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.