Cicada life cycle explained
Sylvia Clarkson, of Careys Bay, asked:
We discovered a few of these bugs, up to 2cm long, in holes in clods of soil when digging in the garden. It was very slow moving. What is it?
Jenny Jandt, a zoologist at the University of Otago, responded:
The insect you have uncovered looks to be a cicada nymph. Cicada nymphs (juveniles) live underground (about 40cm deep), and feed on sap from plant roots.
You can distinguish them because the front pair of legs are much larger than the second and third pair of legs. This is because the nymphs use these legs to manoeuver underground and to dig themselves out from underground after they’ve finished growing.
New Zealand cicadas spend about three years underground. (One North American species lives underground for up to 17 years.) After they emerge from underground at night, they will climb a tree or horizontal structure, where they will molt (shed their exoskeleton) for the last time (see https://youtu.be/ keJpOn1dBc). By morning the wings are hard enough to allow the adult to fly away, leaving behind the exoskeleton still attached to the tree.
Cicada nymphs (and adults) are slow movers. Nymphs spend their whole lives underground, so there is no need for them to be fast. Cicada adults are big and bulky as well.
The adults also feed on sap, but generally they only live about two weeks, during which they mate and the females lay their eggs in plants. On hatching the nymphs drop to the ground and burrow into the soil, and thus the cycle starts over again.
Send questions to: AskAScientist, PO Box 31035, Christchurch 8444 Or email
questions@askascientist.net