It is difficult to account for New Zealand’s unique insect life. There are three possible scenarios:
- The ancestors of today’s insects were present when the New Zealand continent broke away from the supercontinent Gondwana, about 85 million years ago.
- Insects dispersed across the ocean to reach New Zealand after the country became isolated.
- Insect groups once present in New Zealand have become extinct.
Virtually no insect fossils have been found in New Zealand, so it is often a matter of speculation when and how insects arrived. However, DNA research is providing a better picture of the history of certain insect types. Some are claimed as ‘founding’ members of the New Zealand fauna, where the evidence indicates that they were around when New Zealand drifted away from Gondwana. Wētā are one of these groups, and the DNA of a worldwide family of primitive, pollen-feeding moth (Micropterigidae) suggests that they were also in New Zealand when it split from the supercontinent.
Some of the insects that live near water in New Zealand have relatives elsewhere. Cool, clear New Zealand streams and rivers are the habitat of about 350 species of aquatic insect larvae, all of which eventually become winged adults. They include mayflies, stoneflies and caddisflies, and the sandfly Austrosimulium, midge larvae (Chironomidae family), some water beetles, a scorpionfly, Nannochorista and the alderfly Archichauliodes diversus, which has ferocious-looking larvae known as ‘toe biters’. They are the lifeblood of the rivers, providing important food for waterfowl like blue duck, and for native and introduced fish. These insects have their nearest relatives around the southern hemisphere, from Chile to eastern Australia and Tasmania. It is thought that they were formerly part of a freshwater fauna from an area that included Antarctica in the days before New Zealand became isolated and Antarctica became ice-bound.
Many New Zealand insects have their nearest relatives in Australia. There is no certainty when they came, but there can be no doubt that dispersal across the Tasman Sea accounts for a significant number of arrivals.