Beetles belong to the insect order Coleoptera. This name comes from the Greek ‘koleos’ for sheath and ‘pteron’ for wing, and refers to their hardened front wings – called elytra. The elytra meet along the midline of the body, enclosing and protecting the abdomen, and giving beetles their characteristic appearance.
The elytra, an armoured exoskeleton and a compact body allow beetles to live where other insects with more delicate or exposed wings would be damaged – when burrowing in leaf litter or soil, for example. These features also protect beetles from attacks. The elytra help prevent water loss by covering the openings to the breathing tubes on the abdomen.
New Zealand’s longest beetle is the giraffe weevil, also known by Māori as pepeke nguturoa or tūwhaipapa (Lasiorhynchus barbicornis). Males can grow up to 9 centimetres long, in part because of their very elongated head.
The huhu beetle (Prionoplus reticularus), its relative the spiny longhorn (Blosyropus spinosus), and the sand scarab (Pericoptus truncatus) are among the largest.
The tiny feather-winged beetles and the ant-like stone beetles, with species about 0.5 millimetres long, are among New Zealand’s smallest.
Beetles come in a myriad of shapes. Ground beetles and longhorns are cylindrical, while scarab beetles are squat. Diving beetles have streamlined bodies, perfectly adapted to the water, while weevils have comical, snout-like heads. Miniscule clam beetles are almost spherical, while bark mould beetles (Diagrypnodes wakefieldi) are paper thin, to fit beneath loose bark.
Some beetles have characteristic features, like the enlarged mandibles of male Helms’s stag beetles (Geodorcus helmsi), and the shield-like head of male fungus weevils (Hoherius meinertzhageni).
Despite their hardened forewings, called elytra, beetles can fly – often rather clumsily. Unlike most other insects, beetles propel themselves through the air using their membranous hind wings. These reach beyond the elytra when extended for flight, but a complex folding system – a bit like origami – means they can fit under the elytra when at rest. Flightless beetles have reduced hind wings, and in some cases the elytra are fused.
Adult beetles lay eggs, which hatch into larvae. The larvae often live within their food source. Typically they bear no resemblance to their adult form. They have a toughened head capsule and chewing mouthparts, with a body that may be active and hard with sturdy legs, or soft and legless, protected by surrounding plant tissue or soil.
Larvae pass through several stages, called instars, before becoming an inactive pupa. As a pupa, the entire body is reorganised, until it finally emerges as a beetle. Development can take from a few weeks in some species to several years, especially if food is scarce. The adult lifespan is also variable, ranging from days, to months or years for a few larger species.
Some 350,000 beetle species have been named worldwide, making them the largest group of insects. Beetles account for more than 40% of known insect species, and about 30% of all animals. Estimates suggest that as many species again – perhaps more – are yet to be discovered.
New Zealand’s beetle life is diverse – there are more than 4,500 named native species. By way of comparison, this is more than all mammal species worldwide, or all New Zealand plant, fish, amphibian, reptile, bird and mammal species combined. The actual number of New Zealand beetle species is likely to be between 6,000 and 10,000.
About half of the world’s 160 or so beetle families occur in New Zealand, and most of the major families are present. Most biodiversity is within a few families – there are 11 families with over 100 species. The most diverse family is Curculionidae or weevils, with about 1,300 species – nearly one-third of the beetle fauna. This is disproportionately high – three times higher than in the UK, for example.
Other large families in New Zealand are (with approximate species numbers in brackets):
Several families are represented by only one or a few species, such as Cucujidae or bark beetles, whose sole representative, Platisus zelandicus, is confined to the remote Three Kings Islands. The Trogidae or carcass beetles have only one species in New Zealand, accidentally introduced.
Some species are widely distributed, while many are specific to certain regions. Offshore islands, such as the Three Kings, Chatham and subantarctic groups, each have a characteristic beetle fauna.
Some groups show unusual distributions. The family Chaetosomatidae was originally thought to be unique to New Zealand, until new species were discovered on Madagascar. Carrion beetles are confined to the northern hemisphere except for two New Zealand species of the genus Zeanecrophilus.
About 90% of New Zealand beetles are endemic – they are found nowhere else. Two entire families – Cyclaxyridae and Metaxinidae – are endemic. Such a high proportion of endemism is probably the result of New Zealand’s isolation, and its changing climate and geology, which has promoted the evolution of new species.
