Unixenus thailandicus, Huynh & Likhitrakarn, 2023

Huynh, Cuong & Likhitrakarn, Natdanai, 2023, First records of penicillate millipedes (Diplopoda, Polyxenidae) from Thailand, with descriptions of two new species, Zootaxa 5383 (4), pp. 514-536 : 526-533

Unixenus thailandicus sp. nov.

Figs 9–15

Material examined. Holotype. adult male (Location D: PM1.02 ): Khao Cha-ang On , Bo Thong District, Chonburi Province, Thailand. 13.181789, 101.663206; elevation 180 m; coll. S. Panha, N. Likhitrakarn, and C. Sutcharit, 23 May 2007 . Paratypes. 1 male and 4 females, same data, as holotype. Five specimens including the holotype were used for DNA extraction and the preparation of slides, while a female was used for SEM imaging.

Diagnosis. Body length: 2.8–3.0 mm. Posterior vertex trichome sockets with three rows. Antennomere VI with three thick bacilliform sensilla, setiform sensillum between two thick bacilliform sensilla anteriorly, conical sensillum located posteriorly. Labral surface with spinous spherical papillae gradually reducing in size toward posterior margin. Lateral palp of gnathochilarium with 22 conical sensilla in female, 24 in male, and 2.5 times longer than medial palp. Tergal trichome rows form two latero-posterior oval shaped clusters either side of midline, with central gap. Tarsus 2 with setiform seta. Telotarsus with posterior lateral process equal to half-length of the claw, anterior setiform process longer than the claw.

Description. Holotype male body: 2.8 mm; paratypes: 2.8–3.0 mm in both sexes; caudal bundle: 0.7 mm, nest trichomes: 0.2 mm.

Colouration. The ethanol-preserved specimens had lost tergal trichomes, and the original colour faded. The body is yellowish-brown; the trichomes of the latero-posterior rosette on tergites, and the last one, are the darkest black. A broad caudal bundle of trichomes in female silvery black colour and the ventral side of the body white ( Fig. 9 ).

Head: Holotype, post vertex trichomes (pv) formed 3 rows of equal-sized trichome sockets: anterior row, oblique, with large sockets reducing sizes at both ends; middle and posterior rows, undefined, with various socket sizes. Narrow space between anterior and middle rows. Holotype with 17 sockets in anterior row, 7 in middle row, and 5 in posterior row (L); 18 sockets in anterior row, 10 in middle, and 3 in posterior row (R) ( Figs 10A, B ). Paratypes showed various numbers of posterior vertex trichome sockets on both sides, with 16–18 sockets in anterior rows, 7–8 sockets in middle and 3–4 sockets in posterior rows. Clypeo-labrum: Holotype, labral surface with spinous spherical papillae (Psp) gradually reducing in size toward posterior margin. 11 setae at posterior margin of labrum (se) (7 setae and 4 sockets with missing setae), paratypes with 10–11 setae. These setae shorter in length than labrum width. Lateral lamella (Ll) on each lateral side, and two broad lamellae (l) each side of a median cleft (mc) ( Figs 10E , 15B ). Gnathochilarium: Lateral palp 1.5 times medial palp; 24 conical sensilla on the lateral palp and 22 on the medial palp ( Fig. 10D ). paratypes: same 24 in males and 22 in all females. Antennae: Holotype, antennomere VI with three thick bacilliform sensilla, a setiform sensillum between two thick bacilliform sensilla Ti and Ta (Ta, anterior, Ti, intermediate, Tp, posterior position), conical sensillum in posterior position ( Figs 11A, C ; 14A, B, D ). Right article VI with extra sensillum (T*) forming 4 thick bacilliform sensilla in arrangement ( Fig. 11D ). All paratypes showed typical three thick bacilliform sensilla, the holotype had an extra fourth thick bacilliform sensillum which was atypical. Antennomere VII with conical sensillum in posterior position, followed by 2 thick bacilliform sensilla (Ta and Tp), setiform sensillum (s) in between these sensilla ( Figs 11A, B , 14A–C ).

