Bobilla Otte & Alexander 1983
Creators: name: Hegg, Danilo
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Orthoptera
Trigonidiidae
Bobilla

description: Diagnosis of ground crickets in the genus Bobilla Otte & Alexander, 1983
Out of 368 crickets examined, 138 belong to the genus Bobilla. These include 69 Bobilla bigelowi, 65 Bobilla nigrova and 4 specimens from the southern Waikato region that seem to fall in between the two species.
A careful examination of this material supports five out of six of the differences between the two species of Bobilla highlighted by Swan (1972); these differences are reported in Table 1. In addition to these, Swan (1972) had suggested the number of large bristles on each side of the suranal plate in adult females as a useful trait to differentiate between species (2 to 5 in B. nigrova; 6 to 10 in Bobilla bigelowi). These bristles however are prone to rubbing off, and their number seems to be more variable than indicated by Swan. I suggest that there is little merit in referring to this trait when identifying Bobilla to species level; I have therefore omitted it from the summary in Table 1.
Tooth count in the stridulatory file and song analysis yield numbers that are generally in good agreement with the values reported by McIntyre (1977a) (see Tables 1–2). Male crickets are reliably identified to species level by the number of teeth in the stridulatory file (Fig. 3B); this however requires removing the insect’s right forewing.
The songs of the two species of Bobilla differ mainly in their pulse rate, which is around 10 pulses / s in B. nigrova, around 20 pulses / s in B. bigelowi (Table 2; Figs 11, 14). The analysis of Bobilla songs recorded in the field, however, can be difficult; this is explained in detail in this paper’s discussion.
In addition to the differences already identified by Swan (1972) and by McIntyre (1977a, 1977b), here I report four more measurements that are significantly different between the two New Zealand species of Bobilla (Table 1). The first one is the ratio of the short and long diagonals in the right male forewing’s posterior cell of the harp (see Figs 1B, 3A). This ratio PCSD /PCLD is greater than 0.49 in B. bigelowi, less than 0.49 in B. nigrova (Mainland New Zealand only, not including Chatham Islands). The measurements for the two species do not overlap, making this a reliable and not destructive method to differentiate between male B. nigrova and B. bigelowi based on their forewings. The second measurement is the ratio between the maximum eye length and the intraocular distance, in a dorsal view of the head (see Figs 1C, 3C). The ratio EL/IOD is greater than 0.79 in B. bigelowi and less than 0.8 in B. nigrova, with only a small range of values where the two species overlap. The third difference lies in the width of the tympanal openings on the posterior surface of tibia I. While the length of the tympana is the same in the two species, B. bigelowi has narrower openings (Fig. 3E). The fourth measurement is the length ratio of the middle vs dorsal inner apical spurs on tibia III, which is around 0.9 in B. nigrova, 0.8 in B. bigelowi (see Figs 3F, 4). While the width of the tympana and the length of the apical spurs on tibia III differ significantly between species (Table 1), there is too much overlap in their ranges to rely on these measurements for the purpose of species identification.Morphological traitBobilla nigrova(Swan, 1972)Bobilla bigelowi(Swan, 1972)t-test resultFiguresMalesSpike-like tufts of bristles on each side of the suranal plate (Swan 1972). present absent Fig. 10A Fig. 11D Ratio PCSD / PCLD Mainland NZ only 0.478 (0.438–0.488) 0.522 (0.496–0.558)p<10-10 Fig. 1BN= 13N= 30 Fig. 3A Number of teeth in stridulatory file (McIntyre 1977) (188–283) (123–190)N= 85N= 83 Number of teeth in stridulatory file (this study) 204 (180–246) 152 (118–171)p<10-12 Fig. 3BN= 11N= 30 Song: pulse rate (pulses / s) (McIntyre 1977) 13 (11–15) 21 (20–22) Fig. 12N= 36N= 61 Fig. 15 Song: Main frequency (McIntyre 1977) 8.0 kHz at 27°C 7.5 kHz 27°C Fig. 12 Fig. 15FemalesDorsal surface of abdomen (Swan 1972) grey or black, with uniform black Fig. 10B pale markings Fig. 13B Lateral valves of ovipositor (Swan 1972) strongly denticulate weakly denticulate Fig. 11G–H Fig. 14F–G Egg colour (Swan 1972) black pale Fig. 11F Fig. 14E Ovipositor length (mm) (Swan 1972) long (mean 4.9 mm)1 short (mean 4.1 mm) Fig. 3D Ovipositor length (mm) (this study) 5.9 (4.0–6.7) 4.0 (3.1–4.9)p<10-13 Fig. 3DN= 20N= 26Both sexesRatio eye length / intraocular distance 0.754 (0.705–0.806) 0.840 (0.789–0.901)p<10-16 Fig. 1CN= 18N= 37 Fig. 3C Minor axis of tympanum (µm) 119 (105–154) 95 (67–122)p<10-6 Fig. 1EN= 32N= 76 Fig. 3E Length ratio of middle and dorsal inner spurs at apex of tibia III 0.882 (0.833–0.959) 0.822 (0.744–0.912)p<10-5 Fig. 3FN= 18N= 38
1 Note the discrepancy between Swan’s and this study’s measurements. After re-measuring Swan’s paratypes (N = 40) I calculated a mean ovipositor length of 5.3 mm in B. nigrova.
