[The Seventeen-Year Cicada, Magicicada septendecim. a, nymph;
b, cast nymph-shell; c, adult; d, slits in a twig; e, two eggs. a, b, and
c are at 2 x life size; d and e more greatly enlarged. Illustrations from
The Century Dictionary and Cyclopedia, 1911, vol. 2, page 998.]
1996 was once again one of those summers for the Florham-Madison Campus
of Fairleigh Dickinson University! It was a year of the Seventeen-Year Cicada,
otherwise inaccurately known as the Seventeen-Year Locust, and called technically
by its Linnean name Magicicada septendecim.
The cicadas are a group of insects in the Order Homoptera. They have heavy,
subconical bodies, a blunt head, prominent eyes, ridged epistoma, setiform
(hair-like) antennae, short stout legs, bristly hind tibia, and large, fluted
stridulating ("singing") organs at the base of the abdomen. Cicadas
in general are distributed world-wide in warm and temperate regions, with
species as large as 75 mm occurring in the tropics.
The cicada appearing in such numbers on our campus lives most of its life
underground at up to three meters depth, but usually within a meter of the
surface. There the grub-like insects subsist on juices drawn from the roots
of deciduous trees. As time for breeding approaches, they molt to a more
mobile form of the nymph, which eventually burrows upward almost to the
surface, softening the soil as it goes with its saliva. There the nymph
waits until warming temperatures signal that it is the right time to emerge.
It then makes its way to the surface, and crawls to the nearest tree. Using
its hooked and spiney forelegs, it climbs upward, then grips the bark firmly,
and begins the final molt to the adult stage. The exosketon splits along
one medial, longitudinal fissure, and the animal emerges slowly over a period
of about one-half hour. The back arches upward, until the head is free.
Then the body moves forward in the shell until it almost projects at right
angles to the shell and the bark. Finally, with what looks like a great
effort, the insect strains forward and emerges completely, seizing the shell
or the bark with its legs and crawling free. At first it is yellowish white
except for two dark spots which look like eyes, but are not: they serve
perhaps to frighten away casual predators. The wings must then by "inflated":
the veins are pumped with fluid which hydraulically extends them to their
full length and form. As the minutes pass, the exoskeleton hardens and turns
almost black.
The procedure does not always go on without mishap. Frequently a cicada
will emerge too early, and low temperatures will slow its activity, so that
it will not manage to get out of its nymph-shell before its new exoskelton
hardens. Such hapless individuals find themselves encased forever in an
unwanted shell, and if they are not eaten by predators, they eventually
expire. Others may crawl free of the nymph-shell, but not before the wings
have prematurely hardened in uninflated condition: these are the crippled,
flightless individuals, who also have little chance of living a full cicada-life.
A successfully emerged adult, its new exoskeleton and wings adequately seasoned
and hardened, will then crawl upward toward the canopy of the tree. Eventually
it make take off in flight, and within a few days the males among them will
begin their characteristic song. In the Magicicada septendecim the
chorus is a continuous roaring, of constant pitch. The tone is not pure,
however, but is a rattling buzz. By contrast, the usual summer cicada of
August gives a call which rises and falls, each cicada's voice dying down
as the next takes up his part in the round.
Cicadas are eagerly devoured by birds and rodents, and I have known dogs
to learn to enjoy the unusual delicacy. Probably only a fraction of the
individuals emerging as nymphs eventually succeed in surviving to reproduce.
The great French entomologist Henri Fabre doubted that the noise of the
male cicada could be heard by others of the species. He even fired guns
next to singing cicadas, and noted no reaction. Some authorities formerly
stated that no known mechanism exists for the female to hear the calls of
the males. On the other hand, the female abdomen has the same drum-like
form as that of the male, and would resonate (one would suppose) in the
presence of such an adequate stimulus. I am informed that in more recent
times entomologists have located the hearing sense of the female cicada,
and have demonstrated that the male's call is related to courtship. The
male will also buzz if picked up, so we may guess that his noise-making
ability may also serve occasionally to frighten predators.
During adult life, the cicada is able to draw nourishment from plants using
its beak, but apparently very little food is required during the insect's
brief existence in the upper world.
After mating, the female cicada uses her ovipositor (which looks like a
slightly curved hypodermic needle) to deposit groups of eggs in fine slits
in the twigs of the trees. As many as 50 slits are made in twigs, which
hold a total complement of 400 to 500 eggs. Oaks are favored, and we have
plenty on our campus, some of which are old enough to have received such
innoculations for many cicada-cycles. Egg-laying concludes the female's
maternal function, and she then drops from the branch and dies. Neither
do the males survive past early July.
Adult Periodical Cicada Magicicada septendecim with oviposition scars
on twig.
[Drawing by Mr. L. H. Joutel in Felt, Ephraim Porter, 1905, Insects
affecting park and woodland trees. New York State Museum Memoir 8, Plate
11.]
The young cicada nymphs hatch out, scarcely any larger than fleas (about
2.5 mm long). They are slender, grub-like creatures, "as lively as
ants, and after running about on the tree for a short time, drop to the
ground, and bury themselves." They burrow downward to begin their long
subterranean existence. Sixteen summers will pass in the world above before
the cicadas will once again see daylight. "The young grow so slowly
and require so little food, that but slight injury to trees or shrubs results
from their presence." (Felt, 1905, p. 233)
Around the eastern part of the country are broods of Seventeen-Year Cicadas
which are not in phase with ours: they appear in different years. This raises
several good biological questions. Why do different regions generally have
only one or two of these broods? What happened to the broods of intermediate
years? Since they cannot interbreed, are different broods likely to speciate?
A final word on cicada appreciation. These insects are harmless to humans.
They cannot bite, and may readily be handled. Cicadas only very rarely cause
any serious permanent damage to trees. There is in my opinion no reason
or excuse for killing the actors in this spectacular display of nature!
[©1996 by Paul S. Boyer]
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