Male (n=2): idiosoma 435–448×200–215 (length×width) in size (Fig. 1A). Hysterosoma 270 in length. Prodorsal shield triangular-shape occupying entire prodorsum, weakly sclerotized, posterior margin straight, 139–150 in length along the midline, 145–150 in width of the posterior part (Fig. 2A). Setae c2 filiform, 29–31 in length, the distance between setae se 70–71. Subhumeral setae c3 lanceolate, 25–28×5.5–6. Distance between prodorsal and hysteronotal shields 1–6 in length along the midline. Hysteronotal shield anterior margin convex, weakly sclerotized, surface without ornamentation, greatest length 280–290, width at the level of anterior margin 150–155 (Fig. 2A). Bases between setae d2 and e2 with a pair of strongly sclerotized longitudinal sclerites, 80–87 in length. Opisthosomal lobes bluntly rounded; Setae h2 and h3 located on the posterior margin of these lobes. Setae ps2 situated posterior to setae f2; setae ps2 setiform. Dorsal measurements: c2: d2 100, d2: e2 110, d2: gl 37–67, e2: h3 48–49, d1: d2 52, e1: gl 5–6, h2: h2 46–49, h3: h3 35–36, ps1: ps1 19–22. Transventral sclerite absent, inner ends of epimerites IIIa shaped as an oblique T, epiandrum absent (Fig. 2B). Genital apparatus 24–28×13–18, with genital setae g at the midlevel of this apparatus. Setae 3a slightly located posterior to setae 4b. Adanal shield irregular form, with the anterior end of the anal opening. Diameter of anal suckers 18–19. Ventral measurements: 3a: 4a 43–50, ps3: ps3 30, ps3: h3 50. Tarsus III 60 in length; setae r 3–3.5 times longer than this segment (Fig. 2C). Tarsus IV with 2 acute dorsobasal teeth.

Female (n=3): idiosoma 490–510×195–208 (length×width) in size (Fig. 1B). Hysterosoma 320–355 in length. Prodorsal shield (Fig. 3A): Mostly shaped as in a male, length 150–155, width 145–158. Setae c2 filiform, 32–34 in length; subhumeral setae c3 lanceolate, 26–27×5–6. Paired anterior hysteronotal plates situated at the midlevel of hysterosoma, anterior margins extending to the level of setae c3, posterior margins extending beyond the level of setae e2, usually shaped as large longitudinal bends (severe variation in each individual), greatest length 150–190, greatest width 20–24 (Figs. 3A, 4); setae e1 lateral margins of this plates. Pygidial sclerites small longitudinal shaped, located near the bases of setae h2 and h3, greatest length 13–58, greatest width 8–14. External copulatory tube situated between setae h3, small finger-like. Dorsal measurements: c2: d2 103–113, d2: e2 113–138, d2: gl 82–120, e2: h3 81–90, d2: gl 82–120, h2: ps1 6–8, h2: h2 68–72, h3: h3 54–60. Epigynum semicircular, 63–65 long, 65–75 wide (Fig. 3B). Tarsi III and IV 31–33 and 35–36, respectively.

Neopteronyssus bilineatus was originally described by Mironov [25] based on specimens collected from Y. canicapillus (=D. canicapillus) in Thailand. In genus Neopteronyssus, males of group pici have common characteristics of opisthosomal lobes small rounded, transventral sclerite and epiandrium absent, tarsi IV with 2 small dorsobasal teeth, and females have the same features as unpaired or paired hysteronotal shields situated at the midlevel of opisthosoma and not extending to poster end of the body [25]. Among 5 species in the pici group, N. bilineatus is most similar to N. koreanus (Han et al., 2019) and N. yungipicinus (Mironov, 1987) regarding the external traits. However, N. bilineatus clearly differs from N. koreanus and N. yungipicinus by the following features: in males, the dorsal surface of opisthosoma has longitudinal sclerites, and seta r of tarsus III is 3 times longer than this segment; in females, the anterior margins of hysteronotal sclerotized bands extend to the level of setae cp and the posterior margins of these sclerites extend beyond the level setae e2 [25]. In contrast, in males of N. yungipicinus, the posterior part of hysteronotal shield lacks ornamentation, and the length of seta rIII is 2.5 times shorter than the length of this tarsus; and in females, 2 elongated hysteronotal plates are situated between setae d2 and e2 [25,32]. In addition, hysteronotal shield in males of N. koreanus is weakly sclerotized without 2 longitudinal sclerites, and tarsal seta rIII is 1.5 times shorter than tarsus III; in females, pair of anterior hysteronotal plates are located at the level of setae d1 and usually shaped as irregular rectangles (in few specimens, these plates fused by thin sclerite) [23].

In general, N. bilineatus found in Korea were morphologically consistent with the original description and illustrations of Mironov [25]. However, the anterior margins of hysteronotal plates in Korean females did not stretch to the level of setae d1. Furthermore, hysteronotal plates of all Korean females are highly varied in length and shape. As the original description was based on 2 female samples, an explanation of individual variation was insufficient. Although females of N. koreanus, a closely related species, show variation in the anterior hysteronotal plates, there was no significant difference in the partial sequences of mitochondrial COI gene (cytochrome c oxidase subunit I) utilized for a DNA barcode [23]. Therefore, we considered this difference as an intraspecific variation in females of N. bilineatus. Accordingly, the hysteronotal plate length of female was not considered a suitable attribute to distinguish the species.

In this study, N. bilineatus was discovered for the first time in Korea. Although some species of feather mites in the genus Neopteronyssus share multiple hosts, most of them show host specificity [23,25]. Until now, N. bilineatus has been detected only in Y. canicapillus, distributed in Southeast Asia and Korea [33,34]. Therefore, identification of N. bilineatus was chiefly made by the morphological characteristics of mites, and the distribution of host and host specificity were additionally considered in this study.

Conclusively, it was confirmed for the first time that a species of feather mite, N. bilineatus, is indigenously infested in Y. canicapillus in Korea. Additional studies such as DNA barcoding and morphological comparison of specimens are needed to confirm the intraspecific variation of N. bilineatus from Southeast Asia.