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Introductio
n
Almost every year on a
warm spring evening
somewhere in the
eastern United States,
billions of periodical cicadas begin to
emerge from the ground for the first
time in 17 years. After shedding their
exoskeleton, for a few weeks adult
cicadas swarm, sing, reproduce, and
die. It will be 17 years before they are
seen again.
Periodical cicadas are one of the most
unique creatures on Earth. In this
magazine and the video documentary
Return of the Cicadas, you’ll get an
amazing glimpse of the lives of these
fascinating insects, through stunning
close-up video and time-lapse
photography. Return of the Cicadas
focuses on Brood X, which emerged
throughout the Midwest in 2004, and
will again in 2021.
Like a cheap 1950s science fiction
movie, the invasion of billions
of insects taps primeval human
emotions and arouses curiosity
about the natural world. Brood X
also offers a dramatic and visually
compelling lesson about ecology,
animal behavior, biodiversity, forests,
insects, and the influence of humans
on nature.
No insects attract public
attention like cicadas.
What are
Cicadas?
What makes these insects so interesting is their
unique life cycle and strategy for survival. There
are seven species of periodical cicadas, and they
only occur in the eastern half of the United States.
There are many other types of cicadas around the
world, but they are not periodical.
Periodical cicadas (scientific name Magicicada) are
classified in the Homoptera, a large group of insects
that suck sap from plants. Aphids (or plant lice) are
another common type of Homoptera.
Homoptera are just one of many groups, or orders,
that make up the insects, the largest and most
diverse form of life on Earth. Insects, in turn, are
one particular type of arthropod, which include
all animals with exoskeletons, segmented legs and
segmented bodies. All insects specifically have
bodies divided into three parts (head, thorax, and
abdomen), and three pairs of legs.
Annual cicada. Picture by Tom Ingling.
Compound eye
(and antenna)
of the periodical
cicada. Picture
by Rudy Turner.
Life Cycle
A unique feature of
periodical cicadas is
their long lifespan
and unusual life cycle.
While most insects live only a few
days, weeks, or months, periodical
cicadas live for either 13 years or 17
years. This is what distinguishes them
from non-periodical cicadas. We think
that 13-year cicadas descended from
17-year cicadas when their life cycle
shortened by four years.
Newly hatched cicada nymph. Picture by Rudy Turner.
Broods
The broods are given Roman Numerals (like the
Super Bowl) to indicate what year they emerge.
Because there are both 17-year and 13-year
periodical cicadas, there are potentially 30
different broods. However, there are actually
fewer than 30 broods because some have gone
extinct, or never existed.
For example, the 17-year periodical cicadas
Brood X (ten) emerged in 2004 and will emerge
again 17 years later in 2021. Brood XI (2005)
and Brood XII (2006) do not exist, as far as we
know. Brood XIV is scheduled to emerge in
2008 and last emerged in 1991. And Brood I
emerged in 1995 and will again in 2012. So
every 17 years, the brood numbers start over.
We don’t really understand why
the generation time of periodical
cicadas is so long (nearly the same as
for humans), or why they are prime
numbers (1, 2, 3, 5, 7, 11, 13, 17,
19, etc.).
Almost the entire lifespan of
the periodical cicada is spent
underground. Only the last month
occurs above ground, when we first
really notice them. This is equivalent
to a human with an 80-year lifespan
spending 79 years and 7 months
underground.
In different parts of the
country, different broods of
periodical cicadas emerge in
different years.
Map of the distribution of Brood X.
Thirteen-year periodical cicadas are numbered
from XVIII (18) up to XXX (30). Brood XXIII
emerged in 2002 and will emerge again in
2015.
You can see what broods are emerging in
a given year, and where they will emerge,
on our interactive Web site www.indiana.
edu/~preserve/research/CicadasPres/
slide14.html.
Map of the distribution of 17year (blue dots), and 13-year
(red dots) periodical cicadas.
Multiple
es
cicada speci
Another unique feature of
periodical cicadas is that a
brood consists of multiple
species, not just one.
Further, all of the species are emerging on
exactly the same cycle!
The 17-year cicadas include the species
Magicicada septendecim, M. cassini, and
M. septendecula. During 2004 and Brood
X, all three species could be found buzzing
around together. There are four species
of 13-year periodical cicadas (Magicicada
tredecim, M. neotredecim, M. tredecassini,
and M. tredecula). Each 17-year species
has a similar 13-year species. For
example, M. cassini and M. tredecassini
are very similar and may actually be the
same species on a different life cycle.
Why should all three (or four) species in
a brood emerge together instead of each
emerging in a separate year? One idea
is predator satiation. If many cicadas
emerge at the same time, they may be
less likely to be eaten than if only a few
emerge. The different species of cicadas
do better if they all emerge together than
if they emerge independently.
Imagine if you are swimming alone in
a big pool that also happens to have a
hungry great white shark. Compare that
to swimming in the same pool with the
same shark but 100 other people. Where
are you most likely to escape being eaten?
We learned that periodical cicadas
are members of the Homoptera,
or sap-sucking insects.
