Holotype: ♂, Argentina, Salta Province: ~15 km W Escoipe, on Provincial road No. 33, (25°10'25.2"S; 65°49'31.6"W), 2680m (Fig. 14), found on Phyllotis osilae, 17 V 1999, (CML 8044).

The new species can be distinguished from all species of the genus by characteristics of the distal arm of S-IX and the clasper (basimere and telomere). These include the presence of a thick sclerotization along the dorsal margin of distal arm of S-IX (Fig. 5 and 9), and by an oblique angle on the dorso-caudal apex of P1 (process) of the basimere subtended by a deep sinus (Fig. 11). General facies of the aedeagus are most closely akin to those of C. spillmanni; however, the P1 of the basimere of the males of the three known species is rounded at the apex and the dorsal margin of distal arm of S-IX without sclerotizations (Jordan 1931, Johnson 1957, Tipton and Machado-Allison 1972), while in the new species the apex of P1 is oblique with deep sub-tending sinus and dorsal margin of distal arm with thick marginal sclerotization. Furthermore, C. austrinus is separable from C. rex and C. perplexus by the lateral lobe of aedeagus not extended into narrow process, a character shared with C. spillmanni.

Head (Figs 1–3). Frons evenly rounded, thin sclerotization throughout. Preantennal area with two placoid pits, micro-punctuations scattered over surface; two vertical rows of setae: 5–6 small setae in anterior row and three longer setae in posterior row. Eye visible, sinuate, unpigmented. Antennal scape with 27–28 small lateral setae. Pedicel with nine setae, none extending onto clavus; clavus extended beyond antennal fossa. Maxilla acutely sharp; labial palpus of four segments (excluding palp bearing maxillary segment) (Fig. 2). Post-antennal area with three placoid pits; scattered micro-punctuations over surface and several minute triangular punctiform setae between three placoid pits. Occipital area with three rows of setae; anterior row oblique with three small setae; middle row with 4–5 small setae, and main row with seven long setae plus intercalaries.

Figures 1–4. 

Ctenidiosomus austrinus sp. n., male holotype. 1 Head and pronotum 2 Labial palpus and maxilla 3 Pronotum, mesonotum and metanotum 4 Abdominal tergites. Scale: 200 µm.

Thorax. Pronotum with comb of 23 ctenidia (both sides) preceded by three rows of setae; anterior row with three medium setae, middle with 8–9 medium setae, and main row of nine long setae plus intercalaries. Meso- and metanota with a main row of eight setae plus intercalaries (Fig. 3). Mesosternum with long seta at lower margin. Mesepimeron with three long setae. Lateral metanotal area with single long seta at posterior margin. Pleural arch and ridge well developed. Metepisternum with three long setae. Furca short and robust. Metepimeron with two rows of small setae (anterior with five, posterior with three). Rod-like abdominal link plate present as in all Pygiopsyllomorpha (Fig. 10).

Figures 5–10. 

Ctenidiosomus austrinus sp. n., male holotype 5 Terminal segments 6 Penis rods 7 Apex of aedeagus 8 Sensilium and P1 of basimere 9 Sternum VIII 10 Abdominal link plate, furca and pleural ridge. Scale: 200 µm (5, 6, 9); 100 µm (7, 8, 10).

Legs. Fore coxa with small setae scattered over surface, one distinct horizontal row of seven setae near apex, three stout anteroapical setae. Mesocoxa and hind coxa with small setae scattered on lower outer surface. Fore femora with 4–5 minute lateral setae and one long seta at femoro–tibial joint. Meso- and hind femoro–tibial joint with one short lateral and one long mesal seta. Margin of fore, meso- and hind tibiae with seven notches. Hind tibia bearing three setae (two long, one small) in penultimate notch. Long space between notch five and six, heavily sclerotized (Fig. 12). Distotarsomere of hind leg with five lateral plantar bristles and eight preapical plantar bristles arranged in semicircle (Fig. 13).

Figures 11–13. 

Ctenidiosomus austrinus sp. n., male holotype 11 Basimere and telomere 12 Hind tibia 13 Hind distotarsomere. Scale: 200 µm (11, 12); 100 µm (13).

Unmodified Abdominal Segments. Ctenidial combs on terga II–V (T-II–V). The number of ctenidia on two sides as follows: T-II 18, T-III 20, T-IV 18, T-V 15 (Fig. 4). Two antesensilial bristles (Fig. 8).

Modified Abdominal Segments. Sensilium with 18 sensilial pits. Dorsal anal lobe with three long thin setae; ventral with single long seta. (Fig. 8). Tergum VIII with seven long setae at dorso-posterior margin (Fig. 9).

Apex of P1 of basimere with oblique angle on the dorsocaudal margin with deep sub-tending sinus, manubrium with convex anterodorsal margin and apically narrowed. Telomere narrowing gradually to rounded apex. Distal arm of sternum nine (S-IX) distinctly widened apically, with five to six larger setae interspersed with smaller setae on apical margin; dorsal margin with broad marginal sclerotization (Fig. 5). Aedeagus. Median dorsal lobe rounded apically. Lateral lobe nearly parallel with upper margin of median dorsal lobe to level of sclerotized inner tube, then expands caudally into a rounded lobe that envelops inner phallosome (Fig. 7). Long thin sclerite between sclerotized inner tube and ventral margin of phallosome. Minutely membranous pouch ventral to the thin sclerite and dorsal to a thinly sclerotized ventral keel. Ford´s sclerites curved up; hyaline at base and sclerotized on apical half. Crescent sclerite with small satellite sclerite abutted against sclerotized inner tube. Sclerotized inner tube slightly narrowing towards apex; upper margin of apex longer than lower margin. Dorsal armature with minute undulations along distal half of sclerotized inner tube. Ventral armature absent. Aedeagal apodeme narrow; extending to sharp point curved upward at apex. Penis rods coiled multiple revolutions as a watch spring (Fig. 6).

