canterbury’s own lifestyle magazine / a great local read

While New Zealand’s natural history has the potential to make most eyelids droop, the Mt Somers bluff weta has a remarkable story, says Department of Conservation (DOC) invertebrate ecologist Warren Chinn, who has been with DOC’s terrestrial ecosystem unit for the past decade. “We are very lucky to still have them; they really are quite incredible, crazy animals. They are insect dinosaurs.”

Weta are the insect equivalent of the distinctively New Zealand moa, takahe or kakapo. These rugged armour-clad insects represent a past era of animals which were heavier and slower than those of today. A time when New Zealand was home to many large, flightless, ground dwelling animals that had evolved quietly over millions of years with few mammalian predators until humans arrived.

A member of a group of endemic nocturnal insects known as the giant weta Deinacrida elegans (demon grasshoppers), one of five distinct groups of weta found in New Zealand, the Mt Somers bluff weta has endured and thrived here for more than 60 million years, witnessing the rise and fall of many species and the uplift of the islands that make up New Zealand.

Similar types occur in eastern Australia, South Africa and New Caledonia, providing evidence they were once found all over parts of the supercontinent Gondwanaland, which comprises of most of the landmasses in today’s Southern Hemisphere.

And while most of giant weta are now only found on protected islands, the Mt Somers bluff weta’s cryptic and difficult-to-access lifestyle has ensured its survival against the odds, not to mention undoubtedly contributing to its chequered history.

First discovered in Mt Somers in 1957, it was feared extinct until rediscovered in1994 by biologists George Gibbs and Mary Richards. George noticed its uncanny resemblance to one found in the Kaikoura region with similar, if not the same, distinguishing features just six years earlier. Despite its disjunct distribution, it’s since been proven that they are in fact the same species.

Unlike its forest-dwelling relations, the Mt Somers bluff weta is found in narrow fissures on solid, near vertical, rock bluffs between 800-1700m above sea level out of reach of most predators. As well as the rhyolite bluffs around Mt Somers, a small population is also present on the greywacke sandstone ranges in the Seaward and Inland Kaikoura Ranges and the mountains at the head of the Wairau River.

Weighing up to 16 grams (half the size of a mouse) and up to 100mm long (including antennae), the Mt Somers bluff weta is easily identified by its distinct greyish-blue colouration, with striking deep orange, stripy black and white leg joints. “Most of our insects are brown or black and pretty boring, but these are really fantastic. They are very colourful,” says Warren.

Part of its ongoing success story is that Mt Somers bluff weta are cold-blooded and freeze-resilient, meaning they can survive at around 0 degrees and can cope with temperatures down to as low -10 degrees for days at a time. Additionally, they are omnivores, and at times cannibalistic. “They are a jack of all trades when it comes to dinner. It means they can survive anywhere, anytime.”

They are also proficient climbers with slender legs and large tarsal claws. Its natural defence mechanism when disturbed is to project itself off the cliff and tuck its legs up to roll, using its middle legs to stop and run away. They have also been observed “sun-basking” on the warm rocks during the day, he says.

Distinguished by their frightening, protruding, sword-shaped egg layer, female Mt Somers bluff weta lay their broods in soft squidgy soil. First they probe the soil testing for suitability, then they thrust the egg-layer deep beneath their body before releasing several eggs at a time. On average broods are between 18-30. Eggs can hatch in as little as a month.

However, contrary to most insects, Mt Somers bluff weta are semi-metabolous and do not go through a metamorphic lifecycle. “What comes out is a nymph. It’s a huge advantage to them. They start off really small, but they can move out of harm’s way, move around and find food,” explains Warren.

For an insect, the Mt Somers bluff weta enjoys a long lifespan. It takes between 10-12 months to develop from an egg to an adult, and the adult weta may survive up to three years. Growth occurs through a series of moults – seven in this case. Each moult involves splitting their skin, working their way out of it and finally inflating themselves to a bigger size before the outer skin hardens. During this process they are extremely vulnerable, so it occurs under the cover of darkness.

The Mt Somers bluff weta is classified as a “relict” population, but Warren says DOC is concerned about their continued existence going forward. “They are fully protected under the Wildlife Act, but they are currently rated at the lowest level of threatened. We lack a lot of information on them and we’re not confident on the total population count, but in Mt Somers they are stable for now, at least.”

From DOC trapping programmes in the area, he says pests such as possums, hedgehogs, rats, mice and stoats remain their number one threat. Long term though, climbing temperatures could pose a significant problem, they just don’t know yet, which is something they are currently working on. “The rate of climate change during the ice ages was much slower than today’s rate of warming. Only time will tell.”

For now though, if you’re keen to catch a glimpse of these shy, nocturnal creatures head out to the bluffs behind mid-Canterbury’s Woolshed Creek at night from September onwards and you might just be in luck.

[ WORDS Annie Studholme, IMAGES Department of Conservation ]