Do alternative reproductive strategies in the Wellington tree weta represent different behavioural types?

In many animal species, variation in reproductive success among individuals has led to the evolution of alternative mating strategies, which in the case of insects can often be correlated with developmental trajectories. In the Wellington tree weta, Hemideina crassidens, males can mature at the 8th, 9th or 10th instar, while females mature at the 10th instar only. A number of morphological attributes including male head and mandible size correlate with final instar number, and as these attributes represent a form of weaponry, they are often used in mate/site guarding and male–male competition. Tenth instar males have larger head/mandible/body sizes and show a conventional (guarder) reproductive strategy, whereas smaller 8th instar males typically show an unconventional (sneaker) strategy. In contrast, 9th instar males are predicted to adopt a “jack‐of‐all‐trades” strategy whereby they can fight or sneak depending context. Here, we tested whether alternative reproductive morphs exhibit strategy‐specific differences in risk‐taking associated with refuge emergence, activity and antipredator behaviour and further, whether these traits correlate to form a behavioural syndrome. We found that tree weta show consistent and repeatable differences in activity and refuge use at the individual level; however, behavioural covariances suggest that only 8th instar males exhibit a behavioural syndrome. That 9th instar males show high plasticity and variance in their gallery‐related behaviours supports the hypothesis that these males are a “jack‐of‐all‐trades.” Contrary to our predictions, antipredator behaviour was not correlated with other traits, and differences in behaviour overall were consistently more pronounced between individuals rather than between male morphs or sexes.     

Does predation result in adult sex ratio skew in a sexually dimorphic insect genus?

Theory proposes that sexually dimorphic, polygynous species are at particularly high risk of sex-biased predation, because conspicuous males are more often preyed upon compared to females. We tested the effects of predation on population sex ratio in a highly sexually dimorphic insect genus (Hemideina). In addition, introduction of a suite of novel mammalian predators to New Zealand during the last 800 years is likely to have modified selection pressures on native tree weta. We predicted that the balance between natural and sexual selection would be disrupted by the new predator species. We expected to see a sex ratio skew resulting from higher mortality in males with expensive secondary sexual weaponry; combat occurs outside refuge cavities between male tree weta. We took a meta-analytic approach using generalized linear mixed models to compare sex ratio variation in 58 populations for six of the seven species in Hemideina. We investigated adult sex ratio across these populations to determine how much variation in sex ratio can be attributed to sex-biased predation in populations with either low or high number of invasive mammalian predators. Surprisingly, we did not detect any significant deviation from 1 : 1 parity for adult sex ratio and found little difference between populations or species. We conclude that there is little evidence of sex-biased predation by either native or mammalian predators and observed sex ratio skew in individual populations of tree weta is probably an artefact of sampling error. We argue that sex-biased predation may be less prevalent in sexually dimorphic species than previously suspected and emphasize the usefulness of a meta-analytic approach to robustly analyse disparate and heterogeneous data.     

Selected temperatures of an alpine weta Hemideina maori from southern New Zealand

Abstract

We examined the relationship between thermal preference (selected body temperature or “T_sel") and microhabitat temperature of a nocturnal, alpine weta Hemideina maori (Orthoptera: Anostostomatidae), in the southern part of New Zealand. To examine diel variation in thermal preference, we measured T_sel five times over a 24‐h period in the laboratory and compared these temperatures to weta microhabitat temperatures in the wild. T_sel ranged from 4.3 to 29.3°C, but the distribution of temperatures was skewed, towards the cooler end of the thermal gradient, with an overall median and mean of c. 13°C. The frequency of T_sel values was bimodal, with peaks at a relatively narrow range of cool temperatures between 5 and 8°C (33%) and at a slightly broader range of warmer temperatures between 13 and 21 °C (52%). T_sel values did not vary with time of day, but differed significantly between individuals. Comparisons with microhabitat temperature showed that weta at 1250 m a.s.l. on the Rock and Pillar Range, Otago, had only limited opportunity to achieve T_sel and this was only possible during daylight hours, when weta are normally inactive.     

Novel use of hair sensilla in acoustic stridulation by New Zealand giant wetas (Orthoptera: Anostostomatidae)

Sound production in New Zealand giant wetas (Orthoptera: Anostostomatidae) includes a femoro-abdominal mechanism, a ticking sound when alarmed (mechanism unknown) and, in two species (Deinacrida rugosa and Deinacrida parva), a tergo-tergal mechanism on the dorsal overlapping surfaces of abdominal tergites. The tergo-tergal mechanism consists of bilaterally paired patches of short curved spines on the dorsal anterior margins of tergites II–V, rubbed by opposing patches of articulated hair sensilla on the underside of each overlapping tergite. The latter are extremely robust, modified mechanoreceptors inserted at an acute angle onto raised bases which greatly restrict movement. They rub sideways against the underlying spines and produce sound during telescopic abdominal contraction which accompanies defence leg kicking stridulation. Movement analysis showed that the abdominal tergites contract asynchronously during stridulation. Sound is produced during both phases of telescoping. Femoro-abdominal sound comprises loud clicks of broadband sound principally below 10 kHz; tergo-tergal sound is a softer hiss spreading broadly from 10 kHz to the ultrasonic above 20 kHz. We propose that the tergo-tergal mechanism may have evolved under predation pressure by the ground gleaning short tailed bat endemic to New Zealand. The use of mechanosensory hair sensilla for sound production is rare in arthropods.     

