Plagiognathus Fieber, 1858

Introduction, Tomohide Yasunaga, 2023, Two new thermophilic species of the plant bug genus Plagiognathus from Japan and Taiwan (Hemiptera: Heteroptera: Miridae), Bonn zoological Bulletin 72 (1), pp. 25-40 : 26-27

Plagiognathus Fieber, 1858

Diagnosis. East Asian congeners of Plagiognathus are recognized by the following combination of characters: Body elongate and subparallel-sided (♂)/ elongate-ovoid (♀), small to moderate in size (total length 2.9–4.5 mm); coloration variable ( Figs 1–2 ); dorsum generally shining, with uniformly distributed, dark, simple setae, lacking silvery or wooly recumbent setae; vesica always with two apical blades (cf. Figs 6 , 8B–C , 9D–F ); sclerotized rings large, contiguous to each other mesally ( Fig. 7 ); and posterior wall of bursae simple (cf. Figs 8E–F , 9H–I ). As suggested by Schuh (2001), this large Holarctic genus is defined principally by the form of the male genitalia, such as twisted, often sigmoid vesica with two apical blades and rather slender, lanceolate right paramere. Some species of Plagiognathus exhibit great similarity in external appearance to certain members of Europiella Reuter, 1909 ; however, Europiella is distinct in having both silvery and simple setae on dorsum, right paramere remarkably widened and often squared, vesica apically with membranous area, and sclerotized ring small and narrow-rimmed (e.g., Schuh et al. 1995; Yasunaga 2022). Further diagnostic characters and a redescription for Plagionathus were provided by Schuh (2001).

Discussion

Plagiognathus forms one of the largest genera among the plant bug subfamily Phylinae and is known to comprise more than 120 described species; of these about 80 % are Nearctic elements ( Schuh 2001, 2002 – 2013; Aukema 2018). Nonetheless, more than a few species appear to be wrongly placed in Plagiognathus and require further verification by dissecting the male genitalia ( Schuh 2001). For eastern Asia, six congeners were reported by Yasunaga (2001), in addition to P. obscuriceps (Stål, 1858) occurring in northern Russian Far East and eastern Siberia and associated with Salix spp. ( Salicaceae ) ( Kerzhner 1988).

In Japan and adjacent regions, the fauna of Plagiognathus species has previously been the subject of several recent works, and the known congeners were considered to occur predominantly in the temperate or colder climatic zones ( Duwal et al. 2016; Yasunaga & Takai 2016; Yasunaga 2022). Nonetheless, two uniquely thermophilic species (described below) are now confirmed from the warm temperate zone in southwestern Japan as well as subtropical zone in Taiwan.

As pointed out by Schuh (2001), many Plagiognathus species are host-specific and arboreal. However, most of the East Asian congeners were confirmed to be associated with dicot herbaceous plants ( Yasunaga 2001; Duwal et al. 2016), except for P. pini (a specialist of the boreal creeping pine, Pinus pumila ). Several members (e.g., P. arbustorum ) were also documented to prey on aphids or psyllids ( Wheeler 2001). A univoltine life cycle is assumed for these East Asian species, but collection records suggest that a new species from Taiwan, P. chengshingi sp. nov., may have two or more generations per year. Although Kerzhner (1988) reported Artemisia vulgaris ( Asteraceae ) as the host plant of P. amurensis in Russian Primorsky Territory, it does not appear the breeding host of this hop-inhabiting phyline ( Yasunaga 2001; Duwal et al. 2010).

Key to species known in Japan and Taiwan

1. Head and pronotum wholly pale yellow-green (fading to pale brown or stramineous brown in dry-preserved specimens); antennal segment II pale brown, sometimes with dark extreme base [if body widely pale then antennal segment II uniformly darkened: pale variant of P. yomogi (cf. Fig. 1L )] .................... ................................... P. chrysanthemi (Wolf, 1804)

– Head and at least anterior part of pronotum fuscous, brown or reddish brown; antennal segment uniformly darkened ............................................................. 2

2. Currently assumed to be endemic to Taiwan; ventral color pattern on metafemur as in Fig. 1C–E ............ .............................................. P. chengshingi sp. nov.

