Giant spiders and other critters of Rangatira Island, Chatham Islands

Giant spiders and other critters of Rangatira Island, Chatham Islands

Rangatira Island is best known as a refuge for rare birds, but it is also home to a spectacular variety of flightless insects, giant spiders, lizards, and seals. Te Papa natural environment curator Colin Miskelly recently spent a month on the island as a volunteer for the Department of Conservation. The team was mainly focussed on the black robin and Chatham petrel recovery programmes, but made time to appreciate some of the lesser-known inhabitants of the island.

Rangatira (South East) Island Nature Reserve is one of the world’s great wildlife sites. Although farmed until the 1960s, it somehow escaped the introduction of rats and cats that destroyed the ecological fabric of the three other large islands in the Chatham Islands group, 800 km east of mainland New Zealand.

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The northern slopes of Rangatira Island, with Pitt Island in the distance at top left. Photograph by Colin Miskelly

Giant spiders

The island is best known internationally for its rare birdlife, but locally it is also known for its spiders. Many arachnophobic Chatham Islanders have declined opportunities to visit the island due to the infamous Rangatira spiders.

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A female Rangatira spider (Dolomedes schauinslandi) poised on a mahoe trunk at night, Rangatira Island, March 2018. Photograph by Colin Miskelly

Rangatira spiders (Dolomedes schauinslandi) are one of New Zealand’s largest spiders (up to 12 cm across), and are over-sized relatives of the familiar nurseryweb spider (Dolomedes minor) of the New Zealand mainland. Like their smaller relative, the female Rangatira spiders make a conspicuous silk nest among foliage, where they guard their young.

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A female Rangatira spider guards its silken nest among Muehlenbeckia and bracken. Rangatira Island, March 2018. Photograph by Colin Miskelly

These impressive large spiders are endemic to the Chatham Islands, but are now known only from three small islands where rats and mice have never invaded. Although never witnessed, it is assumed that mice hunted the spiders to local extinction on Pitt Island (which has never had rats).

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Slightly smaller and more brightly coloured than the female, a male Rangatira spider hunts on a mahoe trunk at night, Rangatira Island, March 2018. Photograph by Colin Miskelly

Rangatira spiders are most readily found at night, when they emerge from their daytime lairs to hunt. Their large, bright eye-shine allows them to be seen from up to 20 metres away in a headlight beam, as they seek their prey on tree trunks and the forest floor.

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A male Rangatira spider snacks on a Novoplectron wētā at night, Rangatira Island, March 2018. Photograph by Colin Miskelly

The favourite meal of Rangatira spiders is wētā, which they catch by pouncing on them. While they build substantial silken nursery webs, Rangatira spiders do not use silk to ensnare their prey.

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A Chatham Island sheetweb spider (Cambridgea annulata) on an akeake (Olearia traversiorum) trunk at night. Rangatira Island, March 2018. Photograph Colin Miskelly

Another large spider on the island that does spin a prey-catching web is the sheetweb spider Cambridgea annulata. Many New Zealanders will be familiar with related sheetweb spiders that are readily seen in mainland forests at night, poised on their characteristic horizontal webs, waiting for prey to fall on the web. Once again, the Chatham Island species is one of the largest members of the genus.

Nocturnal scavengers

Two species of wētā are extraordinarily abundant on Rangatira Island. Both are super-sensitive to light, and so the night-time forest floor is alive with their hopping forms when a torch or headlamp is shone around. Both are endemic to the Chatham Islands, and both are jumping wētā – though the stocky Talitropsis wētā has a body form approaching that of the much large tree wētā on the New Zealand mainland.

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An adult female Talitropsis megatibia wētā, Rangatira Island, March 2018. Photo: Colin Miskelly

The large, flattened hind-tibiae of the Talitropsis wētā form an effective rear-facing shield when they squeeze head-first into their daytime retreats.

Both the Talitropsis wētā and the more slender Novoplectron serratum are omnivores, but get much of their protein by scavenging. Any bird corpses on the forest floor are covered with a seething mass of wētā at night (until a light is shone on them!), and are reduced to skeletons within a couple of nights.

