Holotype , male (MACN-My 69), from Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Tierra del Fuego, Departamento de Ushuaia, Río Irigoyen, 54.62540; -066.70184 (+/-200 m), elev. 230 m, 20–24 Oct. Berlese funnel and hand collected, A. Porta leg., cleared, preserved in ethanol; • same data, 6 paratypes, 4 males and 2 females (MACN-My 70 to 75), cleared, preserved in ethanol; • 10 paratypes, 7 males and 3 females (MACN-My 76 to 85), cleared, preserved in ethanol; • 1 tube with 4 paratypes (MLP-Ar 20497), not cleared, preserved in ethanol; • 1 tube with 4 paratypes (CADIC-My 01), not cleared, preserved in ethanol; • same data, 3 paratypes (MACN-My 86 to 88), mounted for SEM; • Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Isla Grande de Tierra del Fuego, Departamento de Ushuaia, El Martial, 54.790648, -068.391642 (+/- 100 m); elev. 550 MASL. (+/- 50 m); Berlese funnels, A. Porta leg; • 1 paratype, male (MACN-My 89), mounted on slide; • same data, 1 tube with 2 paratypes (MACN-My 90), cleared, preserved in ethanol; Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Isla Grande de Tierra del Fuego: Departamento de Ushuaia: Bahía Buen Suceso, 54.79349, -065.26397; (+/- 20 m); elev. 12 MASL.; Berlese funnels and hand collecting; A. Porta leg., • 1 paratype male (MACN-My 91), cleared, preserved in ethanol; • same data, 1 paratype (MACN-My 96), mounted for SEM; Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Departamento de Ushuaia: Isla de los Estados: Puerto Hoppner; 54.78344, -064.41387 (+/- 200 m); elev. 20 MASL; Berlese funnels and hand collecting, A. Porta leg., • 3 paratypes, 1 male and 2 females (MACN-My 97 to 99), cleared, preserved in ethanol; same data; • same data, 2 paratypes (MACN-My 100 to 101), mounted for SEM; Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Departamento de Ushuaia: Isla de los Estados: Puerto Parry; 54.81319, -064.37043 (+/- 50 m); elev. 70 MASL; Berlese funnels and hand collecting, A. Porta leg., • 3 paratypes, 2 females and 1 male (MACN-My 104 to 106), mounted; • 3 paratypes, 2 females and 1 male (MACN-My 107 to 109), cleared, preserved in ethanol; • 5 paratypes in 1 vial (MACN-My 110), not cleared, preserved in ethanol; Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Departamento de Ushuaia: Isla de los Estados: Puerto Cook, 54.77711, -064.05438 (+/- 30 m); elev. 40 MASL. (+/-20 m); Berlese funnels and hand collecting, A. Porta leg., • 1 paratype (MACN-My 111), not cleared, preserved in ethanol; Argentina: Provincia de Tierra del Fuego, Antártida e Islas del Atlántico Sur: Departamento de Ushuaia: Isla de los Estados: Puerto de San Juan de Salvamento, 54.753367, -63.889186 (+/- 50 m); elev. 15 MASL; • 2 paratypes, 2 males (MACN-My 112 to 113), cleared, preserved in ethanol; • 4 paratypes in 1 vial (MACN-My 114), not cleared, preserved in ethanol. Chile: Región de Magallanes y de la Antártica Chilena, Provincia de la Antártica Chilena, Isla Navarino, -54.9917, -67.6022, 23 March 2023; hand collecting, under the bark of a dead Nothofagus pumilio tree trunk, L. Fernández leg.,• 3 paratypes, 2 females (MNHNC8446 to 8447), mounted for SEM; • 1 paratype, 1 female (MNHNC8448), preserved in ethanol.

The specific name is a patronymic in honor of Federico Guerrero, captain of the boat “Ocean Tramp,” in recognition of her fundamental contribution to the organization, logistics, and success of the MACN- CADIC 2014 expedition to Península Mitre and Isla de los Estados.