– Known from Japan (and temperate or colder climatic zones in China, Korea and Russian Far East) ......... 3

3. Habitat restricted to Pinus pumila at alpine or boreal zone (currently restricted to Taisetsu Mountains in Hokkaido) .......................... P. pini Vinokurov, 1878

– Associated with herbs or shrubs of dicot angiosperms .................................................................... 4

4. Total body length more than 4 mm; dark spots on metafemur arranged in rows or sometimes partly fused (cf. Fig. 4 ) .......... P. collaris Matsumura, 1911

– Body less than 4 mm (mostly <3.7 mm) in total length; metafemoral dark spots distributed randomly ............................................................................. 5

5. Antennal segment III as long as or shorter than head width across eyes; thermophilic species currently restricted to coastal zones of western Kyushu ............ .......................................... P. marinoccidens sp. nov.

– Antennal segment III longer than head width across eyes; known from temperate and cold temperate zones ....................................................................... 6

6. Dorsum uniformly fuscous, shining; cuneus usually uniformly fuscous; metafemoral dark spots smaller, rather sparsely distributed; associated with Artemisia spp. ............................ P. yomogi Miyamoto, 1969

– Dorsum not much shining, variable in color, pale brown or reddish brown [if dorsum widely darkened, then anterior and posterior margins of cuneus pale brown or yellow-orange]; anterior margin and apex of cuneus (sometimes whole cuneus) pale; dark spots on metafemur larger, densely distributed; Humulus japonicus ......................... P. amurensis Reuter, 1883

Figures

Fig. 1.Dorsal (A–B, D, J) and left lateral (C, E−I, K−L) habitus images of Plagiognathus spp.A. P. chengshingi sp. nov., holotype, ♂. B−C. P. chengshingi sp. nov., paratype, ♂. D−E. P. chengshingi sp. nov., paratype, ♀. F. P. amurensisfrom Kume Town, Okayama, Japan. G. P. amurensis from Taebaek, Kangwon-do, Korea. H. P. marinoccidens sp. nov., holotype, ♂. I. P. marinoccidens sp. nov.,paratype, ♀. J. P. yomogi, ♂ from Hokkaido, Japan. K. P. yomogi, ♀ from Hokkaido. L. P. yomogi, pale variant ♀ from Yamagata, Japan.

Fig. 2. Habitus images of living individuals of Japanese Plagiognathus spp. A−B. P. marinoccidens sp. nov., ♂. C−D..P. marinoccidens sp. nov., ♀. D. P. marinoccidens sp. nov., ♀. E. P. chrysanthemi, ♀. F. P. collaris, ♂. G. P. collaris, ♂, pale variant. H. P. yomogi, ♂. I. P. yomogi, ♀. J. P. yomogi, ♂. K. P. pini, ♂. H–I. Courtesy of M. Takai.

Fig. 6. A–D. Male genitalia (vesica) of Plagiognathus amurensis from Suwon, Korea. E–H. P. marinoccidens sp. nov. from Nagasaki, Japan. Abbreviations: see Material and methods.

Fig. 8. A–C, L–O. Scanning electron micrographs of ♂ genitalia. D–K. Scanning electron micrographs of ♀ genitalia. A–G. Plagiognathus chengshingi sp. nov. H–N. P. yomogi. O. P. amurensis. A, L. Genital segment (pygophore) with phallotheca and parameres, dorsal view. B−C, M−O. Vesica. D, H. Genital chamber, dorsal view. E−F, I−J. Posterior wall. G, K. Apex of ovipositor (gonapophysis I). Abbreviations: see Material and methods.

Fig. 9. A−F, M−O. Scanning electron micrographs of ♂ genitalia. G–L. Scanning electron micrographs of ♀ genitalia. A–I. Plagiognathus marinoccidens sp. nov. J–O. P. amurensis. A, M–N. Genital segment (pygophore) with phallotheca and parameres, dorsal view.B. Phallotheca. C. Left paramere. D−F, O. Vesica.G, J. Genital chamber, dorsal view. H−I, K−L. Posterior wall (I, L for structure of IRS in high magnification).Abbreviations: see Material and methods.

Fig. 7. Female genital chamber of Plagiognathus spp. in dorsal view. A. P. chengshingi sp. nov. B. P. marinoccidens sp. nov. C. P. yomogi. D. P. amurensis. Abbreviations: see Material and methods.

Fig. 4. Metafemoral ventral dark spot pattern of six Japanese Plagiognathus spp.