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A male Novoplectron serratum weta, Rangatira Island, March 2018. Photograph by Colin Miskelly

A diversity of large beetles

Some of the many beetle species on Rangatira join the wētā in scavenging bird corpses, including the large flightless carabid ground-beetle Mecodema alternans. The rodent-free status of Rangatira has provided a refuge for large flightless insects that are now rare or absent from Chatham and Pitt Islands.

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A Mecodema alternans carabid beetle. Rangatira Island, March 2018. Photograph by Colin Miskelly

The most abundant large beetle on the forest floor and tree trunks at night is a species of darkling beetle, Mimopeus pascoei – a favoured food of the sheetweb spider. Other rarer species that mainly emerge on damp nights include the Chatham Island stag beetle Geodorcus capito and the Chatham Island click beetle Amychus candezei.

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A Mimopeus pascoei darkling beetle. Rangatira Island, March 2018. Photograph by Colin Miskelly

The male stag beetles have impressive large mandibles, used to fight other males for access to mates.

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A male Geodorcus capito stag beetle. Rangatira Island, March 2018. Photograph by Colin Miskelly

The click beetle belies its name – it is a ‘clickless’ click beetle. There are three species in the genus Amychus, each of which is found on islands in different parts of New Zealand (the two other species are found on islands in the outer Marlborough sounds and on the Three Kings Islands).

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An Amychus candezei click beetle. Rangatira Island, March 2018. Photograph by Colin Miskelly

Perhaps the most intriguing of the large beetles on Rangatira are the Hadramphus weevils found on soft speargrass (coxella, Aciphylla dieffenbachii) and hoho (Chatham Island lancewood, Pseudopanax chathamicus). Their complex story will be the subject of the next blog in this series.

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Two Hadramphus weevils on a soft speargrass stem at night. Rangatira Island, March 2018. Photograph by Colin Miskelly

These last three beetle species (the stag beetle, click beetle, and Hadramphus weevil) are all protected under the Wildlife Act 1953 – some of the few New Zealand insects to receive this level of protection.

Endemic or not?

One of the rarer inhabitants of Rangatira (or at least rarely seen) is the giant stick insect Argosarchus horridus – the longest New Zealand insect. Until recently considered an endemic Chatham Island species (as Argosarchus schauinslandi), the stick insect is one of many species collected by Professor Hugo Schauinsland, the director of the Bremen Übersee-Museum when he visited the Chatham Islands in 1897. Recent genetic and morphological comparisons have resulted in the Chatham Island stick insects being included in the widespread New Zealand species Argosarchus horridus.

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Giant stick insect Argosarchus horridus. Rangatira Island, March 2018. Photograph by Colin Miskelly

Another widespread native insect more commonly seen on Rangatira is the magpie moth Nyctemera annulata. This day-active moth feeds on species of Senecio and other closely related daisy species, including introduced ragwort and garden cinerarias. Within the Chatham Islands, the woolly-bear caterpillar larvae of the magpie-moth are mainly found on the endemic Chatham Island groundsel Senecio radiolatus subsp. radiolatus.

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Magpie moth Nyctemera annulata on Chatham Island groundsel Senecio radiolatus subsp. radiolatus. Rangatira Island, March 2018. Photograph by Colin Miskelly
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Woolly-bear caterpillar of the magpie-moth Nyctemera annulata on Chatham Island groundsel Senecio radiolatus subsp. radiolatus. Rangatira Island, March 2018. Photograph by Colin Miskelly

A skink and two fur seals

The only lizard known from the Chatham Islands is the Chatham Island skink Oligosoma nigriplantare. Found only on islands lacking rats (i.e. anywhere apart from the main island!), these medium-sized skinks are variable in colour, and are active day or night during warm, fine weather.

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Chatham Island skink Oligosoma nigriplantare. Rangatira Island, March 2018. Photograph by Colin Miskelly

Rangatira Island is a major breeding site for New Zealand fur seals Arctocephalus forsteri, with close to 1,000 pups produced annually along the south coast.

