to live in Missouri: a rare emergence of both of the largest broods of 17-year and 13-year cicadas--Broods IV and XIX, respectively--is occurring here for the first time in 221 years. Brood XIX has emerged in the eastern and central parts of the state and Brood IV in northwestern areas. There are, however, a few small places in central Missouri where the two species overlap.