Polyxenus lankaranensis, Short & Vahtera & Wesener & Golovatch, 2020

Short, Megan, Vahtera, Varpu, Wesener, Thomas & Golovatch, Sergei I., 2020, The millipede family Polyxenidae (Diplopoda, Polyxenida) in the faunas of the Crimean Peninsula and Caucasus, with notes on other European Polyxenidae, Zootaxa 4772 (2), pp. 306-332 : 322-325

Polyxenus lankaranensis new species

Figs 6 , 7 ; Map Fig. 1 .

Material examined: Holotype adult female ( VNR 018346-1 ), Azerbaijan, Lankaran seher, Lankaran, N38º47’22”, E48º51’08”, street trees under the bark, 25 Mar. 2015, leg. H. Reip, D. Antić, F. Walther. Specimen mounted on slide, deposited in SMNG . Paratypes: 1 adult female ( VNR 018347-1 ) , 1 subadult male ( VNR 018347-2 ) both mounted on slides , 4 subadult stadium VII and 2 stadium VI in ethanol ( VNR 018347-3 ), all same location as holotype, deposited in SMNG ; 1 female, ZFMK MYR 6089 ( ZFMK-TIS 2553423 ), same location as holotype, genetic voucher . Other material: Russia, 1 adult male mounted on side, 1 immature stadium VI , Dagestan, Buynaksk , N42º49’, E47º07’, moss under bush, 8 May 1983, leg. D. Krivolutsky ( ZMUM) .

Etymology: adjective, named after the town where it was first discovered.

Diagnosis: Very similar morphology to Polyxenus lagurus , differing in size and colouration, especially when freshly collected. Body length of adults less than 2 mm, white in colour except for the trichomes that varied from black (anterior vertex) to pale brown (all remaining trichomes including those of caudal bundle). Medial anterior vertex trichomes shorter and more bulbous with shorter serrations than those of P. lagurus ( Fig. 6I ). COI sequencing shows separation into a distinct clade from P. lagurus . Differing from P. lapidicola / P. macedonicus in only having two posterior medial vertex trichomes, 6 th antennal article twice the length of 7 th article, pleural trichomes with serrations proximally, a differing arrangement and number of trichomes on the collum and lacking globular anterior tergal trichomes. The width to length ratio of the medial trichomes of tergite 4 is 0.3–0.4 in P. lankaranensis sp. n., while it is 0.5–0. 6 in P. macedonicus .

Description: Measurements: Body length after mounting on slides holotype 1.6 mm, width at widest point on body 0.5 mm, caudal bundle 0.3 mm. Paratypes: adult female 1.8 mm, width 0.6 mm, caudal bundle 0.7 mm; subadult male 1.5 mm, width 0.5 mm, no caudal bundle present. Specimens had been preserved in 80 -100% ethanol.

Head ( Fig. 6C ): Six ommatidia on each side of which one antero-ventral. Vertex with one pair of posterior tufts of 20+21 (holotype) 20+20 (female adult paratype) and 19+17 (subadult male paratype) trichomes arranged in two rows ( Fig. 6C ); the distance between each tuft is about a fifth their length. A narrow gap only to anterior vertex trichome patches. Anterior vertex trichomes medially short and slightly bulbous (35–39 µm) with small serrations (in contrast to those of P. lagurus which larger, 90–120 µm, and with more feathered larger serrations) ( Fig. 6I ), gradually becoming more elongate laterally. Three trichobothria arranged in an isosceles triangle with the most internal (immediately adjacent posterior tufts of vertex) much smaller than the other 2 ( Fig. 6C ). Gnathochilarium with lateral palp 2.5 times medial palp, with 12 long sensilla, 17 sensilla on medial palp. Clypeo-labrum with nine setae along posterior margin, seven lamellar plates each side along anterior edge, three rows large papillae anteriorly with remaining labrum covered in papillae gradually reducing in size towards the posterior of the labrum ( Fig. 7B ). Antennae: proportions of antennal articles as in Fig. 6C with 7 th article 2/3–3/4 the length of 6 th article. Details of sensilla based on holotype and adult female paratype. Antennal article VI with 1 thick basiconic sensillum, 6–7 thin basiconic sensilla with 1–2 thin anterior to the single thick sensillum and 5–6 arranged in a crescent to the posterior, one posterior coeloconic sensillum. ( Figs 6B , 7E ); antennal article VII with two thick basiconic sensilla to posterior of 4-6 thin basiconic sensilla arranged in an arc curving distally, one setifom sensillum between the two posterior basiconic sensilla and one posterior coeloconic sensillum ( Fig. 6A , 7E ).

Trunk: Collum with almost symmetrical arrangement of 56 (holotype), 48 (subadult male paratype) barbate trichomes with lateral rosettes of barbate trichomes and three trichomes internally either side. Trichomes forming a single continuous row anteriorly and posteriorly ( Fig. 6D ). Lateral protuberances of collum with 4–6 barbate trichomes. Remaining tergites with two even posterior rows of trichomes. Tergite 2 with 65 trichomes (adult), 44 (subadult male). Trichomes are barbate with posterior row 1/3 longer, anterior row varying in size with medial trichomes shortest (30–40 μm) (6E–G, 7A). The trichomes of the lateral tufts are longer, barbate with serrations to base. Legs ( Fig. 6J ): naming of leg segments is after Manton (1956). Legs 1 and 2 without trochanter, leg 1 also lacks tarsus 1. Trochanter, post-femur and tarsus 1 lack setae. Coxae I with one seta and coxae II with three setae; all other coxae without seta. Prefemora and femora with one seta; these large biarticulate setae having an elongate funicle furnished with a long acute process at apex (7C), single tiny elongate seta on distal edge of each tibia. Tarsus II spine ( Fig. 6J ) longer than telotarsus (ratio 1.2:1). Telotarsus bearing an anterior process with a spinous projection longer than claw, a single large latero–posterior process associated with the claw and posterior lamellar process longer than the claw (7F). Coxal glands in males on leg pairs 8 and 9.

