Cicadas are of the Family Cicadidae, Order Homoptera. They
represent the genera Magicicada and Tibicen. About 1,500 species of cicadas are
known, usually occupying deserts, grasslands and forests. More than 100 species
are found in North. The Dog-Day Cicada (Tibicen) appears yearly in midsummer,
but there are also periodic cicadas. The most common of these is the black and
green Harvest Fly, which matures in two years.
The best-known of these is the 17-Year Cicada (Magicicada), which lives only in
the United States. After 17 years of dormancy underground, this species emerges
for 5 weeks of activity in the sunlight, and then dies. With the exception of
the termite queen, this cicada is probably the longest living insect. The
17-year Cicada is often incorrectly called the 17-year Locust. True locusts are
grasshoppers.
Magicicada
Periodical cicadas are found in eastern North America and belong
to the genus Magicicada. There are seven species -- four with 13-year life
cycles (including one new species just described), and three with 17-year
cycles. The three 17-year species are generally northern in distribution, while
the 13-year species are generally southern and midwestern. Magicicada are so
synchronized developmentally that they are nearly absent as adults in the 12 or
16 years between emergences. When they do emerge after their long juvenile
periods, they do so in huge numbers, forming much denser aggregations than those
achieved by most other cicadas. Many people know periodical cicadas by the name
"17-year locusts" or "13-year locusts", but they are not true locusts, which are
a type of grasshopper.
Magicicada adults have black bodies and striking red eyes and orange wing veins,
with a black "W" near the tips of the forewings. Most emerge in May and June.
Some of the annual cicada species are sometimes mistaken for the periodical
cicadas, especially those in the genera Diceroprocta and Okanagana; these other
species emerge somewhat later in the year but may overlap with Magicicada. The
Okanagana species are the most potentially confusing because of their similar
black-and-orange coloration. The best way to identify cicada species is by the
sounds that they make, because cicada songs are nearly always species-specific.
Magicicada Life Cycles
Cicada juveniles are called "nymphs" and live underground, sucking root fluids for food. Periodical cicadas spend five juvenile stages in their underground burrows, and during their 13 or 17 years underground they grow from approximately the size of a small ant to nearly the size of an adult.
In the spring of their 13th or 17th year, a few weeks before
emerging, the nymphs construct exit tunnels to the surface. These exits are
visible as approximately 1/2 inch diameter holes, or as chimney-like mud
"turrets" the nymphs sometimes construct over their holes. On the night of
emergence, nymphs leave their burrows around sunset, locate a suitable spot on
nearby vegetation, and complete their final molt to adulthood. Shortly after
ecdysis (molting) the new adults appear mostly white, but they darken quickly as
the exoskeleton hardens. Sometimes a large proportion of the population emerges
in one night. Newly-emerged cicadas work their way up into the trees and spend
roughly four to six days as "teneral" adults before they harden completely
(possibly longer in cool weather); they do not begin adult behavior until this
period of maturation is complete.
It appears that the particular night of emergence may be determined by the soil
temperature; nymphs emerge when the soil temperature inside the exit tunnel (and
therefore the body temperature of the nymph) exceeds approximately 64 degrees F
(Heath 1968). Because emergence is temperature-dependent, periodical cicadas
tend to emerge earlier in southern and lower-elevation locations. For example,
cicadas in South Carolina often begin to emerge in late April, while those in
southern Michigan do not appear until June. The best way to predict the time of
emergence for your area is to check records from the prior emergence in that
location, by asking longtime residents or by searching local newspaper archives.
The date of emergence does not vary much between generations, although unusual
springtime weather conditions may accelerate or delay the emergence by a week or
so.
After their short teneral period, males begin producing species-specific calling songs and form aggregations (choruses) that are sexually attractive to females. Males in these choruses alternate bouts of singing with short flights until they locate receptive females. Contrary to popular belief, adults do feed by sucking plant fluids; adult cicadas will die within days if not provided with living woody vegetation on which to feed. Magicicada feed from a wide variety of deciduous plants and shrubs, but usually not from grasses. The picture below shows Magicicada septendecula feeding, with the piercing-and-sucking mouthparts visible just behind the forelegs.
Mated females excavate a series of Y-shaped eggnests in living twigs and lay up to twenty eggs in each nest (Marlatt 1923). A female may lay as many as 600 eggs (Marlatt 1923). Below is a photograph of Magicicada eggnests.
After six to ten weeks, in midsummer, the eggs hatch and the new
first-instar nymphs drop from the trees, burrow underground, locate a suitable
rootlet for feeding, and begin their long 13- or 17-year development.
Why are there so many of them?
Periodical cicadas achieve astounding population densities, as high as 1.5
million per acre (Dybas 1969). Densities of tens to hundreds of thousands per
acre are more common, but even this is far beyond the natural abundance of most
other cicada species. Apparently because of their long life cycles and
synchronous emergences, periodical cicadas escape natural population control by
predators, even though everything from birds to spiders to snakes to dogs eat
them opportunistically when they do appear. Magicicada population densities are
so high that predators apparently eat their fill without significantly reducing
the population (a phenomenon called "predator satiation"), and the predator
populations cannot build up in response because the cicadas are available as
food above ground only once every 13 or 17 years. Periodical cicadas do have a
specialized fungal parasite, but its effects on Magicicada population density
are not well understood. Individual periodical cicadas are slower, less flighty,
and easier to capture than other cicadas, probably because the safety afforded
by their great numbers means that the risks of predation for an individual are
low. Explaining the evolution of such an unusual life strategy is one of the
most difficult problems for periodical cicada biologists.
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