Petrobiellus sachalinensis, Kaplin, 2020

Kaplin, V. G., 2020, A new species of bristletails of the genus Petrobiellus (Microcoryphia: Machilidae) from Sakhalin, Zoosystematica Rossica (Zoosyst. Rossica) 29 (1), pp. 17-22 : 18-22

Petrobiellus sachalinensis sp. nov.

( Figs 1–13 )

Holotype. Female (slide-mounted), Russia, Sakhalin Prov., western shore of northern Sakhalin I. , near Aleksandrovsk-Sakhalinsky , 50°54′N 142°09′E, shoreline, under stones, 8.VIII.2019, V. Kaplin leg.

Paratypes. 11 females (1 female on slide), same data as for holotype .

Description. Female. Body length (not including antennae): 8.4–11.6 mm; body width: 2.9–3.1 mm; cercal length: 5.5–5.7 mm; total width of compound eyes: 0.94–1.00 mm; eye length: 0.54– 0.57 mm; paired ocelli width: 0.49–0.51 mm, length: 0.17–0.18 mm. Middle and hind coxal styli length: 0.60–0.65 mm. Ovipositor length: 3.35– 3.50 mm. General body colour whitish or slightly yellowish, almost without pigment. Antennal base, frons, lateral parts of clypeus, basal part of first and third articles of maxillary palps, mandibulae, hypopharynx, and first, third and basal half of second tarsomeres with brown or brown-violet pigment of weak or medium intensity. Maxillary and labial palps, flagellum and pedicellus of antennae, legs, coxal and abdominal styli without scales. Scapus with very sparse scales. Colour of scales whitish or light gray. Metanotum also with one pair, urotergites with three pairs of large spots of brown scales ( Fig. 1 ). Antennae very long, 1.5– 1.9 times as long as body. Distal chains of flagellum divided into 14–23 annuli ( Fig. 2 ). Clypeus without long thin bristles. Cercus approximately 0.48– 0.55 times as long as body, with about 21–24 divisions. Apex of cercus with one apical spike ( Fig. 3 ). All divisions of cerci, except for apical one or two, with 1–3 colorless spines on inner side supporting cerci above substrate. Divisions in middle part of cerci also with one outer lateral spine ( Fig. 4 ).

Compound eyes dark, with bluish tinge (in ethanol). Ratio of length to width of compound eye about 1.13–1.15; ratio of contact line to length of eyes about 0.45. Paired ocelli sole-shaped, brown, with narrow white bordering, 2.8–3.0 times as wide as long. Distance between inner margins of paired ocelli 0.13–0.15 and between their outer margins 0.96–0.98 times total width of compound eyes ( Fig. 5 ). Frons clearly convex between paired ocelli.

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Apical article of maxillary palp 0.60–0.62 times as long as preceding one. Fifth article 1.04– 1.06 times as long as 4th article ( Fig. 6 ). Dorsal surface of 7th, 6th and 5th articles of maxillary palp with 13–16, 10–13 and 3 hyaline spines, respectively. Apical article of labial palp fingerlike, 3.3–3.4 times as long as wide ( Fig. 7 ). Mandibles with 2–3 distal teeth ( Fig 8 ). Femora slightly widened. Ratios of length to width of femur, tibia and tarsus as shown in Table 1 . Fore and middle legs shorter than hind legs. Fore and middle tarsus and tibia 0.77 and 0.83 times as long as those of hind legs, respectively. Ratio of length of apical hind tarsomere to total length of hind tarsus about 0.32. Ventral surface of legs without long thin bristles. Fore, middle and hind tarsi and hind tibia with slightly pigmented spines ( Fig. 9 ). Number of spines as shown in Table 2 .

Middle and hind legs with coxal styli. Ratio of length of styli to width of middle and hind coxae 1.03–1.06 and 1.09, respectively.

Urites I, VI and VII with 1 + 1, urites II–V with 2 + 2 eversible vesicles ( Fig. 10 ). Posterior angle of urosternites II, III–V, VI, and VII approximately 118°, 110–112°, 107°, and 120°, respectively. Inner posterior lobes of urocoxites VII protruding. Ratio width to length of protruding lobe about 1.0. Ratios of lengths of stylus (without apical spine), urosternites and urocoxites II–IX as shown in Table 3 . Ratio of lengths of urosternite and urocoxites I about 0.35. Urocoxites IX with 1 + 1 outer sublateral spines.

Ovipositor slender, elongate, significantly protruding beyond apices of styli IX. Anterior and posterior gonapophyses with 52 or 53 divisions ( Fig. 11 ). Four basal divisions of anterior gonapophyses and about 11 basal divisions of posterior gonapophyses glabrous. Apical spines of gonapophyses as long as three apical divisions combined. Distal divisions of anterior and posterior gonapophyses with seven or eight setae (not counting sensory setae and apical spines) ( Figs 12, 13 ).

Males are unknown; probably the species is parthenogenetic.