Telson: Medial cluster of 37 (holotype), 25 (subadult male paratype) dorso-medial (ornamental) trichomes with insertion points forming a ventrally pointing triangular shape, with a medial gap ( Fig. 6K ), dorsal to two bundles of caudal trichomes positioned laterally. Ornamental trichomes darkly pigmented, barbate, varying in length, and straight forming a fan-like shape typical of the subfamily Polyxeninae (the genera Propolyxenus , Polyxenus , Typhloxenus ). Longest ornamental trichomes to tip of caudal bundle, hooked trichomes with three or four hooks.

Remarks: Initially the collected specimens of P. lankaranensis sp. n. were examined and identified as a population of very small P. lagurus . The specimens shared most characters except for the tiny size and lower number of trichomes, as well as shorter and more bulbous anterior vertex trichomes. The tiny polyxenids also lacked cuticular pigment and were white in colour except for the trichomes. Genetic analysis confirmed the distinctness of this species. Due to their size, this species is no doubt being under-collected. There are two very similar small Polyxenus species already described: Polyxenus lapidicola Silvestri, 1903 , and Polyxenus macedonicus Verhoeff, 1952 . The former species, collected near Portici, Italy was no more than 2 mm in length, and incompletely described. In describing P. macedonicus, Verhoeff (1952) compared the two species and size was not mentioned, so one must assume this was not a point of difference and later literature, in which specimens initially identified as P. lapidicola were reassigned to P. macedonicus , makes no mention of any size differences ( Condé 1953). It is unfortunate that the types appear to be lost (Nguyen Duy-Jacquemin pers. comm.) as it is possible the figures ( Silvestri 1903), from which the diagnostic differences have been determined, are misleading. Condé (1953) commented that the shape of the trichomes illustrated in the description of the species ( Silvestri 1903, Fig. 4 ) is unlikely to be correct and does not agree with the shape of trichomes in figure 2 ( Silvestri 1903). However, it should be noted that trichome shape is often not a good diagnostic character on its own as even in Polyxenus lagurus some trichomes in anterior rows of tergal trichomes can be distinctly globular ( Humbert 1893, plate XII).

Figures

FIGURE 1. Map to show localities of all specimens identified in this study. Map created using SimpleMappr, http://www.simplemappr.net, (Shorthouse 2010). Legend: Stars = Propolyxenus argentifer (Verhoeff, 1921) comb. n.; purple star = Verhoeff types; red star = identified in this study; orange = formerly identified as P. trivittatus (Verhoeff, 1921); green inverted triangle = Polyxenus lagurus (Linnaeus, 1758); green circle = Polyxenus lankaranensis sp. n.; white circle = Polyxenus sp.; yellow star = Propolyxenus aegeus Silvestri, 1948.

FIGURE 4. Scanning electron microscope (SEM) images of Propolyxenus argentifer (Verhoeff, 1921) comb. n. adult female: A. Dorsal view of head; B. Trichomes on tergite 2. C. labrum; D. Scale-like trichomes on tergites 8, 9 and 10; E. Portion of 8th leg showing setae: biarticular setae on prefemur (a) and femur (b) small setiform seta on tibia (c), spine on tarsus 2 (d); F. Hooked caudal trichome; G. Distal section of right antenna showing sensilla on articles 6, 7 and 8: thick basiconic sensilla (T), thin basiconic sensilla (t), conical sensilla (c), setiform sensilla (s); H. Lateral palp of gnathochilarium; I. Telotarsus showing claw (c) with posterior process (p), anterior spinous process (s). and lamella (l); J. telson showing pattern of insertions of ornamental trichomes. Scale bars: A, D, J = 200 µm; B, C, E = 50 µm; E, F, G, H = 20 µm; I = 10 µm.

FIGURE 6. Polyxenus lankaranensis sp. n. Holotype, adult female; A. Sensilla of left 7th antennal article; B. Sensilla of left 6th antennal article; C. Head; D. Collum; Tergites showing pattern of trichome insertions: E. Tergite 2; F. Tergite 4; G. Tergite 10. I. Comparison of medial anterior trichomes, Left: P. lankaranensis, Right: P. lagurus, adult female, Croatia. J. 9th leg showing pattern of setae and tarsal spine; K. Same species, paratype adult female, pattern of dorso-medial trichomes anterior to caudal bundle. Polyxenus sp., adult male; H. Sensilla of left 6th antennal article. Scale bars: A, B, H, K = 10 µm, C, D, E, F, G = 100 µm, I, J = 50 µm.

FIGURE 7. Scanning Electron Microscopic (SEM) images of Polyxenus lakaranensis sp. n., adult female: A. Dorsal view; B. Labrum; C. Setae on coxa of leg 2; D. Ventral view showing pair of vulvae (see arrow) associated with 2nd pair of legs; E. Distal section of right antenna showing articles 6, 7 and 8; F. Telotarsus showing claw (c) with posterior process (p), anterior spinous process (s) and lamella (l). Scale bars: A, B, D = 200 μm, E = 20 μm, C, F = 5 μm.