Unixenus intragramineus, Huynh, 2018

Huynh, Cuong, 2018, Two new species of penicillate millipedes (Diplopoda, Polyxenidae) from Phu Quoc Island in southern Vietnam, Zootaxa 4402 (2), pp. 283-302 : 285-292

Unixenus intragramineus sp. n.

( Fig. 2 )

Holotype. Male, Tranh River shore (Cua Lap hamlet, Duong To commune, Phu Quoc Island , Vietnam), 10°10'22.82"N, 103°58'08.14"E; elevation 5 m; the specimen was collected by the first author from within the stem of a creeping grass ( Poaceae ) ( Fig. 3 ) growing within the intertidal zone on the beach; on 1 st July, 2015. Queensland Museum accession number: QMS 108520 (deposited in Queensland Museum, Brisbane, Australia).

Paratypes. 4 males and 6 females were collected in the same place and on the same date as the holotype. Queensland Museum accession numbers of 10 paratypes: QMS 108521–108530 (deposited in Queensland Museum, Brisbane, Australia) .

Etymology. The species is named intragramineus, noun in apposition, as it was found living in the stem of a creeping grass growing in the intertidal zone.

Diagnosis. U. intragramineus sp. n. has the general characteristics of Unixenus and it shares most characters with U. mjoebergi . Chaetotaxy: one seta present on prefemur and two setae on femur, a single seta on tibia and tarsus 2. The claw of U. intragramineus sp. n. is different from that of U. mjoebergi in being slender, with a posterior lateral process longer by half the length of the claw. A small anterior lateral process is present, anterior setiform process longer than the claw and with enlarged base.

Description. Measurements: Holotype male body length 1.7 mm, males (paratypes) ranging from 1.7–2.0 mm (n = 10) and females (n = 10) from 2.2–2.5 mm. Caudal bundle of male slightly narrower in width and longer (0.6 mm) compared to female caudal bundle, the latter wider and shorter (0.5 mm).

Colouration: Head dark brown in eye area connected to 2 brown transverse bands in vertex area. Body milky white in colour, contrasting with light brown pleural trichomes and darker colour in caudal bundle; dark brown marks on latero-posterior rosette trichomes forming a dark band along each side of body laterally; last tergite darkest in colour. Ventral side of body white in colour ( Fig. 2 ).

Head: 8 ommatidia on each side: 4 dorsal, 4 lateral (1 anterior, 2 medial and 1 posterior). Vertex with 2 posterior trichome groups and a large gap between them, each group with 2 rows. Anterior, oblique row with trichome sockets of similar size. Posterior row with slightly smaller trichome sockets. A narrow space between anterior and posterior rows ( Figs 4A and 5A ). Holotype with 13+13 trichome sockets in anterior row and 7+7 trichome sockets in posterior one. Paratypes with variations common in this species regardless of sex, from 10–16 (anterior rows) and 5–8 (posterior rows). Trichobothria: trichobothrium a (posterior position) and trichobothrium b (lateral position) typically thin sensory hairs with narrow cylindrical funicles; trichobothrium c (anterior position) similar in structure to trichobothria a and b, but its funicle slightly enlarged distally. All these trichobothria (a, b and c) equal in socket size and forming an isosceles triangle with equal distance between ab and bc ( Figs 4E and 5B ).

Antennae: 8 articles and 4 sensory cones ( Fig. 6A ), characteristics typical of Polyxenidae . Antennal article VI with 3 thick bacilliform sensilla (T) of different lengths: a short sensillum anteriorly (T1), the longest intermediate sensillum (T2), and a sensillum of medium length (T3). A setiform sensillum (s) present between T1 and T2, and a conical sensillum posteriorly (c) ( Fig. 6C ). Antennal article VII with 2 thick bacilliform sensilla (T), anterior one being slightly longer than the one located posteriorly, with one setiform sensillum (s) between them and a conical sensillum (c) located in posterior position ( Fig. 6B ). This pattern of sensilla on the antennal article VII is commonly seen in all Unixenus species.

Clypeolabrum: Holotype with 10 setae at posterior margin, these setae being half the width of labrum. Setae in paratypes ranging from 10–15 in both sexes. Labral surface with spherical papillae with pointed ends, reduced in size posteriorly. Anterior margin of labrum with 4+4 lamellae on each side of median cleft ( Figs 4G and 5C, D ).

