Periodical cicadas
The term periodical cicada is commonly used to refer to any of the seven species of the genus Magicicada of eastern North America, the 13- and 17-year cicadas. They are called periodical because nearly all individuals in a local population are developmentally synchronized and emerge in the same year. Although they are sometimes called "locusts", this is a misnomer, as cicadas belong to the taxonomic order Hemiptera, suborder Auchenorrhyncha, while locusts are grasshoppers belonging to the order Orthoptera. Magicicada belongs to the cicada tribe Lamotialnini, a group of genera with representatives in Australia, Africa, and Asia, as well as the Americas.
Magicicada species spend around 99.5% of their lives underground in an immature state called a nymph. While underground, the nymphs feed on xylem fluids from the roots of broadleaf forest trees in the eastern United States. In the spring of their 13th or 17th year, mature cicada nymphs emerge between late April and early June, synchronously and in tremendous numbers. The adults are active for only about four to six weeks after the unusually prolonged developmental phase.
The males aggregate in chorus centers and call there to attract mates. Mated females lay eggs in the stems of woody plants. Within two months of the original emergence, the life cycle is complete and the adult cicadas die. Later in that same summer, the eggs hatch and the new nymphs burrow underground to develop for the next 13 or 17 years.
Periodical emergences are also reported for the "World Cup cicada" Chremistica ribhoi in northeast India and for a cicada species from Fiji, Raiateana knowlesi.
Description
The winged imago periodical cicada has two red compound eyes, three small ocelli, and a black dorsal thorax. The wings are translucent with orange veins. The underside of the abdomen may be black, orange, or striped with orange and black, depending on the species.Adults are typically, depending on species, generally about 75% the size of most of the annual cicada species found in the same region. Mature females are slightly larger than males.
Magicicada males typically form large aggregations that sing in chorus to attract receptive females. Different species have different characteristic calling songs. The call of decim periodical cicadas is said to resemble someone calling "weeeee-whoa" or "Pharaoh". The cassini and decula periodical cicadas have songs that intersperse buzzing and ticking sounds.
Cicadas cannot sting and do not normally bite. Like other Auchenorrhyncha bugs, they have mouthparts used to pierce plants and suck their sap. These mouthparts are used during the nymph stage to tap underground roots for water, minerals and carbohydrates and in the adult stage to acquire nutrients and water from plant stems. An adult cicada's proboscis can pierce human skin when it is handled, which is painful but in no other way harmful. Cicadas are neither venomous nor poisonous and there is no evidence that they or their bites can transmit diseases.
Oviposition by female periodical cicadas damages pencil-sized twigs of woody vegetation. Mature trees rarely suffer lasting damage, although peripheral twig die-off or "flagging" may result. Planting young trees or shrubs is best postponed until after an expected emergence of the periodical cicadas. Existing young trees or shrubs can be covered with cheesecloth or other mesh netting with holes that are in diameter or smaller to prevent damage during the oviposition period, which begins about a week after the first adults emerge and lasts until all females have died.
Life cycle
Nearly all cicadas spend years 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 members of the population are developmentally synchronized—they emerge as adults all at once in the same year. This periodicity is especially remarkable because their life cycles are so long—13 or 17 years.In contrast, for nonperiodical species, some adults mature each summer and emerge while the rest of the population continues to develop underground. Many people refer to these nonperiodical species as annual cicadas because some are seen every summer. This may lead some to conclude that the non-periodic cicadas have life cycles of 1 year. This is incorrect. The few known life cycles of "annual" species range from two to 10 years, although some could be longer.
The nymphs of the periodical cicadas live underground, usually within of the surface, feeding on the juices of plant roots. The nymphs of the periodical cicada undergo five instar stages in their development underground. The difference in the 13- and 17-year life cycle is said to be the time needed for the second instar to mature. When underground the nymphs move deeper below ground, detecting and then feeding on larger roots as they mature.
The nymphs seem to track the number of years by detecting the changes in the xylem caused by abscission of the tree. This was supported experimentally by inducing a grove of trees to go through two cycles of losing and re-growing leaves in one calendar year. Cicadas feeding on those trees emerged after 16 years instead of 17.