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New Zealand fur seals Arctocephalus forsteri on the south coast of Rangatira Island (note one animal showing wounds from a close encounter with a great white shark). Photograph by Colin Miskelly

Among the more familiar New Zealand fur seals, we saw two bull subantarctic fur seals Arctocephalus tropicalis – rare visitors from the southern Indian Ocean or South Atlantic Ocean. In addition to their pale faces and bellies, short snouts, and distinctive top-knots, subantarctic fur seals have a deep dog-like bark, quite different from the huffing and whinnying of New Zealand fur seals.

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Subantarctic fur seal Arctocephalus tropicalis. Rangatira Island, March 2018. Photograph by Colin Miskelly

With many thanks to the Department of Conservation for the opportunity to assist with Chatham threatened bird recovery programmes and to visit Rangatira Island Nature Reserve.

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14 Comments

  1. Very special place which will always be in my thoughts. And never forget the huge spider crawling up my crutch while my colleagues were too afraid to tell me!!

    1. Colin Miskelly
      Author

      Thanks very much for sharing this encounter Christine. I bet the spider has grown with the telling!
      Ngā mihi
      Colin

  2. Again, a fascinating easily-read account of your visit to Rangitira Island. Thanks Colin.

    1. Colin Miskelly
      Author

      Thank you very much for your comments Ian.
      Ngā mihi
      Colin

  3. Really excellent to see those photos of endemic Chatham Islands taxa. In scientific terms the interest of these taxa is not just their spectacular variety, but in the nature of their phylogenetic relationships with New Zealand and other regions.

    A number of studies of taxa with Chatham endemics have misrepresented molecular divergence dates as actual or maximal to claim that these endemics are the result of waifs or strays that someone made their way to the islands, but molecular dates are only minimal estimates and cannot preclude earlier origins. In addition, a number of endemics do not nest within a larger widespread New Zealand clade as would be expected of such derivation. The totality of phylogenetic and geographic evidence supports the origin of Chatham Island endemics as fragments of a formerly widespread biota that included New Zealand and other currently isolated areas. Hopefully future TePapa studies will highlight this outstanding, and spectacular, biogeographic correlation.

    1. Colin Miskelly
      Author

      Thanks very much for your comments John.
      You are correct that individual divergence times reveal only the time of most recent genetic contact, rather than the timing of initial physical separation of populations. But it is these accumulated individual data points that reveal likely geological processes. It is well known that the basement rocks of the Chathams are ancient, but the recurring theme of recent phylogenetic studies of the Chathams biota is an incremental loss of claimed deep endemism, pointing to a history of recent submergence and re-emergence of the land surface. Within birds, several former endemic genera are now known to be nested within recent radiations of fast-evolving rails and waterfowl. And plants and terrestrial invertebrates show strikingly similar patterns. There are still some intriguing mysteries out on the Chathams, including the origin of the striking Chatham Island forget-me-not (Myosotidium). However, the overwhelming pattern within the Chatham Islands terrestrial biota is of recent colonisation and genetic isolation from a mainland New Zealand ancestor.
      Ngā mihi
      Colin

  4. I’m loving your stay on Rangatira, and look forward to the next tale

    1. Colin Miskelly
      Author

      Thanks very much Maureen
      Ngā mihi
      Colin

  5. As always, a fascinating read with superb images. I always appreciate the focus on the incredible invertebrates in these places that you visit. Looking forward to news on Hadramphus!

    1. Colin Miskelly
      Author

      Kia ora Mark
      Thanks very much for your feedback – I figured you would be eager for the latest weevil news!
      Kind regards
      Colin

  6. Everything you see is absolutely fascinating, Colin, and is brought vividly to life by your excellent writing. Thank you.

    1. I totally agree, thanks.

    2. Colin Miskelly
      Author

      Thanks very much for your kind feedback Alison, Olwen and Denis.
      Ngā mihi
      Colin

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