Hemideina crassidens

Hemideina crassidens
Female
Male
Conservation status
Not Threatened (NZ TCS)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Orthoptera
Suborder:	Ensifera
Family:	Anostostomatidae
Genus:	Hemideina
Species:	H. crassidens
Binomial name
Hemideina crassidens (Blanchard, 1851)
Synonyms
Anostostoma crassidens Blanchard, 1851 Deinacrida megacephala Buller, 1867 Hemideina abbreviata Walker, 1869 Hemideina capitolina Walker, 1869 Hemideina figurata Walker, 1869 Hemideina fusifera Walker, 1869 Hemideina producta Walker, 1869 Hemideina tibialis Walker, 1869 Deinacrida armiger Colenso, 1886 Deinacrida ligata Brunner von Wattenwyl, 1888 Hemideina brevaculea Salmon, 1950

Hemideina crassidens, commonly known as the Wellington tree wētā, is a large, flightless, nocturnal insect in the family Anostostomatidae. This wētā species is endemic to New Zealand and populates regions in the southern half of North Island/Te Ika a Maui and the north-west of the South Island/Te Wai Pounamu. They forage arboreally during the night and are most likely polyphagous. There is obvious sexual dimorphism in adults. Individuals are reliant on tree cavities for refuge, social interactions and mating.

The conservation status of H. crassidens is "not threatened".

This species was first described as Anostostoma crassidens in 1851 by Émile Blanchard. Although Hemideina crassidens is endemic to New Zealand, the first known specimen was collected in Chile, presumably by Claude Gay.^[1] It has been suggested that this species was accidentally introduced to Chile due to trade routes (although no more specimens have been found).^[2] In 1935, it was recognized as belonging to Hemideina by Heinrich Hugo Karny and subsequently moved to this genus.^[3]

There have been eleven previous descriptions of H. crassidens that were later synonymized. Of these, eight were in the Hemideina and three were in Deinacrida. These synonyms were created by Francis Walker, Walter Buller, Carl Brunner von Wattenwyl, John Salmon and William Colenso.^{[4][5][6][7][8]}

Hemideina crassidens is endemic to New Zealand.^[9] Populations are distributed between the Ruapehu district and Wellington district of the North Island, and in the Westland district of the South Island.^[9][10][11] A closely related native tree wētā, Hemideina thoracica, is widely distributed in the northern two-thirds of the North Island New Zealand.^[12] It has been suggested that H thoracica competitively excludes H. crassidens from warmer northern regions of the North Island.^[12] On Mt Taranaki, H. crassidens is found only at elevations above ~700 m asl, and H. thoracica is found lower down the slopes where temperatures are slightly warmer.^[10][12]

Hemideina crassidens is a nocturnal and arboreal.^[9] This species uses holes in tree branches to hide during the day.^[13] They usually live in tree holes, suggesting a reliance on forest. However, many extant tree wētā populations occupy scrub habitats or even use rock refuges when trees are not available.^[14]

In the central North Island populations of H. crassidens have become isolated within areas mostly populated by H. thoracica suggesting previous range contraction due to interactions with competitors and the environment.^[12] This contraction is likely to continue southwards during global warming, resulting in the displacement of H. crassidens from many lowland areas of central and southern North Island.^[12]

Being a tree wētā, H. crassidens spend most of their time foraging arboreally.^[9] They are generally herbivorous, feeding on the leaves, fruit and flowers of a wide range of trees and shrubs.^[9][15] However, polyphagy is common among wētā species and it is likely that H. crassidens supplement their diet with animal matter.^[15] Individuals have been documented feeding on living or recently dead invertebrates.^[16]