During their long life-cycle underground, the
juvenile cicada nymphs attach to the roots of
trees and shrubs and suck xylem fluids from the
roots using a straw-like mouthpart.
Xylem fluid is basically the water trees take
up from the soil along with dilute nutrients
dissolved in the soil water. The poor food value
of xylem might be one reason that it takes them
17 years to mature.
During their underground phase, periodical
cicadas construct burrows or tunnels that allow
them to move around underground fairly easily.
These tunnels are well constructed and remain
watertight during floods for example. When
their 17-year life cycle begins to draw to a close,
they build tunnels upwards to the soil surface so
they can quickly emerge from the ground when
conditions are right.
Fe e d i n g
Emergence
The mass emergence
of periodical cicadas
from the ground is one
of the most dramatic
biological events that
anyone will ever see.
In late spring of the year
they are scheduled to
emerge, often following
a warm rain,
something triggers the periodical cicadas
to abandon their underground homes
and emerge into a completely different
world aboveground. This trigger is
related to soil temperature.
When the soil temperature reaches 64
degrees Fahrenheit, it is time to go.
Periodical cicadas know to emerge when
the soil reaches 64°, but how do they
know if it is the right year? If you emerge
a year too early or a year too late, you
have missed the big party and are likely
to get eaten!
We don’t know definitely how periodical
cicadas tell time, but it appears that
they are counting the years and know
to emerge in the seventeenth year when
the soil is 64°. They are probably not
counting years directly, but rather
the seasonal growth cycles of their
host trees. And some do get mixed
up and come out a year early or late.
But the great majority count correctly
and emerge from the ground right on
schedule!
The first nights, the emergence is slow.
But then more and more cicadas begin to
emerge day after day until the ground is
literally erupting with cicada nymphs.
Attack of
the cicadas
As the day turns to dusk, the cicadas
begin to come out in ever-increasing
numbers. The twilight emergence
probably is also a way to avoid
predators that are most active during
the daylight hours.
The density of cicadas can be
staggering. We have counted over
200 nymphs emerging per square
meter of ground in the forest. This is
equal to about five million per acre. In
certain suburban or park-like areas,
the numbers can be even higher. And
periodical cicadas are not tiny insects
like gnats or mosquitoes. They are the
size of small shrimp. That is a lot of
cicada biomass and a lot of potential
food for other animals.
During periodical cicada emergences
the ground begins to look like Swiss
cheese. The bodies surface. It is an
invasion, and it will happen again in
17 years.
The process of
eclosion, or emerging
from their exoskeleton,
is a very risky time for
periodical cicadas.
Once the nymphs emerge from the
ground, they search for and climb up
any vertical surface. This is often a
tree trunk but can also be fence posts,
telephone poles or even the sides of
houses.
Life in the
suburbs
Before European
colonization of North
America, most of the
eastern landscape was
forest. Periodical cicadas are
forest insects, but they have adapted
well to the suburbanization of our
landscape. Housing developments,
college campuses, parks, golf courses,
and cemeteries support very high
densities of cicadas, probably far
higher than in the past. Scattered,
well-maintained trees and shrubs­
—such as those found in developments
and parks—are ideal cicada habitats,
especially if they are close to forest
habitats and migrant cicadas.
However, all landscape changes are
not good for cicadas. Forest clearing
and agriculture can lead to the local
extinction of periodical cicada broods.
For example, Brood X once occurred
throughout Indiana but is now gone
from most of the north-central part of
the state. This area is now vast fields of
corn and soybean with few trees
in sight.
The landscaping in new developments
takes time to mature. If there are
reasonably large trees and shrubs
present when a brood emerges,
they may be colonized by periodical
cicadas, especially if there are nearby
areas with dense populations. During
2004 and Brood X in Bloomington,
Indiana, developments built in
the 1960s had huge numbers of
periodical cicadas while developments
built in the 1970s and 80s had fewer.
Developments built in the 1990s
and 2000s had virtually none at all.
Historic college campuses, which
have been stable for a long time, are
especially good habitats for periodical
cicadas.
When they find a likely spot, the
cicada nymph grabs a secure hold and
then begins to pulsate in a rhythmic
fashion. The back of the exoskeleton
then splits open and the undeveloped
white adult stage climbs out. The
periodical cicada then flips upside
down and will hang motionless
overnight until its wings expand and
its body hardens and turns black.
If all goes well, a fully developed and
healthy periodical cicada is ready
for its first flight the next morning.
Unfortunately it does not always go
well. Many cicadas become infected
by a pathogenic fungus while they
are emerging from the ground. The
sheer numbers of cicadas leads to
competition for an unoccupied perch
with no disturbance while their body
develops. Many cicadas get knocked
off their perches by other cicadas,
and often the crowding results in
malformed and misshapen wings that
will never fly.
Further, while the nymphs are
crawling over the ground and climbing
up trees, and when the immature
adults are handing upside down, they
are sitting ducks for predators.
Eclosion
Mating
Predation
The numbers of periodical
cicada nymphs and adults
during an emergence
is staggering.