Captain Thomas Broun (1838–1919) was New Zealand’s most prolific beetle taxonomist – he described and named 3,538 new species. The quality of his work, however, did not always match his output. His descriptions were often basic, and they were rarely illustrated. Many species were described several times over – the ground beetle Megadromus meritus was given 14 different names. Controversially, Broun bequeathed his main collection to the British Museum in London.
Until the 1990s it was thought that New Zealand beetles were an ancient lineage, and had become isolated when New Zealand split and drifted away from the Gondwana supercontinent, around 85 million years ago. Others had arrived by dispersing across the ocean, or had been introduced through human activities. But research now suggests that New Zealand was largely submerged around 35 million years ago, and many plants and animals have arrived since then.
New Zealand’s beetles, like many of its plants and animals, are most closely related to species in Australia and South America.
Saprophagous species – those that feed on dead plants or animals – live in leaf litter, rotting logs and animal carcasses. They include members of the genus Saphobius or dung beetles, which eat decomposing vegetation, and the tiny, beach-dwelling Phycosecis limbata, which scavenges washed-up carrion.
Numerous species are fungus feeders, many of which are found in the forest – like the tiny, spore-feeding mildew beetles, and members of the Ciidae family. Pinhole borers (also known as ambrosia beetles) are weevils that grow fungus in galleries which they form by boring into wood. They inoculate the wood with spores they carry in specialised cavities. Both adults and larvae eat the fungi.
Predatory beetles include most species of ground beetles. Among the most voracious of these are the tiger beetles, such as the boldly-patterned common tiger beetle or pāpapa (Cicindela tuberculata), often seen on clay banks. The adults are fast runners, darting about to catch prey in their massive, sickle-shaped mandibles. The larvae, in contrast, sit anchored to their burrows – heads near the entrance, jaws open – waiting to seize any prey that blunder past.
Other predators are the rove beetles, identified by their shortened elytra (hardened forewings) and flexible abdomens.
A few of New Zealand’s beetles are parasitic, and in New Zealand two families – Bothrideridae and Rhipiphoridae – have species that parasitise wood-boring beetle larvae.
One of New Zealand’s most intensive beetle surveys was made in the Auckland suburb of Lynfield. Fifteen years of collecting from the beaches, back yards and bush yielded 982 species, 753 of which were unique to New Zealand. The study showed that most native beetles kept to remnants of native bush, and very few had adapted to urban areas.
The majority of beetles are herbivores – they only eat plants. They may eat leaves, stems or roots, or they may bore into wood. Dominant among the herbivores are the weevils, longhorns and leaf beetles.
Because they are under constant threat of attack, beetles have many ways to avoid capture. Ground beetles use chemical defences, and rove beetles and darkling beetles secrete foul smells to ward off predators. Some species are camouflaged, such as Pristoderus bakewelli, whose knobbly upper surface blends in with the surrounding bark. Others rely on mimicry. The longhorn beetle Drototelus elegans imitates the bold warning coloration of the parasitic wasp Xanthocryptus novozealandicus.
Beetle behaviour is poorly understood, although it is known that complex relationships exist. Some beetles can be social, such as ground beetles of the genus Megadromus, some of which tend their young. Stag beetles (Holloceratognathus passaliformis) have been found in native ant nests, but the nature of the relationship is not known. In other countries, many beetle species infiltrate ant colonies and prey on the ants.
Beetles can be found throughout New Zealand’s mainland and offshore islands, in virtually all habitats.
Few species are true marine-dwellers, but several inhabit rocky shores or sandy beaches – for example, plump, C-shaped sand scarab larvae live under driftwood.
A small group of beetles are aquatic. Pond dwellers include the predatory diving beetles. Algae-grazing riffle beetles and cascade beetles are found in fast-flowing streams. Semi-aquatic species of ground beetles and mud beetles live in sand, stones and mud at stream edges.
Grassland and shrubland beetles tend to be reclusive. Among them are the iridescent blue or orange flower longhorns, mānuka beetles or kekerewai (Pyronota festiva), and pintail beetles, which may be seen feeding on flowers. The whirr of chafer beetles can be heard as they fly about at dusk.
