Archived content - Cicada (4 of 46)
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A. Introduction (Continued)
2. Strange Lives
"First among the (Homoptera) come the cicadas, in which the babies live to old age and the adults die young ....... The larva of the cicada lives underground for years; then the complete insect comes out and runs its full course of adult life in a few days, or at most a few weeks... (Its) eggs hatch into larvae, which forthwith burrow down into the soil and remain, the world forgetting, by the world forgotten, till the Earth and its family have grown 17 years older. This is the longest known period of youth ....... the elephant alone take(s) longer to attain complete maturity." - Arthur Mee, The Children's Encyclopedia (1923).
2A. Underground young.
Eggs that are laid in summer hatch later the same year. Their grublike young ("nymphs") immediately drop to the ground and burrow into the earth. For this they are well equipped. Their enormous front legs are fitted for excavating and working soil mixed with saliva. The leg tip (tarsus) folds back out of the way into a groove on the pick-shaped preceding segment (tibia). The greatly enlarged part (femur) of the front leg bears a comb row for raking and packing, and teeth at the base fit against the tibia like a crab claw.
Burrowing nymphs search for roots. Once a healthy food source is found the grub packs the earth outside its body to form a roomy chamber, and begins feeding. Young cicadas within their soil chambers are safe from most enemies. Fungus kills a few, and pigs may root up nymphs living near the surface. One might suppose that, in such a protected site, they would become inactive parasites of the plants to which they attach. Apparently this is far from true. Using their admirably adapted front legs, they move from place to place, making new chambers and even revolving within their subterranean homes. This activity seems much in excess of what is needed for mere survival and host selection. Whether it expresses joie de vivre or boredom, or has another source, remains problematical.
Nymphal feeding goes on for some time; in fact, for years. Just how long the majority of cicadas need to complete development is unknown. Circumstantial evidence such as cyclical emergence of large numbers of adults indicate 3-6 years as the most common situation, with records of 8, 9 or even 10 years in a few cases, but the shortest life actually recorded in a laboratory setting is 4 years Footnote 1 for the tiny grass-feeding Cicadetta (Melampsalta) calliope, and the longest life cycle is 19 years for Okanagana synodica (reported here for the first time).
The periodical cicadas of the eastern USA appear with great precision every 13 or 17 years, depending on locality. Only small numbers of individuals may be found 1 year before the local brood is expected (or rarely, 1 year after). Strangely, the precision of this system suggests that cicadas have acquired their long lives as multiples of 4 years added to a 1 year life cycle. Thus, life cycles of 5, 9, 13 and 17 years should be the norm, and records indicate that cicadas appearing yearly actually have 9 year cycles for Okanagana rimosa and up to 13 years for Tibicen canicularis (the Dog-day Cicada) in Canada. Such "annual" cicadas are apparently the result of overlapping generations (for example, 5-year cicadas could produce many individuals with 4 and 6 year-long lives). Differences of 4 years (e.g., in 13 and 17-year cicadas) are most probably derived from one another by a four-year shortening of the life cycle Footnote 2. Nymphs of 17-year periodical cicadas grow more slowly during the first four years of life than do 13-year cicadas. This four-year acceleration hypothesis was unexpectedly supported by a massive emergence in suburban Chicago of hundreds of thousands of periodical cicadas in 1969, four years ahead of schedule. A similar four-year acceleration was again observed in that area in 1986. A similar situation was noted in 1983 and again in 1987 in suburban Washington, DC; and likewise in Cincinnati, Ohio in 1983. Such accelerations have been verified from extensive historical records in Indiana, Ohio, and Long Island but have not, as yet, resulted in the establishment of new breeding populations. Genetic evidence also gives evidence of massive brood accelerations Footnote 3, with the northern half of one brood of 13-year cicadas being genetically identical to, and therefore derived from, adjacent 17-year cicada broods by shortening of their life cycle.
Extra long lives caused by inhibition in growth - which is also called a dormancy period - in both the periodical and Dog-day cicadas may be an adaptation to cooler northern temperatures. If so, its loss, leading part of the population to emerge years ahead of schedule, may be a symptom of global warming.
When the time comes for transformation to the adult, nymphs gather near the surface and await the appropriate time of day, temperature or humidity conditions. Some kinds, such as the periodical cicadas, actually construct capped "chimneys" of packed earth cemented by plant juices and lie in wait there. Usually they emerge from the soil at night. Then they climb up trees and bushes, dig in their claws and split the juvenile cuticle down the back. When the adult emerges and flies away, the tough cuticle (perfectly preserving the form of the nymph) remains clinging to its perch. Such cast "skins" gathering in windrows beneath trees are familiar sights where cicadas are common. Periodical cicadas, emerging in incredible numbers in a single night, may leave their casts clinging to everything in sight, including sleeping animals!
Emergence from the ground exposes cicadas to their most dangerous time. Helpless on the tree until their wings are hard enough to sustain flight, they are for some hours completely at the mercy of birds and other animals in search of tasty animal protein. This may account for "swarms" of cicadas. Appearing without warning, and vulnerable for only a few hours, only a small proportion are eaten by predators lucky enough to happen by at the right time. Then the cicadas disperse, or lay their eggs and die within a short time. It will be years before such a feeding opportunity happens again in the same place.
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