1996: a Year of the Cicadas

[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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