In late April to early June of the emergence year, mature fifth-instar nymphs construct tunnels to the surface and wait for the soil temperature to reach a critical value. In some situations, nymphs extend mud turrets up to several inches above the soil surface. The function of these turrets is not known, but the phenomenon has been observed in some nonperiodical cicadas, as well as other tunneling insects.
The nymphs first emerge on a spring evening when the soil temperature at around of depth is above. The crepuscular emergence is thought to be related to the fact that maximum soil temperatures lag behind maximum insolation by several hours, conveniently providing some protection for the flightless nymphs against diurnal sight predators such as birds. For the rest of their lives the mature periodical cicadas will be strongly diurnal, with song often nearly ceasing at night.
During most years in the United States this emergence cue translates to late April or early May in the far south, and late May to early June in the far north. Emerging nymphs may molt in the grass or climb from a few centimeters to more than 100 feet to find a suitable vertical surface to complete their transformation into adults. After securing themselves to tree trunks, the walls of buildings, telephone poles, fenceposts, hanging foliage, and even stationary automobile tires, the nymphs undergo a final molt and then spend about six days in the trees to await the complete hardening of their wings and exoskeletons. Just after emerging from this final molt the teneral adults are off-white, but darken within an hour.
Adult periodical cicadas live for only a few weeks; by mid-July, all have died. Their ephemeral adult forms are adapted for one purpose: reproduction. Like other cicadas the males produce a very loud species-specific mating song using their tymbals. Singing males of the same Magicicada species tend to form aggregations called choruses whose collective songs are attractive to females. Males in these choruses alternate bouts of singing with short flights from tree to tree in search of receptive females. Most matings occur in so-called chorus trees.
Receptive females respond to the calls of conspecific males with timed wing-flicks which attract the males for mating. The sound of a chorus can be literally deafening and depending on the number of males composing it, may reach 100 dB in the immediate vicinity. In addition to their "calling" or "congregating" songs, males produce a distinctive courtship song when approaching an individual female.
Both males and females can mate multiple times, although most females seem to mate only once. After mating, the female cuts V-shaped slits in the bark of young twigs and lays about 20 eggs in each, for a total clutch of 600 or more. After about 6–10 weeks, the eggs hatch and the nymphs drop to the ground, where they burrow and begin another 13- or 17-year cycle.
Predator satiation survival strategy
The nymphs emerge in very large numbers at nearly the same time, sometimes more than 1.5 million individuals per acre. Their mass emergence is, among other things, an adaptation called predator satiation. Although periodical cicadas are easy prey for reptiles, birds, squirrels, cats, dogs and other small and large mammals, there are after synchronized emergence simply too many individuals for the predators to consume; many individuals thus remain behind to procreate.It has been hypothesized that the prime-number development times improve avoidance of predators with shorter reproductive cycles and for this reason have been selected for. A predator with, for example, a three-year reproductive cycle, which happened to benefit from a brood emergence in a given year, will have gone through either four cycles plus one year or five cycles plus two years by the next time that the same brood emerges. In this way cicada generations always emerge when some portion of the predators they will confront are sexually immature and therefore incapable of taking maximum advantage of the momentarily limitless food supply.
A second hypothesis posits that the prime-numbered developmental times are an adaptation that prevents hybridization between broods. Under extremely harsh conditions, mutations producing extremely long development times are selected for. A mechanism, such as reproducing only after prime-numbered intervals, that reduces the frequency of cicadas mating with cicadas that may lack the long-development trait will also be selected for. The North American Pleistocene glacial stadia are instances of such extremely harsh conditions. On this hypothesis, predator satiation reinforces a longer-term survival strategy of protecting the long-development trait from hybridizations that might dilute it. This hypothesis has been supported by mathematical modeling.
The length of the cycle was hypothesized to be controlled by a single gene locus, with the 13-year cycle dominant to the 17-year one, but this interpretation remains controversial and unsubstantiated at the level of DNA.