Lateral palp of gnathochilarium: Lateral palp 2.5 times as long as medial palp. 13 conical sensilla on lateral palp, medial palp with 22 slender conical sensilla observed in all paratypes and holotype ( Fig. 4F ).

Trunk: Body with 10 segments, 9 pleural projections, excluding both telson and caudal bundle; 13 pairs of legs. Collum (tergite 1, smallest) with trichome sockets arranged in 2 oval shapes laterally, opposite each other connected by a posterior row of trichome sockets forming a line with a gap in middle. Collum, a tergite only with lateral protuberances, bearing a small number of trichome sockets. In holotype, collum with 46 (L) or 48 (R) trichome sockets, lateral protuberances with 5 trichome sockets on each side ( Fig. 4B ). Trichome sockets on collum varying in paratypes and ranging from 42–60, trichome sockets ranging from 4–7 on lateral protuberances. All other tergites, from 2 to 10, each with a pair of pleural projections located anterolaterally. Tergal trichome socket arrangements from tergites 2 to 9 typically with 4 undefined rows of tergal trichomes arranged in two lateroposterior clusters, either side of midline. Anterior row often uneven, intermediate rows rarely in defined rows and posterior row of trichomes continuous and evenly distributed, forming a line with a medial gap. Tergite 10 exceptional in trichome sockets being smaller and denser. Almost no space between lateral rosette trichome sockets and those in posterior row. Trichome sockets of tergite 2 in holotype numbered 52 (L) and 54 (R) ( Fig. 4C ), tergite 10 with 50 (L) and 49 (R) ( Fig. 4D ). In contrast, trichome sockets on tergite 2 in paratypes ranging from 46–67, whereas tergite 10 containing between 53–66 trichome sockets.

Legs: Leg segments are named following Manton (1956). Legs 1 and 2 without trochanter, leg 1 lacking tarsus 1 as well. Chaetotaxy: coxa 1: 2 setae, coxa 2: 2 setae, coxae 3–13: 3–4 setae; prefemur, tibia and tarsus 2 with 1 seta; femur with 2 setae ( Fig. 7A ). Setae on coxa and prefemur, and at distal edge of femur: bi-articulated with longitudinal ridges on basal funicle, each ridge extending distally into a long and thin spine, with the spines surrounding a base of flagellum ( Fig. 7B ); seta of mid femur similar, but smaller ( Fig. 7C ), tibia and tarsus 2 each with a setiform seta ( Figs 7D and 7E ). Posterior edge of last sternite with 2–4 setae similar to those present on coxa, number of these setae varying; two present in holotype and 2–4 in paratypes. Sex organs in male: A pair of penes present on coxae 2 and 2 pairs of coxal glands located on coxal plates of legs 8 and 9.

Telotarsus-Claw: A slender structure bearing a posterior process, the latter longer by half the length of claw. Both a small anterior lateral process and a lamella process present, anterior setiform process enlarged at base and longer than the claw ( Fig. 7F ).

Telson: Dorsal ornamental trichome sockets arranged symmetrically with 4 sockets of trichomes a in holotype; paratypes with 5–7 sockets of trichomes a. A single trichome b and 3 large protruding base sockets of trichome c: c 1, c 2 and c 3, forming a triangular shape, on each side of telson. Circular indentation d apparent near exterior side of trichomes c ( Fig. 7G ).

Caudal bundles: In males, caudal bundle comprised of a group of caudal trichome sockets forming a single structure of uniform size; this structure split ventrally with trichome socket-free tissue and extending towards the centre with a small gap dorsally. Three rows of the largest sockets of barbate trichomes forming slightly uneven lateral rows, the latter extending towards the centre of caudal structure ( Fig. 5E ). In females, caudal bundle different from those in males, with two obvious distinguishing features: caudal bundle structure, the main dorsal structure similar to that of male, and 2 laterosternal bundle structures with smaller sockets of nest trichomes. These smaller sockets located internally and surrounded by 2 rows of caudal trichome sockets externally. Trichome socket-free tissue present ventrally, extending with a small gap and connecting to central tissue where some barbate trichome sockets present. No separation between caudal and nest trichome sockets, all fused into a unified structure of a single caudal bundle structure ( Fig. 5F ). Arrangement of caudal bundle of U. intragramineus sp. n. similar in structure to that observed in U. broelemanni ( Condé and Nguyen Duy-Jacquemin, 1992) , classified as type II.

