The loud and often shrill singing of cicadas – a sound synonymous with summer – makes them one of New Zealand's most familiar insects. Cicadas belong to the insect order Hemiptera, a group with piercing and sucking mouthparts. It also includes insects such as aphids, scales, plant hoppers and spittle bugs.
Cicadas are common throughout the warmer parts of the world, particularly the tropics.
Cicadas have broad, blunt heads with prominent compound eyes, a tapering body, four large membranous wings and six small legs. They should not be confused with those other insect songsters, the grasshoppers, locusts and crickets, which may be identified by their large, jumping, hind legs.
The New Zealand cicada fauna consists of 42 species and subspecies in five genera, although additional species are yet to be formally described. All are unique to New Zealand.
The largest is the chorus cicada (Amphipsalta zelandica), with a wingspan of 80 millimetres, and the smallest are species of Maoricicada, with a wingspan of around 29 millimetres and a body length of 14 millimetres. The most closely related species are found in Australia, Norfolk Island and New Caledonia. Studies show that the New Zealand fauna came about from several invasions across the Tasman Sea from Australia and perhaps New Caledonia. They arrived within the last 11 million years, well after New Zealand became isolated after separating from Australia.
In New Zealand, cicadas are found in a variety of habitats, including tall forest, scrub, grasslands, swamps, riverbeds and sand dunes. Their distribution extends from the coast up to the mountains. Species such as the chorus cicada and the clapping cicada (Amphipsalta cingulata) are often found in towns and cities, where they perch on fences, buildings and lamp posts. In contrast, some Maoricicada species are restricted to rocky mountain tops. They are the only cicadas known that live in the high alpine zone. Other species in the same genus uniquely inhabit stony riverbeds.
The common impression of cicadas as clamorous, singing insects flitting from tree to tree is deceptive. In fact, most of a cicada’s life is spent in darkness, deep underground.
After mating, the female lays her eggs on plant tissues using a sword-like ovipositor. Depending on the species, these may be laid in distinctive herringbone cuts on branches and twigs, or in grass stems where their presence is revealed by tufts of plant fibres.
Wingless cream-coloured nymphs emerge from the eggs, armed with two claw-like forelegs and four smaller legs. They drop to the ground, where they seek out cracks and crevices and burrow into the soil using their powerful, digging forelegs. Cicada nymphs may dig down as deep as one metre, but generally range down to 40 centimetres.
Nymphs feed on sap from plant roots, using their needle-like mouthparts, within which are two tubes. Saliva is pumped down one tube to partially digest sap, which is then sucked up the other. Nymphs may feed at the same site for several days or even weeks before moving to a new location. They excrete large amounts of fluid from their liquid diet, which they use to compact and waterproof their burrows, especially the feeding cells where they spend a long time attached to a root to feed.
As the nymphs grow they shed their skins, usually going through five stages (known as instars). The life span of New Zealand cicadas is not well known, but some species live underground as nymphs for at least three years and probably up to five. In North America, one remarkable species – Magicicada septendecim – lives for 17 years, all but a few weeks of them underground.
As the cicada nymph reaches maturity it burrows upwards, until it is near the surface. When the conditions are suitable, it emerges from the ground under the cover of darkness. Having climbed up a support such as a tree trunk, the nymph then enters its final moult.
In its final moult, the cicada changes from a drab, ground-dwelling nymph to an often colourful, energetic, winged adult. The exoskeleton is entirely shed, including the linings of the breathing tubes, which can often be seen poking out from the cast skin. In areas dense with cicadas, dozens of skins can be seen on tree trunks.
The nymphal skin splits along the back of the thorax and an adult gently emerges and hangs to allow its soft and crumpled wings to be pumped up to their full size. By the next morning the wings are hard enough for the adult to fly away, leaving behind the empty case of the nymph.
Like the immature stages, adults feed on sap. But generally they only live for two to four weeks. During this time they mate and the females lay their eggs.
In New Zealand, enemies of cicadas include tiny wasps that infest cicada eggs, predatory beetles that attack nymphs, and fungal diseases that destroy both nymphs and adults. As they migrate towards the surface, near-mature nymphs can become food for kiwi. Adult cicadas may be taken by birds or caught in spider webs.
The most distinctive feature of cicadas is their ability to sing. During the peak of summer their massed chorus can be deafening. Only the males sing, mainly to court females.
Male cicadas use their tymbals to produce sound. These are ribbed membranes on each side of the base of the abdomen. Each tymbal is attached by a tendon to a powerful muscle. As the muscle contracts it buckles the shape of the tymbal, much as when the domed lid of a jar is first unsealed, causing a burst of sound called a pulse. When the muscle is relaxed the tymbal pops back into shape. Rapid and repeated muscle contraction produces the distinctive cicada call.
The digestive and reproductive organs are reduced in the male, making room for large air sacs in the abdomen that amplify the song. Variations in songs are the result of differences in the sound frequency (which is dependent on the tymbal structure), the sound pulse rate and pattern, and the volume.
Cicada songs vary widely between species, ranging from raucous screeches to more restrained, faint chirps. These are sometimes so distinctive that individual species may be identified by song alone. Some New Zealand cicadas (genus Amphipsalta) also make a sharp clapping or clicking sound by rapidly tapping their wings against the branch on which they are perched.
The widespread chorus cicada is New Zealand’s largest species (wingspans can reach 8 centimetres), and its loudest, with the adults emerging en masse and singing in chorus. Their Māori name is kihikihi wawā – wawā meaning ‘to roar like the sound of heavy rain’.
Sound is received by males and females through a pair of membranous plates at the base of the abdomen called tympana. These are covered by protective plates called opercula, which extend from the base of the thorax. Capsules containing hearing organs are connected to the opercula.
Females are able to detect the direction of sound, and so can track the singing males. Males, in contrast, are only able to detect the closeness of competing males.
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