Comparison. Petrobiellus sachalinensis sp.nov. can be distinguished from its congeners by the following characters: the general body colour whitish or slightly yellowish (vs yellowish or rufous); colour of scales whitish or light gray (vs brownish or dark brownish); body (excluding head and tarsi) almost without pigment (vs significantly pigmented); and paired ocelli almost sole-shaped (vs pistiliform, strongly narrowed in middle parts). The new species also differs from all other species of the genus in the most elongated compound eyes; relatively short coxal styli; long 4th article of maxillary palp; small number of short supporting spines in ventral part of tarsi; ratios of lengths of urosternite and urocoxite, stylus and urocoxite; posterior angle of urosternite; and number of divisions of ovipositor. The main morphological differences of females of P. sachalinensis sp. nov. from those of the other species of Petrobiellus are given in Table 4 . According to the brief description of P. takunagae by Silvestri (1943), the distance between the paired ocelli in this species is about half the length of ocellus, and in P. sachalinensis sp. nov., this distance is almost equal to the length of ocellus. The new species differs from P. takunagae also in femora without pigment and tibiae poorly pigmented (vs femora and tibiae with intensive blackish-violet pigment); and four basal divisions of anterior gonapophyses glabrous (vs about 12 basal divisions of anterior gonapophyses glabrous).

Etymology. The species name is derived from the toponym of Sakhalin.

Habitats. Western shores of the northern part of Sakhalin Island. Under stones near the line of seashore (Fig. 14; see Addenda section).

Figures

Fig. 1. Petrobiellus sachalinensis sp. nov., general view.

Figs 2–9. Petrobiellus sachalinensis sp. nov., holotype (female). 2, distal chain of flagellum; 3, apex of cercus; 4, divisions in middle part of cercus; 5, anterior part of head (vertex, compound eyes, paired ocelli, frons, antennae bases, median ocellus, clypeus, and labrum); 6, maxillary palpus; 7, labial palpus and labium (part); 8, apex of mandible; 9, fore leg (part). Scale bar: 0.1 mm.

Figs 10–13. Petrobiellus sachalinensis sp. nov., holotype (female). 10, urosternite and urocoxites (part) V; 11, urocoxites VIII (part) and IX, with posterior gonapophyses; 12, distal part of anterior gonapophy- ses; 13, distal part of posterior gonapophyses. Scale bar: 0.1 mm

Tables

Table 1. Ratios of the length to width of the leg parts in females of Petrobiellus sachalinensis sp. nov.

Segments	fore	Pair of legs middle	hind
Femur	2.64–2.83	2.68–2.74	2.83–2.91
Tibia	3.00–3.22	2.82–3.12	3.68–3.74
Tarsus	6.50–6.67	6.40-6.65	9.12–9.36

Table 2. Number of spines at the legs in females of Petrobiellus sachalinensis sp. nov.

Segments	fore	Pair of legs middle	hind
1st Tarso- meres 2nd	2 4	2 6	4–5 7
3rd	0	0	0
Tibia	0	0	1–2
Femur	0	0	0

Table 3. Ratios of the lengths of some abdominal structures in females of Petrobiellus sachalinensis sp. nov.

Urites	Urosternite / urocoxite	Stylus / urocoxite	Apical spine / stylus
II	0.69	0.85	0.34
III	0.71	0.84	0.34
IV	0.70	0.81	0.34
V–VII	0.67–0.68	0.74–0.76	0.36–0.37
VIII	–	0.86	0.31
IX	–	0.78	0.28

Note. Length of the stylus does not include apical spines.

Table 4. Main morphological differences between females of the species of Petrobiellus.

Morphological characters	P. kusakini Kaplin, 1980	P. curvistylis Uchida, 1954	P. sachalinensis sp. nov.
General body colour	yellowish	rufous	whitish or slightly yel- lowish
Colour of scales on body surface	dark brownish	brownish	whitish or light gray, with several relatively large spots of brown scales
Distribution of hypodermal pigment	head, maxillary palps, mandibles, maxillae, thorax and legs highly pigmented	scapus and pedicellus of antenna, trochanter, femur and tibia of all legs with blackish purple pigment	head and tarsi with brown or brown-violet hypoder- mal pigment of weak or medium intensity
Colour of paired ocelli	dark, without white bordering	dark	brown, with white bor- dering
Shape of paired ocelli	pistiliform, strongly nar- rowed in the middle parts	pistiliform, either of them almost separated in mid- dle into two parts	almost sole-shaped
Ratio of length to width of com- pound eye	1.09	1.00–1.02	1.13–1.15
Ratios of lengths of apical and pre- ceding articles of maxillary palps	0.83	0.57	0.60–0.62
Ratios of lengths of 5th and 4th articles of maxillary palps	1.20	1.10	1.04–1.06
Number of spines on 2nd tarsomere	16–26	?	4–7
Ratio of length of styli to width of coxae	1.61	1.14	1.05–1.09
I Ratios of lengths of urosterni- II te and urocoxite VII	0.32 0.39 0.53	0.29 0.52 0.40	0.35 0.69 0.67
Ratios of lengths of stylus II (without spine) and urocoxite VII	0.91 0.77	0.77 0.61	0.85 0.74
Posterior angle of urosternite II	128°	108°	118°
Number of ovipositor divisions	55	?	52–53

Note. Petrobiellus takunagae Silvestri, 1943 is not included in the table due to insufficient data in its original description; the characters distinguishing this species from P. sachalinensis sp. nov. are given in the Comparison section after the description of the new species.