H. audax Clark & Greenslade, 1996 differs from H. guerreroi sp. nov. by a greater length (8.7 mm), longer setae on the tergites, both claws of 12 pairs of similar size, and by the morphology of the cerci; H. conveniens Clark & Greenslade, 1996 differs by the chaetotaxy and morphology of the cerci; H. insequens Clark & Greenslade, 1996 differs by the dorsum of the head with fine pubescence; H. madecassa Aubry & Masson, 1953 differs by the chaetotaxy of the first tergite and the apical antennomere with only 2 spined organs; H. mutila, Adam & Burtel, 1956 differs by the central rod of the head well-marked behind the ovoid knob, where it is produced forward into 2 lateral branches, claws of leg 12 being of similar size, and by the distribution of the outgrowths (microsetae) in the tergites; H. nivea (Scopoli, 1763) differs by the shape of the central rod of the head, the chaetotaxy of the first tergite, and by the relative size of the claws of the 12^th leg pair; H. pluvialis Clark & Greenslade, 1996 differs by the claws of leg 12 of similar size and the tergal setae being more elongated and of different size; H. proxima Adam & Burtel, 1956 differs by the central rod of the head well marked behind the ovoid knob, where it is produced forward into 2 lateral branches and 2 branches delimiting an indistinct posterior area, leg 12 with an elongated anterior seta, and by the first tergite with 2+2 setae; H. pyrethrata Clark & Greenslade, 1996, by the cuticle on the posterior part of tergites glabrous and the apical antennomere with only one terminal spined organ; H. vulgata Adam & Burtel, 1956 differs by the central rod of the head well-marked before the ovoid knob and behind this where it is produced into 2 branches which delimit a posterior area, the presence of additional setae on the styli (particularly in leg 12), and by the different chaetotaxy on the cerci.

Length of body (Fig. 1) without antennae and cerci 3.4 (3.1–5.4) mm.

Head (Fig. 2). Head short, 1.38 (1.05–1.44) broader than long, frontal margin convex with a prominent lateral angle at point of articulation of mandible (Fig. 2A), posterior margin concave with rounded posterolateral angles. Central rod posteriorly ovoid (Fig. 2A, cr), 0.24 (0.19–0.36) the length of the head, other parts of the rod and its branches lacking. Dorsal surface of head (Fig. 2A–C) covered with straight, very thin setae of lengths not significantly different. 3+3 distinct large setae near the antennal base. Longest seta of lateral head angles 0.9 (0.8–0.9) of the length of the diameter of the first antennomere and 3.5× longer than shortest setae. Each anterior plate (m) of the second maxillae (Fig. 2D) with 3 proximal setae, external side almost straight. External-distal corner of these plates with 5 sets of sensilla with a typical chandelier shape (Fig. 2D, arrow) decreasing in size proximally in addition to 2 elongated setae inserted on conic protuberances, the most posterodistal one with a contiguous small tooth; second-maxillary proximal arms (pa) not much wider than anterior plate. Each of the three terminal protuberances (tp) (Domínguez Camacho 2009) distal to transversal groove of the second maxilla (tg) with a characteristic set composed of 2–4 setae inserted on cylindrical cuticular protuberances plus 1 large distal sensillum, which is contiguous to a small tooth. First maxillary palp (Fig. 2D; ml and p, 2F) large, conical, and pointed. Head cuticle glabrous (Fig. 2B). Tömösvary organ circular (Fig. 2E, To) 30–44 µm wide. Both maxillae bearing several setae on surface.

Antennae (Fig. 3). Antennae with (21–38) antennomeres; length (0.37–0.6) of length of body. Distal antennomeres much thinner than proximal antennomeres, diameter of distal antennomere (0.55–0.64) of that of first. First antennomere (Fig. 3A, B) 1.5 (1.4–1.82) × as wide as long with a single apical whorl of 5–6 setae: 2–3 thick-based dorsal setae and 2 inner setae and 0–1 ventral setae. Longest dorsal seta, 0.33 (0.38–0.48) × the diameter of antennomere. Second antennomere (Fig. 3A) 2.16 (1.61–2.6) × as wide as long with 9–10 setae: 6 (5) thick-based setae and 4 thin setae. Third antennomere (Fig. 3A) 1.77 (1.41–1.9) × as wide as long with 8 (6–10) setae; longest seta is dorsal, 0.46 (0.44–0.52) of diameter of antennomere. Small trifid spined organ begins on outer part of the tergal side of the third or fourth antennomere (Fig. 3A, arrow). Second medial whorl of setae begins with 1 seta on inner side of antennomere 4–5 and is complete in antennomere 10 (12–15). Tenth antennomere 1.3 (1.05–1.57) wider than long with 10–20 setae: 8–15 of about the same length, 5–6 very short and thin setae. The antennomere next to the apical one (Fig. 3D) 1.13 (1.09–1.38) × as long as wide. An additional third whorl never completes, but traces occur from ventral side of 6–12 antennomeres below the apical whorl. Apical antennomere (Fig. 3C–F) 1.1 (1.31–1.5) × as long as wide with 24–28 setae, 2 small spined organs, and one large spined organ. The latter (Fig. 3F) 0.3 (0.21–0.31) of length of antennomere and consisting of a central straight rod surrounded by 5 (4) somewhat curved bracts. First antennomere glabrous with scaly cuticular pattern; distal antennomeres from the second with a sparce, evenly distributed pubescence; apical surface of distal antennomere (Fig. 3C, E) with fine pubescence.