The forest is a beetle stronghold – the greatest diversity occurs there. The soil is home to root-feeding larvae of mumu and tanguru chafers (Stethaspis longicornis and S. suturalis) – the bright-green adults emerge en masse in summer. Leaf litter hosts a multitude of species, most of them tiny. Predatory ground beetles scuttle across the forest floor at night, and include species of the genera Mecodema and Megadromus, which reach 4 centimetres in length.
Log-dwellers include wood borers like elephant weevils (Rhyncodes ursus) and metallic-tinged jewel beetles (Nascioides enysi). Fungus-feeders and predators such as click beetles of the genus Thoramus also live in logs.
Māori ate the large white larvae of huhu beetles (Prionoplus reticularis), which they called tunga haere and tunga rākau. The larvae feed on the dead wood of native trees and introduced pine. When mature larvae or tataka have emptied their gut contents before pupating, they are considered a delicacy. Adult beetles or tunga rere can be seen flying around lights on spring or summer evenings, and can give a nasty nip when handled.
A range of species can be found high in the forest canopy. These include foliage, branch and stem borers, and predators.
Mountainous areas are home to black-and-white striped speargrass weevils, whose larvae feed on the tap roots of speargrass or Spaniard plants (Aciphylla). Flightless chafers (Scythrodes squalidus and Prodontria species) and moss beetles inhabit high-altitude grasslands and herbfields.
Some beetles have adapted to extreme environments. Subantarctic rove beetles (Baeostethus chiltoni) can survive periods of immersion in sea water, and some water scavenger beetles are found swimming in thermal pools in temperatures as high as 45°C. Blind ground beetles live deep in caves, and diving beetles live metres below the surface in alluvial groundwater.
European settlers brought predatory species such as rats, mice and hedgehogs, which found beetles to be easy prey. As with New Zealand’s native birds, the large, flightless beetles are particularly vulnerable. Some species have vanished altogether, such as the large weevil Tymbopiptus valeas, known only from fossil specimens. Others have become locally extinct, like the Cook Strait click beetle (Amychus granulatus), which is confined to predator-free island refuges, although fossils show that it was once present on the mainland.
Forest clearance by Māori and European settlers has reduced species diversity. Deforestation has decreased the numbers of many species, and there is little doubt that many more species have been lost. Although widespread forest clearance has now stopped, the continued fragmentation of forests may cause the larger, flightless, and less common beetles to become extinct.
Few native beetles have adapted to pastureland and towns – introduced species tend to dominate these environments.
Legal protection has been given to 17 of the most endangered beetle species, and the Department of Conservation has identified many more that are threatened. Conservation efforts are hampered by the poor knowledge of beetle life histories and by difficulty identifying many species.
Nevertheless, there is a growing awareness of the importance of conservation. In what was probably a world first, a reserve was set up in 1979 to protect the rare Cromwell chafer beetle (Prodontria lewisi). This species is restricted to the tussock-covered inland dunes of Central Otago, many of which have been lost to pasture, and it is now found only in the 81-hectare reserve.
Some species presumed extinct have been rediscovered, like the Pitt Island longhorn (Xylotoles costatus) and the Canterbury knobbled weevil (Hadramphus tuberculatus).
About 350 species of beetle are introduced. Many arrive almost unnoticed in New Zealand and have little impact. A few beneficial species have been deliberately introduced for the biological control of pests such as the aphid-feeding 11-spotted ladybird (Coccinella undecimpunctata). Others are pests, such as pasture-eating sitona weevils (Sitona discoideus) and house borers (Anobium punctatum), infamous for eating through the wood of houses and furniture.
The whirligig beetle (Gyrinus convexiusculus) is a self-introduced Australian species which lives in ponds and lakes in Waikato and Northland. These beetles use their broad, paddle-like legs to scoot about on the water’s surface in search of prey. Their eyes are divided into upper and lower parts – one for viewing above the water, the other for below.
Native beetle pests are relatively rare. One is the grass grub or tūtaeruru (Costelytra zealandica) – the glossy brown adults are often seen flying around lights on warm summer evenings. Damage caused by the root-feeding larvae, which can number several hundred per square metre, make this one of New Zealand’s most serious pasture pests.
Beetles can play a vital role in helping forest litter to decompose, and as predators of pest species. They can be useful indicators of forest health. Beetles are also used to estimate historical environmental conditions – fossil beetles indicate past types of forest, which are evidence of a changing climate.
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