Genus Isoplectron Hutton, 1896

Isoplectron Hutton, 1896: 237.

Setascutum Richards, 1972: 163. Syn. nov.

Petrotettix Richards, 1972: 166. Syn. nov.

Isoplectron – Karny 1937: 229–230, pl. 6 fig. 5. –– Ward 1997: 13–15. –– Johns & Cook 2013: 1. –– Hegg et al. 2022: 51–52.

Setascutum – Ward 1997: 13–15. –– Johns & Cook 2013: 1.

Petrotettix – Ward 1997: 13–16. –– Johns & Cook 2013: 1. –– Hegg et al. 2022: 51.

Type species

Isoplectron armatum Hutton, 1896.

Diagnosis

A genus of small to mid-sized Rhaphidophoridae (adult body length typically between 9 and 13 mm; up to 18 mm in the larger species), with hind tibiae armed above with two rows of linear spines that are very similar in size. These spines are fused to the shaft of the tibia and are never socketed or articulated. Fore femora always without apical spines; mid femora usually armed with one retrolateral spine at the apex. Male subgenital plate broadly speaking triangular, with an elongated central lobe; female subgenital plate bilobed. Upper valve of the ovipositor always serrated above.

While some species of Isoplectron are morphologically very similar and are easily mistaken for one another, other species are morphologically and ecologically quite distinct. This makes it somewhat difficult to characterise the genus, yet some traits are common to all species. A detailed description of these common traits follows; individual species descriptions focus on those traits that differ among species.

Etymology

Not explained by Hutton. From the Greek ʻ Isos ʼ = ‘equal’, ʻ plectron ʼ = ʻplectrumʼ (probably referring to the dorsal spines on the hind tibiae, which are shaped like a plectrum). The hind tibiae are armed with spines that are equal in shape and size. Isoplectron is neuter gender.

Description

Adult

MESUREMENTS. See Table 1. Sexual dimorphism in body length, with females being larger than males by up to 10% in most but not all species.

HEAD (Fig. 11A–B). Oval in shape. Eyes rounded, but with a straight inner edge facing the scapes of the antennae. Eye colour green, wholly or partially, in all species. Fastigium divided by a deep median groove; shaped like an isosceles triangle with a rounded pale patch in the middle when seen from the side. No visible sexual dimorphism in scapes of antennae or any other head-part. Labial and maxillary palps pale, of varying length, with moderately dense covering of hair.

THORAX. Pronotum, mesonotum and metanotum all covered in dense, fine tomentum. A pale, thin median dorsal line is generally inconspicuous or absent (see Fig. 12A–I). Lateral edges of pronotum with a pronounced rim. In dorsal view, the pronotum is up to 80% wider at the posterior end than at the anterior end (Fig. 12A–I).

LEGS. Moderately long. Hind femora slender; sexual dimorphism evident, with all leg segments longer in males than in females in most but not all species. Coxae and trochanters generally of uniform pale colour. Fore and mid femora and tibiae may be uniform pale or variegated; hind legs variegated. Fore coxae with a pronounced lateral anterior spine. Fore tibiae approximately half of body length, and 10% longer in males than in females in most species; on average 5% longer than fore femora in both males and females. Fore femora without linear spines above or below, and always without apical spines. Fore tibiae armed below, generally with two linear spines on both anterior and posterior edge in all species. The number of apical spines on the fore tibiae varies by species. Mid legs 2% to 5% shorter than fore legs, otherwise with the same proportions in male and females. Mid femora without linear spines above or below, but usually armed with one retrolateral spine at the apex. A prolateral spine at the apex of the mid femur is always absent. Mid tibiae armed below, generally with two linear spines on both anterior and posterior edge in all species. Dorsal linear spines on the mid tibiae are rare but possible. The number of apical spines on the mid tibiae varies by species. Hind tibiae approximately of same length as body in females, up to 50% longer in males in most but not all species. Hind femora approximately 10% shorter than hind tibiae. Hind femora generally armed with few but at times very conspicuous linear spines below on both anterior and posterior edges. Hind tibiae armed with anything between 11 and 37 linear spines above (number varies both within and between species), of similar size, on both anterior and posterior edges (Fig. 13A–I). The spines are fused to the shaft of the tibia and are never socketed or articulated. The number of apical spines on the hind tibiae varies by species. Hind tarsi with four segments; first and second segments with a pair of spines on distal end. First and second tarsal segment may be armed or unarmed above, depending on species.

ABDOMEN. Tergites always covered in dense, fine tomentum. Colour of tergites varies by species; a pale, thin median dorsal line is generally inconspicuous or absent (see Fig. 12A–I). In some species, the seventh and eighth sternites are equipped with 3 to 5 very conspicuous protuberances, in females only (see Fig. 18G, J and Fig. 19A).

MALE TERMINALIA. Cerci between 10% and 25% of body length depending on species; pointed at apex, variable in colour, clothed in setae. The subgenital plate looks somewhat similar in all species, with an elongated median lobe; it is at least twice as long at centre as on the sides. Paraprocts armed with strong, stout spinules in most species. The difference in male terminalia between species is pronounced enough to provide one of the strongest characters for species level identification (see Figs 14–16).

FEMALE TERMINALIA. Subgenital plate consists of two small, rounded lobes, separated by a gap in the middle. Ovipositor reddish-brown, moderately to strongly curved upwards at apex, terminating in a sharp point; approximately three quarters of body length in most species. Upper valve always serrated above, strongly so in some species; lower valve with 5 to 10 strong teeth at apex on ventral edge (Figs 17–19).

Nymph

Generally look similar to adults. Due to the small size of the insects and the lack of developed terminalia, nymphs may be next to impossible to differentiate from their equivalents in Praecantrix gen. nov. or Neonetus.

1 For the number of males and females in each sample, refer to Table 1.

Distribution

New Zealand, all of South Island; limited to southern regions in North Island (Fig. 3).