Body: 10 segments, 9 pleural projections, excluding telson and caudal bundle; 13 pairs of legs. Collum with a pair of lateral protuberances (Lp), each with 7 trichome sockets ( Figs 12A , 15A ), paratypes with 7–9 trichome sockets. Tergites 2 (T2) to 10 (T10) with a pair of pleural projections (pp) located antero-laterally. Tergal trichome rows form two latero-posterior oval shaped clusters either side and a posterior row slightly upwardly curved towards the centre with a small gap between these trichome socket rows ( Figs 12B , 15A ); tergite T10—last tergite, trichome sockets dense and smaller ( Fig. 12C ). Legs: naming of leg segments after Manton [1956]. Legs 1 and 2 without trochanter; leg 1 without tarsus 1. Chaetotaxy: coxa (co) 1 and 2: 2–4 biarticulated setae with spinous projections, coxae 3–13: 3–5 biarticulated setae with spinous projections; Last sternite with 4 similar setae ( Fig. 13G ); pre-femur (pf), post-femur (pof), tibia (tib) with 1 setiform sensillum, except femur with 1 biarticulated setae with spinous projections and setiform sensillum at intermedio-posterior position; tarsus 1 (T1) without seta, and T2 (T2) with a setiform sensillum ( Figs 13A–E ).

Telotarsus: Claw slender with anterior lateral process (a), posterior lateral process (p) equal to half the length of the claw (c); anterior setiform process (s) longer than the claw; lamella process (la) present ( Figs 13F , 15C, D ). Sex organs in the male: A pair of penes on coxae 2 ( Fig. 13A ) and two pairs of coxal glands located on the 8th–9th coxal plates.

Caudal bundles: Similar to congeners ( Huynh & Veenstra 2021) and classified as caudal bundle type I arrangement ( Condé & Nguyen Duy-Jacquemin 2008). Caudal trichomes with a series of 2–6 backward hooks ( Huynh & Veenstra 2013).

Telson: Dorsal ornamental trichome sockets symmetrically arranged either side of telson; Holotype with 6 trichome a sockets on each side of the telson (paratypes with 5–7). Trichome a sockets form row with various socket sizes, one trichome b socket, 3 large trichome c sockets with protruding base: c1, c2 and c3, forming triangular shape located in ventroposterior positions in each arrangement, circular d apparent on each side near the exterior side of trichome c ( Fig. 13H ).

Remarks: Unixenus thailandicus sp. nov. has characteristics shared by all species within the genus Unixenus ( Polyxenidae ). These characteristics include the typical arrangement of sensilla on the antennal articles, labral structures, and the presence of a setiform sensillum on tarsus 2 of the leg. However, U. thailandicus sp. nov. exhibits a distinct difference from its congeners in terms of the number of conical sensilla on the lateral palp of the gnathochilarium. While most Unixenus species typically have 13 sensilla, the presence of 24 conical sensilla in males and 22 in females suggests that the new species may belong to a new genus. It is important to note that due to the limited number of specimens and the degradation of DNA in the preserved specimens, genetic information is lacking, which hinders the comparison with sequences of other species in the genus.

Etymology: Unixenus thailandicus sp. nov. is named after the country where the new species were collected.

Figures

FIGURE 9. The alcohol preserved Unixenus thailandicus sp. nov. from Thailand.

FIGURE 10. Unixenus thailandicus sp. nov., holotype. A. The head capsule. B. The enlarged images of posterior vertex. C. Trichobothria. D. Right gnathochilarium. E. The clypeo-labrum. (Explanation of figure labels in the text)

FIGURE 11. Unixenus thailandica sp. nov., holotype. A. Antennomeres VI–VIII (left antenna). B. Sensilla on antennomere VII. C. Sensilla on antennomere VI. D. Antennomere VI (right antenna): four bacilliform sensilla with an extra Ti*.

FIGURE 12. Patterns of trichome sockets of Unixenus thailandicus sp. nov. A. The collum, and lateral protuberances. B. Tergites 2 with pleural projections. C. Tergite 10.

FIGURE 13. Unixenus thailandicus sp. nov., holotype. A. Second leg with the chaetotaxy. B. Biarticulated setae with spinous projections. C & D. The setiform setae. E. A seta on tarsus 2. F. Telotarsus and its processes; a setiform is longer than the claw (dot line indicated). G. The last sternite. H. Ornamental trichome sockets.

FIGURE 14. SEM images of Unixenus thailandicus sp. nov. A & B. Antennomeres IV–VIII, and the sensilla. C. Sensilla on antennomere VII. D. Sensilla on antennomere VI.

FIGURE 15. SEM images of Unixenus thailandicus sp. nov. A. Body structures. B. The clypeo-labrum. C. Telotarsus (lateral view), and the setiform process that is longer than the claw. D. Telotarsus (ventral view), and the setiform that appears to be cut off.