The relationship between male head size and harem size in the sexually dimorphic mountain stone weta Hemideina maori

Abstract 1. Tree weta are a group of large, flightless orthopterans with pronounced sexual dimorphism. Males have enlarged heads that are used in fighting for possession of cavities in trees or under rocks where females shelter during the day.
2. The fieldwork reported here examined the relationship between male head size and mating success in Hemideina maori , an alpine tree weta that shelters under rock slabs that have broken off isolated outcrops or tors.
3. The relationship between male head size and harem size in H. maori is not as clear-cut as thought previously. First, overall body size is a better predictor of male mating success than head size per se . Second, both body size and head size explained a relatively low percentage (19.8%) of the overall variation in mating success. Third, despite the intensity of directional selection being estimated to move the frequency distribution of head size and femur size 0.49 and 0.54 standard deviations from the mean, male heads and femurs were ≈ 2 mm smaller at the main study site than at a second site 100 m higher in elevation. A similar pattern was found for adult females. Additional surveys have indicated that body size in H. maori decreases with decreasing altitude, which is correlated with increasing night-time temperature.
4. Although there are reasons why natural selection might favour weta maturing earlier and at smaller body sizes in warmer environments, relatively large males would still have a mating advantage over smaller males under such conditions. This sexually dimorphic alpine insect might be a good example of the trade-offs and conflicting demands that sexual selection versus natural selection can place on organisms.     

Diet of the ground weta Zealandosandrus gracilis (Orthoptera: Stenopelmatidae)

Abstract

The diet of the ground weta Zealandosandrus gracilis Salmon was investigated by examining the crop contents of 68 individuals collected throughout the year from the Cass area, Canterbury. Specimens representing all instars of both sexes, as well as adults, were obtained. Z. gracilis is carnivorous, preying on immature and adult invertebrates (mostly insects) of the forest litter. Similar prey was taken by early instars of both sexes; late instars captured a wider range of prey, including larger and more active species. Late-instar and adult females preyed more on adult insects such as mycetophilid flies than did males, which took more insect larvae.     

Metapopulation dynamics of a flightless alpine insect Hemideina maori in a naturally fragmented habitat

Abstract 1. Despite widespread acceptance of metapopulation theory, the effects that inter-patch dispersal and variability in patch size have on metapopulation dynamics in insects are two issues that require further study. In addition, previous studies of metapopulations have tended to focus on organisms with high dispersal capabilities such as some species of butterfly and bird.
2. Mountain stone weta Hemideina maori are a long-lived, flightless orthopteran that live on island rock outcrops or tors in the alpine region of southern New Zealand. A total of 480 adults and 789 juveniles was marked over three seasons on four large and 14 small tors to assess the effects of habitat fragmentation on the population dynamics of H. maori .
3. Only 12 adults (2.5% of marked adults and 4.0% of recaptured adults) and two juveniles (0.3% of marked juveniles and 0.7% of recaptured juveniles) dispersed between tors. The mean dispersal distance was 361 m (range = 36–672 m). Larger tors supported larger populations and had a higher number of emigrants and immigrants while smaller tors had proportionally higher emigration and immigration rates. Although adults on large and small tors had similar mean lifespans, five extinction events and three recolonisation events occurred during the study period, all on small tors.
4. Hemideina maori conform to many of the predictions of metapopulation theory even though they are flightless, show relatively low dispersal rates, and occur at low densities. Extinction and colonisation events are more common on small tors but may be relatively unimportant for the long-term survival of the metapopulation because they occur on the smallest habitat patches, which support the smallest proportion of the overall population.     

A new species of tusked weta from the Raukumara Range,North Island,New Zealand (Orthoptera: Anostostomatidae: Motuweta)

Abstract

A new species of weta from mainland New Zealand is described and assigned to the previously monotypic genus Motuweta Johns and the genus redefined. Mandibles of adult males bear prominent horns. Motuweta riparia n. sp. is a moderate‐sized species (36 mm) found on the margins of first or second order streams in the eastern mid North Island ranges. Its escape reaction is to leap into the stream and remain underwater until danger has passed, a strategy which has probably enabled it to survive in predator‐infested areas.     

A comparison of five hybrid zones of the weta Hemideina thoracica (Orthoptera: Anostostomatidae): degree of cytogenetic differentiation fails to predict zone width

Abstract Tension zones are maintained by the interaction between selection against hybrids and dispersal of individuals. Investigating multiple hybrid zones within a single species provides the opportunity to examine differences in zone structure on a background of differences in extrinsic factors (e.g., age of the zone, ecology) or intrinsic factors (e.g., chromosomes). The New Zealand tree weta Hemideina thoracica comprises at least eight distinct chromosomal races with diploid numbers ranging from 2n = 11 (XO) to 2n = 23 (XO). Five independent hybrid zones were located that involve races differing from one another by a variety of chromosomal rearrangements. The predicted negative correlation between extent of karyotypic differentiation (measured in terms of both percent of genome and number of rearrangements) and zone width was not found. Conversely, the widest zones were those characterized by two chromosome rearrangements involving up to 35% of the genome. The narrowest zone occurred where the two races differ by a single chromosome rearrangement involving approximately 2% of the genome. The five estimates of chromosomal cline width ranged from 0.5 km to 47 km. A comparative investigation of cline width for both chromosomal and mitochondrial markers revealed a complex pattern of zone characteristics. Three of the five zones in this study showed cline concordance for the nuclear and cytoplasmic markers, and at two of the zones the clines were also coincident. Zones with the widest chromosomal clines had the widest mitochondrial DNA clines. It appears that, even within a single species, the extent of karyotypic differentiation between pairs of races is not a good predictor of the level of disadvantage suffered by hybrids.