Tergites (Figs 1, 4, 5). First tergite (Fig. 4A, C) rudimentary, with 2 distinct lanceolate setae and 0–2 small setae (Fig. 4A, arrow). Second tergite complete (Fig. 4A, C), 2.52 (2.13–2.6) × as broad as long: posterior margin straight in the middle; anterolateral angles distinct with macrochaetae directed outwards and slightly forwards. These macrochaetae are 1.72 (1.25–1.9) of diameter of first antennomere, 21 (16–29) posteromarginal setae; longest posteromarginal seta, 0.5 (0.39–0.57) of the length of anterolateral macrochaetae. Inner setae of tergite short, subequal in length, lanceolate, similar to posteromarginal setae. Pubescence short, reaching posterior margin, mostly in short transverse bows (Fig. 4B), reaching up the anteroposterior margins, posterior margin glabrous. Third tergite (Fig. 4A) 2.46 (2–2.85) × as broad as long with straight posterior margin; anterolateral macrochaetae (Fig. 4D) as on preceding tergite, 1.6 (1.31–1.6) of diameter of first antennomere; 30 (22–37) marginal setae between macrochaetae. Posteromarginal setae and pubescence similar to second tergite; longest posteromarginal setae, 0.56 (0.4–0.58) of the length of anterolateral macrochaetae. Fourth tergite (Figs 4A, 5A) much broader than preceding one, 2.81 (2.63–3.68) × as broad as long, posteriorly somewhat emarginate; with anterolateral macrochaetae directed outwards and backwards (Fig. 3E), 1.65 (1.07–1.81) of diameter of first antennomere; 31 (28–34) marginal setae between macrochaetae; inner setae as on third tergite. Tergite fifth to thirteen (Figs 1A, B, 5A, D) emarginated, pubescence as on anterior tergites. Fifth tergite with posterior concavity less pronounced than the preceding one and significantly narrower than the fourth and the sixth (Fig. 1A, B). Penultimate tergite (Fig. 5D) with broad and shallow emargination; setae lanceolate, as on anterior tergites; pubescence as on anterior tergites. Last tergite (Figs 5D, 9B) with a convex posterior margin well projected behind tergal margin bearing 4 recognizable setae, pubescence reaching posterior margin. In addition to macrosetae on tergites 2, 3, and 4, lateral macrochaetae present on tergites 6 (Fig. 5B), 7, 9 (Fig. 5C), 10 (Fig. 5D, E), 12 (absent in populations of State Island), and 13 (Fig. 5D, F).

Ventral surface. Mainly covered by microsetae born from small knobs, last segment surface with longer pubescence born at the posterior end of scale-like cuticular structures. Male genital segment (Fig. 6A–D) with sternal plates with 9–13 setae, 2 of which are 2 (1.45–2.1) × longer than the rest. Female genital segment (Fig. 6F) with sternal plate with 13–18 setae, 2 or 3 of which are longer than the rest, 1.65–2.7 longer than the other setae.

Coxal sacs. Typical for the genus, fully developed at bases of legs 3–9, usually bearing 7–10 setae (Fig. 6E).

Male organs. Genital opening (Fig. 6A–C) valves with 7–10 setae on each of the internal borders and 7 (4–13) setae, which are subequal in length to those of the border.

