periodical cicada

Periodical Cicada
Magicicada sp. from Brood XIII, 2007
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hemiptera
Family:	Cicadidae
Genus:	Magicicada
Species
See text.

Magicicada is the genus of the 13- and 17-year periodical cicadas of eastern North America. These insects display a combination of long life cycles, periodicity, and mass emergences. They are sometimes called "seventeen-year locusts", but they are not locusts; locusts belong to the order Orthoptera.^[1]

Taxonomy

There are seven recognized species. Three species have a 17-year cycle:

• Magicicada cassini (Linnaeus, 1758)
• Magicicada septendecim (Fisher)
• Magicicada septendecula (Alexander and Moore, 1962)

Four more species follow a 13-year cycle:

• Magicicada neotredecim (Marshall and Cooley, 2000)
• Magicicada tredecim (Walsh and Riley, 1868)
• Magicicada tredecassini (Alexander and Moore, 1962)
• Magicicada tredecula (Alexander and Moore, 1962)

Generally, the 17-year cicadas are distributed more in the northern states of the eastern United States, while the 13-year cicadas occur in the southern states.

Description

Nearly all cicadas spend multiple years growing underground as juveniles, before emerging above ground for a short adult stage of several weeks to a few months. The seven periodical cicada species are so named because, in any one location, all of the members of the population are developmentally synchronized—they emerge as adults all at once in the same year. This periodicity is especially amazing because their life cycles are so extremely long—13 or 17 years. Cicadas of all other species (perhaps 3000 worldwide) are not synchronized, so some adults mature each summer and emerge while the rest of the population continues to develop underground. Many people refer to these non-periodical species as annual cicadas since some are seen in every summer. The life cycles of most annual species range from two to ten years, although some could be longer.

Periodical cicadas are average-sized for cicadas, but they are slightly smaller than the annual cicada species found in the same regions of the United States. Imagines (or adults) have a size of 2.5 to 3 cm (1.0 to 1.2 in). They are black, with red eyes and yellow or orange stripes on the underside. The wings are translucent and have orange veins.

They are harmless insects; they neither bite nor sting. They are not venomous, and there is no evidence that they transmit diseases. They generally do not pose a threat to vegetation, but young plants may be damaged by excessive feeding or egg laying. It is thus advised not to plant new trees or shrubs just before an emergence of the periodical cicadas. Mature plants usually do not suffer lasting damage even from a mass emergence.

Broods

Magicicadas (both teneral and fully developed). Photo by Arthur D. Guilani

Magicicada septendecim

Cicada prior to final molt

Newly molted Brood XIII

Periodical cicadas are grouped into 30 broods, based on the year they emerge. Broods are numbered using Roman numerals; broods I–XVII are the seventeen-year cicadas, while Broods XVIII–XXX are the thirteen-year cicadas. Some broods are known not to exist, but they are retained in the numbering scheme for convenience. This scheme was put forth by C.L. Marlatt in his classic study of 1907. Since then the actual number of broods has been recognized as 15 rather than 30.^[2]

Life cycle

Transformation of the periodical cicada from the mature nymph to the adult

The nymphs of the periodical cicadas live underground, often at depths of 30 cm (1 ft) or more, feeding on the juices of plant roots.^[4] They stay immobile and go through five development stages before constructing an exit tunnel in the spring of their 13th or 17th year. These exit tunnels have a diameter of about 1–1.5 cm (0.4–0.6 in).

Emergence holes

Magicicada in final molting stage prior to hardening of exoskeleton.

The nymphs emerge on a Spring evening when the soil temperature at about 20 cm (8 in) depth is above 17 °C (63 °F). In most years, this works out to late April or early May in far southern states, and late May to early June in the far northern states. Emerging nymphs climb to a suitable place on the nearby vegetation to complete their transformation into an adult cicada. They molt one last time and then spend about six days in the leaves waiting for their exoskeleton to harden completely. Just after this final molt, the teneral adults are white, but darken within an hour.

The nymphs emerge in large numbers at about the same time, sometimes more than 1.5 million individuals per acre (>370/m²).^[5] Their mass-emergence is a survival trait called predator satiation: for the first week after emergence, the periodic cicadas are an easy prey for reptiles, birds, squirrels, cats, and other small and large mammals.^[6][7] The cicadas' survival mechanism is simply to overwhelm predators by their sheer numbers, ensuring the survival of most of the individuals. It has been hypothesized that the emergence period of large prime numbers (13 and 17 years) is also a predatory avoidance strategy adopted to eliminate the possibility of potential predators receiving periodic population boosts by synchronizing their own generations to divisors of the cicada emergence period.^[8] The length of the cycle appears to be controlled by a single gene locus, with the 13-year cycle dominant to the 17-year one.^[9]

Magicicada molting

Adult periodical cicadas live only for a few weeks—by mid-July, all have disappeared. Their short adult life has one sole purpose: reproduction. The males "sing" a mating song; like other cicadas, they produce loud sounds using their tymbals. Receptive females respond to the calls of conspecific males with timed wing-flicks, which attract the males for mating. The sounds of a "chorus"—a group of males—can be deafening and reach 100 dB.

Magicicada egg slits (circled in red)

Both males and females can mate multiple times, although most females seem to mate just once. After mating, the female cuts V-shaped slits in the bark of young twigs and lays approximately 20 eggs in each, for a total of 600 or more eggs. After about six to ten weeks, the eggs hatch and the newborn nymphs drop to the ground, where they burrow and begin another 13 or 17-year cycle. Uneaten carcasses of periodic cicadas decompose on the ground, providing a resource pulse of nutrients to the forest community.^[10]

Cycles in cicada populations are significant enough to impact other animal and plant populations. For example, tree growth has been observed to decline the year before the emergence of a brood because of the increased feeding on roots by nymphs.^[10] Moles, which feed on nymphs, have been observed to do well during the year before an emergence, but suffer population declines the following year because of the reduced food source. ^[11] Wild turkey populations respond favorably to increased nutrition in their food supply from gorging on cicada adults on the ground at the end of their life cycle. However, all animal responses are not positive—squirrel populations have been decimated because the egg laying activity of female cicada damaged upcoming mast crops.

Media

Wikimedia Commons has media related to: Magicicada

Hundreds of cicadas, Brood X

References

External links

• Cicada Mania
• Massachusetts Cicadas: Brood XIV Magicicadas in Massachusetts in 2008
• Magicicada Mapping Project Homepage
• Comprehensive website with reprints, distribution records, databases, and maps at Cicada Central.
• Comprehensive web site University of Michigan
• Bibliography
• Link list
• Image gallery
• Roar of the Cicada