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Bugs That Count

Brian Hayes

A Clockwork Insect

Cicadas spend almost all of their lives underground, as "nymphs" feeding on xylem sucked out of tree roots; they come to the surface only to mate. Most species have a life cycle lasting a few years, but individuals are not synchronized; all age groups are present at all times, and each year a fraction of the population emerges to breed. True periodicity, where an entire population moves through the various stages of life in synchrony, is extremely rare. Of 1,500 cicada species worldwide, only a handful in the genus Magicicada are known to be periodical; all of them live in North America east of the Great Plains.

Figure 1. Synchronization gradually decays...Click to Enlarge Image

The taxonomy of the Magicicada group is somewhat controversial and more than a little confusing. If you sort a collection of specimens by appearance or mating call or molecular markers, they fall into three sets; but it turns out that each of these sets includes both 13–year and 17–year forms. Are there six species, or only three? Complicating matters further, John R. Cooley, David C. Marshall and Chris Simon of the University of Connecticut have recently identified a seventh variety that has a 13–year period but shows genetic affinities to a 17–year group.

Then there is the division into broods. A brood is a synchronized population, in which all individuals are the same age. Generation after generation, they go through life in lockstep. One might expect that a brood would consist of a single species, but that's generally not the case. Brood X, for example, includes the 17–year forms of all three species. Geographically, adjacent broods tend to have sharp boundaries, with little overlap. Where two broods do share the same real estate, they are chronologically isolated, typically with four years between their emergences.

If you are a cicada trying to emerge in synchrony with all your broodmates, there are two problems you need to solve: First you must choose the right year, and then the right day (or night, rather) within that year. The latter task is easier. Cicadas synchronize the night of their emergence by waiting for an external cue: They crawl out of their burrows when the soil warms to a certain temperature, about 64 degrees Fahrenheit.

Keeping track of the years is more challenging. First you need an oscillator of some kind—a device that goes tick–tick–tick at a steady pace. The cicada oscillator presumably ticks once per year. Second, you need to tally the successive ticks, like a prisoner scratching marks on the wall of a cell. Finally you have to recognize when the tick count has reached the target value of 13 or 17.

The cicada oscillator is probably an annual variation in some property of the xylem the insects consume, reflecting a deciduous tree's yearly cycle of growing and shedding leaves. Support for this hypothesis comes from an ingenious experiment conducted by Richard Karban, Carrie A. Black and Steven A. Weinbaum of the University of California, Davis. They reared cicadas on orchard trees that can be forced to go through two foliage cycles in a single year. Most of the cicadas matured after 17 of the artificially induced cycles, regardless of calendar time.

The cicada's tally mechanism remains unknown. One example of a biological counting device is the telomere, a distinctive segment of DNA found near the tips of chromosomes in eukaryotic cells. Each time a cell divides, a bit of the telomere is snipped off; when there's none left, the cell ceases to replicate. Thus the telomere counts generations and brings the cell line to an end after a predetermined number of divisions. Perhaps the cicada employs some conceptually similar countdown mechanism, although the biochemical details are surely different.

There is no reason to suppose that cicadas count strictly by ones. Indeed, the coexistence of 13–year and 17–year periods suggests other possibilities. For example, the two life cycles might be broken down as (3x4)+1=13 and (4x4)+1=17. In other words, there might be a four–year subcycle, which could be repeated either three or four times, followed by a single additional year. An appealing idea is to identify such subcycles with the stages, or instars, in the development of the juvenile cicada. And it's notable that what distinguishes 17–year from 13–year forms is a four–year prolongation of the second instar. Unfortunately for the hypothesis, the rest of the nymphal stages are not uniform, four–year subcycles. Nymphs pass through them at different paces. Only at the end of the cycle do  the members of a brood get back in synch.

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