Remarks. Unixenus species show very similar taxonomically important morphological characteristics: the number of sensilla on antennal article VI, the structure of the gnathochilarium and the chaetotaxy. Caudal bundle type II is common to the genus Unixenus , as well as in other genera in the subfamily Monographinae ( Condé and Nguyen Duy-Jacquemin 2008) . If only these characteristics are used for identification, it can be difficult to confidently identify Unixenus specimens to the species level. The telotarsus has proved to be a useful additional structure that can help distinguish between species in this genus. Unixenus intragramineus sp. n. has characteristics that are very similar to those of U. mjoebergi ; both species have the same chaetotaxy which makes separating these species difficult. However, the structure of the telotarsus in these two species differs. U. mjoebergi has a robust claw, while the claw of U. intragramineus sp. n. is a slender structure. U. intragramineus sp. n. is the only penicillate millipede found living within a grass stem, which is a highly specialized habitat that provides a food source and a refuge from the intermittent tidal inundation.

Figures

FIGURE 2. Unixenus intragramineus sp. n., A. a digital image and B. a SEM image.

FIGURE 3. The typical beach habitat of Unixenus intragramineus sp. n. lacking leaf litter. This species was found living within the stem of a creeping grass (Poaceae) growing on an unstable, sandy substrate within the intertidal zone of Tranh River beach, where it is subject to intermittent tidal inundation and is often covered by sand particles. A. The foreshore where U. intragramineus sp. n. was found, the white arrow indicates the grass habitat; B. The grass species showing an aggregation of its runners forming a small clump; C. The grass stems were buried in sand in a beach habitat where no leaf litter was present.

FIGURE 4. Holotype Unixenus intragramineus sp. n., A. Head capsule with 8 ommatidia on each side, the posterior vertex trichome groups (pv) and 3 sockets of trichobothria (trichobothria a, b and c). B. Collum (Co) and lateral protuberances (Lp) showing the arrangement of trichome sockets; C. Tergite 2 (T2); D. Tergite 10 – the last tergite; E. trichobothria a, b and c (right hand side) (trichobothrium a, posterior position, trichobothrium b, lateral position and trichobothrium c, anterior position); F. Gnathochilarium: the lateral palp (LP) with 13 sensilla and medial palp (MP) with 22 sensilla; G. Labrum showing the pointed spherical papillae on its surface and the structures 4 + 4 lamellae.

FIGURE 5. Scanning Electron Microscope images of Unixenus intragramineus sp. n.: A. Head, the posterior vertex trichomes group (pv), collum (Co) and tergite 2 (T2); B. Trichobothria a, b and c; C. Labrum showed 4 + 4 lamellae (l); D. Labrum with the pointed spherical papillae (Psp) on its surface; E. The male caudal bundle structure showing the dorsal ornamental trichome sockets (ots), barbate trichome sockets (bts), trichome socket free tissue (tsft) and caudal trichome sockets (cts) with some caudal trichomes (ct) attached. F. The caudal bundle structure of females showing a difference in the arrangement with caudal trichome sockets (cts) located dorsally, barbate trichome sockets (bts) with the largest size in the centre, trichome socket free tissue (tsft) and nest trichome sockets (nts) that were smaller located centro-ventrally.

FIGURE 6. A. Unixenus intragramineus sp. n., scanning electron microscope image of the antennal articles VI, VII and VIII with 4 sensory cones (sc); B. Arrangement of sensilla on the antennal article VII from the holotype; C. Sensilla on the antennal article VI. (c: conical sensillum, T: thick bacilliform sensillum, s: setiform sensillum)

FIGURE 7. A. Holotype Unixenus intragramineus sp. n., the 2nd leg showing seven leg articles (Co: coxa, pre.fe: prefemur, fe: femur, post.fe: post-femur, tib: tibia, T1: tarsus 1, T2: tarsus 2, C: claw) and sex organ – penis (P); (B) A biarticulated seta with a ridged funicle present on the coxa, prefemur, the distal edge of femur and the posterior edge of last sternite; a similar smaller version of a biarticulated seta with a ridged funicle is present in the middle of the femur (C); seta on tibia (D); the setiform seta on tarsus 2 (E). F. Telotarsus–Claw showing all processes: the anterior lateral process (lp), the posterior process (p), the claw (c), the setiform process (s) and the lamella process (l), SEM image of the telotarsus (right hand side). G. The dorsal ornamental trichome sockets of trichomes a (a), trichome b (b), trichomes c (c) and the circular indentation d.