Table 4. Main morphological differences between females of the species of Petrobiellus. Morphological charactersP. kusakini Kaplin, 1980P. curvistylis Uchida, 1954P. sachalinensis sp. nov.General body colouryellowishrufouswhitish or slightly yel- lowishColour of scales on body surfacedark brownishbrownishwhitish or light gray, with several relatively large spots of brown scalesDistribution of hypodermal pigmenthead, maxillary palps, mandibles, maxillae, thorax and legs highly pigmentedscapus and pedicellus of antenna, trochanter, femur and tibia of all legs with blackish purple pigmenthead and tarsi with brown or brown-violet hypoder- mal pigment of weak or medium intensityColour of paired ocellidark, without white borderingdarkbrown, with white bor- deringShape of paired ocellipistiliform, strongly nar- rowed in the middle partspistiliform, either of them almost separated in mid- dle into two partsalmost sole-shapedRatio of length to width of com- pound eye1.091.00–1.021.13–1.15Ratios of lengths of apical and pre- ceding articles of maxillary palps0.830.570.60–0.62Ratios of lengths of 5th and 4th articles of maxillary palps1.201.101.04–1.06Number of spines on 2nd tarsomere16–26?4–7Ratio of length of styli to width of coxae1.611.141.05–1.09I Ratios of lengths of urosterni- II te and urocoxite VII0.32 0.39 0.530.29 0.52 0.400.35 0.69 0.67Ratios of lengths of stylus II (without spine) and urocoxite VII0.91 0.770.77 0.610.85 0.74Posterior angle of urosternite II128°108°118°Number of ovipositor divisions55?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.

Published as part of Kaplin, V. G., 2020, A new species of bristletails of the genus Petrobiellus (Microcoryphia: Machilidae) from Sakhalin, pp. 20 in Zoosystematica Rossica (Zoosyst. Rossica) 29 (1) on page 20, DOI: 10.31610/zsr/2020.29.1.17, http://zenodo.org/record/10124220