Hemideina crassidens are relatively large at maturity (> 6.5 cm body length).^[16] Their bodies are smooth and shiny, the abdomen is ringed with contrasting bands of dark brownish to black and yellow, with a pale underside.^[16] This colouration is reminiscent of uniform of the Wellington rugby team, the Hurricanes, and can be used to distinguish it from the Auckland tree wētā (Hemideina thoracica). They have heavily spined hind tibiae that are used in defence postures.^[16][17] The antennae are long and mobile to help with sensing and navigation, particularly in the dark.^[9] The Wellington tree wētā and the Hawkes Bay tree wētā (Hemideina trewicki) look the same but have different numbers of chromosomes.^[18] In South Island H. crassidens have 19 (male) or 20 (female) chromosomes and in North Island this species has 15 or 16 chromosomes.^[19] The Hawkes Bay tree wētā has 17 (male) or 18 (female) chromosomes.^[18]

Males display highly exaggerated, positively allometric mandibles.^[20][21] Accelerated maturation of males gives rise to three different head sizes that correlate their maturation on either the 8th, 9th or 10th instar.^[20] 8th instar males have small heads, 10th instar males have particularly elongated jaws and a large vertex and 9th instar males have an intermediate from.^[21][16][20] Females only reach maturity at the 10th instar.^[20] In this wētā species, females can be identified by their small head and long ovipositor.^[16]

Rather than bore their own tree holes, H. crassidens inhabit natural crevices and cavities, or pre-existing tunnels that have been excavated by large wood-boring larva such as Aenetus virescens (Lepidoptera: Hepoalidae).^[16] They prefer tree holes in living timber and avoid fallen or rotten logs.^[22] Native trees and shrubs such manuka (Leptospermum scoparium), kanuka (Kunzea ericoides), ngaio (Myoporum laetum), kohekohe (Dysoxylum spectabile) and mahoe (Melicytus ramiflorus) are favoured by H. crassidens.^[22] Suitable galleries have a narrow entrance hole (~20mm in diameter) in order to provide protection from predators such as birds.^[23] The holes are entered head first and exited in reverse so that the spines on their hind tibiae point outwards to defend from intruders.^[23][16] Inhabited galleries can often be identified by the neatly nibbled entrance which is kept clean.^[16] Individuals often occupy the same gallery for an extended period of time.^[13][24] Galleries buffer against environmental variation, especially humidity.^[24][16]

Hemideina crassidens is a polygynous insect in which males guard females that reside in tree cavities termed 'galleries'.^[20][21] Galleries are used for breeding and males compete for control of them and access to the females within.^[20][22] Male head size is an indicator of combative ability as the head is used as weaponry in combat for dominance and control of a harem.^[20] In cases where the gallery is sufficiently large, multiple males can occupy a single gallery, suggesting that a dominant male defends a harem rather than a gallery per se.^[20] Males can avoid combat by facing off, opening their jaws and assessing the gape of their opponent's mandibles.^[17][20] In a non-violent contest the male whose mandibles span the furthest is considered dominant.^[20][21] Small headed males can mate by wandering nocturnally and copulating with females outside of a gallery, or by sneaking into a gallery occupied by a dominant male and copulating with females within.^[16] A small headed male is able to control a harem within a gallery if the entrance is too small for large headed males to fit through.^[20][21]

Most matings and ovipositions occur over the summer and autumn.^[16][9] Females lay their eggs vertically in the soil approximately 10mm under the surface.^[9][16] Hatching occurs in spring and individuals take about 18 months to become mature, reaching sexual maturity in the following summer.^[9] Hemideina crassidens do not show any parental care.^[16] Their normal lifespan is about 3 years, with about 18 months as immatures.^[16]

The number of suitable galleries in an area is considered to be a major limiting resource of H. crassidens.^[25][22] The construction and distribution of artificial refuges has been suggested as a possible conservation strategy to try and increase populations of H. crassidens and H. thoracica.^[26]

Under the New Zealand Threat Classification System, this species is listed as "Not Threatened" as of 2022. The population is considered stable.^[27] H. crassidens has been reported as "Not Threatened" in previous assessments.^[28][29]

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