Dimensions: Holotype male: 3.7 mm

The specific epithet is derived from the Latin term austrinus or “southern” because this new species represents the southern-most record of any known species of Ctenidiosomus.

The single male holotype was collected from a juvenile male specimen of the sigmodontine rodent, Phyllotis osilae, during the dry season in the month of May. Ctenidiosomus perplexus and C. rex, were recorded on sigmodontines rodents, while C. spillmanni was collected not only on sigmodontine rodents but also on Hystricomorpha rodents, and C. traubi on Caenolestes obscurus, Order Paucituberculata (Johnson 1957, Mardon 1981) (Table 1).

The type locality of Ctenidiosomus austrinus corresponds to the Ecoregion “Monte desert of Mountains and Isolated valleys” (Burkart et al. 1999), where the vegetation is characterized by small and medium shrubs and cacti called “cardones” (Trichocereus atacamensis). Also, some scattered trees typical of the ecoregions “algarrobos” (Prosopis alba) are present. The soil is stony, and formed by rocks of all sizes (Burkart et al. 1999). Similar to all other species of Ctenidiosomus, the new species was collected at a high elevation (2680 m above sea level) (see Fig. 14 for the localities). With this report, the geographical distribution of the genus Ctenidiosomus is extended ~2600 km further South from its previously known southern limits of El Tambo, Huancayo Province, Department Junín, Peru (Mardon 1981). The presence of this species in Bolivia is highly probable due to its location between Peruvian and Argentine records.

There have been far fewer species of fleas described from northwestern Argentina than other regions of the country (e.g. Patagonia; see Hastriter and Sage 2009, 2011, Sanchez and Lareschi 2014), primarily due to the lack of sampling efforts. We predict that with increased surveillance in Northern Argentina and bordering countries (Bolivia, Paraguay), numerous new taxa will be discovered. Although in recent years, the sampling of small mammals has increased in Northwestern Argentina (Díaz and Barquez 2007, Ojeda et al. 2008, Ferro and Barquez 2009, Díaz and Barquez 2009, Jayat and Ortiz 2010), this region still represents one of the least studied areas in South America relative to Siphonaptera and other ectoparasites.

Figure 14. 

Distribution of the species of Ctenidiosomus. Ctenidiosomus austrinus: 1 Type locality: Argentina, Salta Province, Dept. Chicoana, app. 15 km W Escoipe, on Provincial road N°33 (25°10'25.2"S; 65°49'31.6"W), 2680 m. Ctenidiosomus spillmanni: 2 Peru, Dept. Junín, Huancayo Province, El Tambo (12°01'37.88"S; 75°12'46.46"W), 3327 m; 3 Peru, Depto. Ancasch, Huaylas Province, Huaylas (8°54'7"S; 77°50'21"W), 2067 m; 4 Peru, Dept. Piura, Huancabamba Province, Huancabamba (5°14'22"S; 79°26'59"W), 1930 m; 5 Ecuador, Loja Province (no specific locality) I; 6 Ecuador, Loja Province, Alamor (4°01'14.29"S; 80°01'21.97"W), 1279 m. 7 Type locality: Ecuador, Pichincha Province (no specific locality). Ctenidiosomus rex: 8 Type locality: Colombia, Dept. Huila, San Agustin (1°52'39.20"N; 76°15'11.08"W), 1625 m; 9 Colombia, Dept. Huila, San Agustin, left bank of Rio Magdalena (1°55'01.10"N; 76°20'59.06"W), 1748 m†. Ctenidiosomus traubi: 10 Type locality: Colombia, Dept. Antioquia, Sonson‡, 7 km E of Paramo (5°43'23.23"N; 75°11'11.36"W), 2049 m. Ctenidiosomus perplexus: 11 Type locality: Venezuela, Trujillo State, Trujillo Township, Trujillo, 14 km. E, near Misisi. (9°21'51.23"N; 70°18'30.89"W), 2210 m. Other distribution records: 12 Venezuela, Merida State, near Santa Rosa (8°53'52.11"N; 70°36'55.77"W), 2040 m; 13 Venezuela, Merida State, near Middle Refugio (8°36'07.36"N; 71°04'17.00"W), 2550 m; 14 Venezuela, Merida State, La Coromoto (8°6'48.31"N; 71°29'54.1"W), 2500 m§. 15 Venezuela, Tachira State, San Cristobal Township, 35 km S and 22 km W of San Cristobal, Buena Vista (7°29'43.29"N; 72°26'52.45"W), 2386 m; 16 Venezuela, Tachira State, Buena Vista (7°29'43.29"N; 72°26'52.45"W), 2386 m; †The elevation was incorrectly cited as “2300m” in Johnson 1957; ‡The city was incorrectly cited as “Sanson” in Mardon 1981; § The elevation was incorrectly cited as “3170” in Tipton and Machado Allison 1972; I Both, the specific and unspecified localities are indicated by a point, the unspecified locality is placed in an approximate point.