Catching them is easy and almost every
animal around will take advantage and
gorge on cicadas. Birds, snakes, squirrels,
raccoons, fish, dogs and cats all start
eating cicadas. Even animals like field mice
that normally eat plants and seeds will start
eating cicadas. Quite a few people also
eat cicadas. Newly eclosed adults fried in
a little butter are quite tasty, if you like
eating insects.
A common complaint during Brood X was
that people’s pet dogs and cats had eaten so
many cicadas they would not eat their own
food. And fishermen know not to bother
during an emergence because the fish are
all stuffed with cicadas.
But there are so many periodical cicadas,
they can’t all be eaten. Many, if not most,
escape, find a mate and reproduce. The
mass emergence of periodical cicadas
totally overwhelms the capacity of predators
to eat them all.
The huge bolus of food can have a big
effect on wildlife populations. For example,
the year after Brood X there were record
numbers of wild turkeys hatched in
southern Indiana because the adult turkeys
were so healthy and well-fed.
Another dramatic
feature of periodical
cicada emergences is the
overwhelming volume
and incessant singing.
The sounds of thousands or millions
of periodical cicadas singing
simultaneously have a strange science
fiction quality. There is a constant ebb
and flow of buzzing that starts at dawn
and continues all day.
Only male cicadas sing. The songs are
produced by a pair of organs called
tymbals, which are found on either
side of their abdomen. Each tymbal
consists of a ribbed membrane that
stretches like a drumhead across
an opening in the abdomen, which
is hollow and acts as a resonating
chamber to amplify the sound. The
sound can be very loud, with
background noise levels during Brood
X exceeding OSHA standards for the
workplace. Another type of cicada
from Australia produces the loudest
sound by an insect ever recorded.
The males gather in sunlit trees to
sing. Singing takes a lot of energy so
the bright sun raises their collective
body temperature and energy level.
Thousands of singing males in the same tree form a
chorusing center. Female cicadas, which do not sing, are
attracted by the sound and go in search of a mate. They
seem to find most attractive the individual males with the
loudest and clearest songs. In many other animals, like
birds and frogs, females will also choose mates based on
their singing ability.
Once the female identifies a possible mate, she will flick
her wings to show interest. The male responds with
another type of call to acknowledge the female. Then a
series of back-and-forth wing flicks and male calls help
to confirm that the female has made the right choice.
Then the cicadas couple rear to rear and the first step of
reproduction occurs.
A tymbal (noisemaking organ) of a
periodical cicada.
Picture by Rudy
Turner.
Egg laying
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Fi
Fertilized females lay
their eggs in pencilsized branches of trees.
They have a special organ called an
ovipositor that can cut into hard
wood and lay the eggs inside the
branch. Each female can lay several
hundred eggs in batches of 10-20 into
individual slits on branches. After 6-8
weeks the eggs hatch and the young
nymphs emerge. They then drop to
the ground and burrow, not to be seen
again for 17 years.
The decision of what tree to lay
her eggs in is an important one
for a female cicada because the
belowground phase of the life cycle
will likely be spent feeding on that
same tree. Some tree species like
apples, maples, and sycamores seem to
Female cicada laying eggs.
be highly preferred while others like sumac, sassafras,
and walnut are mostly avoided. It may be that trees with
oils and resins are poor hosts for eggs and nymphs.
Female cicadas prefer to lay eggs in smaller, younger
trees in sunny locations. They are trying to predict
which trees have the best change of survival for the
next 17 years. But a windstorm or chainsaw can kill a
tree, and eventually all of the cicada nymphs attached
to its roots.
The damage to small branches from egg-laying can
cause them to die or snap off. Trees with heavy egglaying appear sick with hundreds or thousands of dead
branches. Small trees can be killed outright or severely
deformed. Many people purchase nets to drape over
prized specimens and protect them from cicadas.
Ovipositor of female periodical
cicada. Picture by Rudy Turner.
Scarring on twigs from egg laying (left).
Periodical cicadas are unique,
but illustrate many more
common biological interactions
and relationships.
Unlike many pests, periodical cicada emergences
are completely predictable. For us, the year 2021
is circled on the calendar. That is when Brood X
will next emerge in the Midwest.
But you don’t have to wait until then to
see periodical cicadas. Different broods
are emerging almost every May and June
somewhere. Many people make travel plans
specifically to go to an area at the time when
periodical cicadas are emerging.
It is worth the trip.
And if you can’t experience the wonders of
a periodical cicada emergence in person,
this video documentary will have to suffice.
But if you are lucky enough to experience
an outbreak in person, this documentary
and publication provide a lot of background
information for a better understanding and
appreciation of what you are experiencing.
Return of the Cicadas was filmed by Samuel Orr, produced by Samuel
Orr and Keith Clay, presented by WFYI of Indianapolis, and funded by
the National Science Foundation’s Communicating Research to Public
Audiences (CRPA) program with a grant to Keith Clay and the Indiana
University Research and Teaching Preserve.
You can learn more about cicadas at
www.indiana.edu/~preserve/research/cicada/index.html
and you can contact us at [email protected].
© 2008 Indiana University and Keith Clay