Nine specimens of Bobilla collected on Pitt Island (Chathams), 800 km east of New Zealand, all fit the description of B. nigrova in their morphology, song, and male genitalia. The mean ratio PCSD / PCLD, however, is 0.511 (min 0.493, max 0.576, N = 5), typical of B. bigelowi on mainland New Zealand. This suggests that the Bobilla population on Chatham Islands may have been isolated for some time.
Two adult male Bobilla bigelowi collected at Lake Ōhau, Mackenzie Country, had fully developed hindwings (Fig. 12A). This contradicts Swan’s (1972) statement that “[Bobilla bigelowi] males are always apterous, but macropterous females with hindwings extending beyond the tip of the ovipositor are not uncommon (3–6 percent of a population)”. Fully winged specimens occur in both sexes and in both species but are rare.
Of interest is a population in the southern Waikato region, where nine out of twelve female specimens of Bobilla collected had long forewings, the remaining three having short forewings. This contrasts with 1 out of 16 B. bigelowi with long forewings and 1 out of 14 B. nigrova with long forewings from the rest of the country, the morph with short forewings being by far the most common in both species. Four of the specimens from this southern Waikato region could not be assigned with certainty to either of the two species; rather, they seem to fall in between. Females have the dark colour and white eggs of B. bigelowi, but the long ovipositor with denticulate lateral valves of B. nigrova. The number of teeth in the stridulatory file and the ratio of eye length over intraocular distance lye half-way between the typical ranges for the two species. It seems most likely that this would be a hybrid population arising from interbreeding between the two species of Bobilla. For more information about the origin of these specimens, see Supp. file 1: Table S3.
Silent ground crickets
Out of 368 crickets examined, 41 are ground crickets (subfamily Nemobiinae) lacking a stridulatory apparatus or tympana and belonging to two clearly distinct taxa, one from the forests of New Zealand’s South Island and from Mt Ruapehu, the other one exclusively from North Island. No silent ground crickets have previously been described from New Zealand.
Several genera of silent nemobiine crickets have been described from other regions in the Southern hemisphere. These include seven genera from Australia (Rentz & Su 2019: 319): Narellina Otte, 1994; Eumarinemobius Gorochov & Tan, 2018; Tincanita Otte &Alexander, 1983; Silvinella Otte &Alexander, 1983; Nambungia Otte & Alexander, 1983; Territirritia Rentz & Su, 1996; and Calperum Rentz & Su, 1996; six genera from South America: Absonemobius Desutter-Grandcolas, 1993; Phoremia Desutter-Grandcolas, 1993; Amanayara de Mello & Jacomini, 1994; Monopteropsis de Mello & Jacomini, 1994; Kevanemobius Bolfarini & de Mello, 2012; and Pepoapua Jesus & Pereira, 2017; one genus from Vanuatu and Samoa (Desutter-Grandcolas 2009): Cophonemobius Chopard, 1929; three genera from New Caledonia (Anso et al. 2016; Desutter-Grandcolas et al. 2016): Paniella Otte, Alexander & Cade, 1987; Orintia Gorochov, 1986; and Kanakinemobius Desutter-Grandcolas, 2016.
The two New Zealand taxa have been carefully compared with descriptions and photographs of all of the above genera (Table 3), and don’t match any of these. The southern taxon is apterous in both males and females and has characteristic male genitalia with articulated pseudepiphallic parameres, modified for clasping. While it looks deceptively similar to Absonemobius in photograph, the latter genus has a head that is wider than it is high in frontal view (Desutter-Grandcolas 1993) and macroscopically different male genitalia (see Desutter-Grandcolas & Hugel 2016). Among Australian genera, it most resembles Territirritia Rentz & Su, 1996, but has a different number of sub-apical spines on the hind tibiae and again different male genitalia. Here, the southern species is assigned to a new monotypic genus, Austronemobius gen. nov.
The northern taxon is noticeably larger and has reduced forewings that look identical in males and females, without a stridulatory apparatus in males. While superficially similar to New Caledonia’s Kanakinemobius, the structure of the male forewings places it in a separate genus. The male genitalia of the northern taxon resemble those of Amanayara from South America, but the latter genus is entirely apterous and has two glandular subapical spurs on the inner side of the hind tibia in males (de Mello & Jacomini 1994). The structure of the wings closely resembles that of the South American genus Pepoapua, but the latter genus has different male genitalia and a serrated ovipositor in females (Jesus et al. 2017). Here, the northern taxon is assigned to a new monotypic genus, Mutonemobius gen. nov.
a individual variation within species and population.
b varies between species.
Both new genera, Austronemobius gen. nov. and Mutonemobius gen. nov., are placed in the tribe Nemobiini Vickery, 1973 based on the absence of a glandular subapical spur on the inner edge of Tibia III in males (Vickery 1973).
description: Published as part of Hegg, Danilo, 2024, Small crickets of New Zealand (Orthoptera: Grylloidea: Trigonidiidae and Mogoplistidae), with the description of two new genera and species, pp. 1-87 in European Journal of Taxonomy 955 (1) on pages 9-17, DOI: 10.5852/ejt.2024.955.2655, http://zenodo.org/record/13742408