Legs (Figs 7, 8). Tarsus of first pair of legs (Fig. 7A, B) 3.85 (3.38–4.53) × as long as wide, strongly tapering distally. Longest dorsal row with 5 (4–6) setae, longest ventral row with 4 (4–5) setae; distal setae longer than proximal ones; the longest one most distally on dorsal side, 0.9 (0.81–1.19) × as long as greatest diameter of tarsus. Anterior claw acuminates (Fig. 7C, D), almost straight, its length 0.21 (0.19–0.29) of the length of tarsus and 1.93 (1.7–2.5) × as long as posterior claw. Posterior claw basally suddenly thicker (Fig. 7D); frontal seta lanceolate and subequal in length to posterior claw. Pubescence short but distinct; trochanter with 13–23 setae subequal in length, the longest seta 2× longer than the shortest; coxa with (2–4) setae. Tarsus of 12^th leg pair (Fig. 8A–F) 5.5 (4.3–5.6) × as long as wide, slowly tapering distally. Setae (Fig. 8B) arranged in rows lengthways, longest dorsal row with 6 (5–7) setae, longest ventral one with 7 (5–7) setae. Dorsal setae longer, longest of these 0.85 (0.61–0.95) of greatest diameter of tarsus. Anterior claw slender, somewhat curved, 0.14 (0.18–0.29) of the length of tarsus; length of posterior claw 0.52 (0.63–0.84) of the length of anterior claw, basally suddenly thicker; frontal seta (Fig. 8D) lanceolate and longer than posterior claw, with an aristate tip. Tibia (Fig. 8A, C, E) 2.23 (1.9–2.71) × as long as wide, its length 0.75 (0.72–0.83) of the length of tarsus; longest row of dorsal side with 5 (4–6) setae, the one on ventral side with 4 (2–5) setae; length of setae decreasing in length proximally but inconsiderably; posterior side with few, 6 (4–7), setae. Femur very short with 5 (2–5) rows of 2–5 setae on dorsal and anterior sides; 0–1 ventral and 0–1 posterior seta. Trochanter with 22–35 thin subequal setae on dorsal and anterior sides only. Pubescence on tarsus, tibia, and femur short but distinct, sparser in the posterior facies of femur and tibia.

Styli. 3.2 (2.5–3.8) longer than wide, with 2 terminal setae, the larger 2.14 (1.71–2.33) longer than the shorter. On 12 leg well developed, elongated, 3.3 (2.91–4) × longer than wide, their length 1.42 (1.1–1.5) of the width of tarsus and densely covered with a short pubescence, basally glabrous; with 2 apical setae, the longer 1.5 (1.4–2.66) longer than the other, and 0.53 (0.33–0.57) the length of the stylus (Fig. 8G).

Sense calicles. Typical for the genus (Domínguez Camacho 2009) (Fig. 9F).

Cerci (Fig. 9A–E). Conical, proportionately short, 0.12 (0.75–0.12) of the length of body and 3.1 (2.75–4.5) × as long as wide. They have a moderate number of somewhat arched setae, the most distal ones longer than proximal ones; longest distal setae 0.43 (0.37–0.63) of greatest diameter of cercus. Longest dorsal row with 7 (6–9) setae (Fig. 9C), ventral 8 (5–9) (Fig. 9E), longest inner row with 8 (4–9), outer (Fig. 9A), with 9 (5–10) setae. The proximal 0.81 (0.7–0.84) of the tergal side covered with dense pubescence, as in tergites, mainly in short transverse bows following posterior borders of scale-like cuticular structures. Apical end glabrous. 2 apical setae (Fig. 9C, D) of different lengths, the longer one being (0.19–0.29) × the length of the cerci and (1.8–2.75) × the length of shorter seta.

We found this species on three islands of the Tierra del Fuego archipelago, which is divided between Argentina and Chile. On the Argentinean side, we found it in the south of the Isla Grande de Tierra del Fuego and in the Isla de Los Estados. On the Chilean side, we found it on the Isla Navarino, one of the southernmost islands of the archipelago. This species seems to be common in the Argentinean islands explored, but less common in Navarino.

H. guerreroi inhabits the Nothofagus forests of the Argentine and Chilean parts of the Tierra del Fuego archipelago (Fig. 10). This species could correspond to the previous records attributed to S. immaculata by Attems (1897) from Navarino and Ushuaia, and most probably also from Lapataia.

Twelve species of the genus Hanseniella have macrochaetae on tergites 2, 3, 4, 6, 7, 9, 10, and 13, putatively on 12, but not on 5, 8, 11, and 14 (Soesbergen 2019): H. audax Clark & Greenslade, 1996; H. conveniens Clark & Greenslade, 1996; H. insequens Clark & Greenslade, 1996; H. madecassa Aubry & Masson, 1953; H. mutila, Adam & Burtel, 1956; H. nivea (Scopoli, 1763); H. pluvialis Clark & Greenslade, 1996; H. proxima Adam & Burtel, 1956; H. pyrethrata Clark & Greenslade, 1996; and H. vulgata Adam & Burtel, 1956.