A genus of the Neelidae Folsom, 1896 with the following diagnostic characters:

Habitus of Neelidae. Small size, about 0.6 mm. Color white. Tegumentary grain fine and uniform. Apex of head with sword-like spines, body with several such spines, mainly around sensorial fields. Ant. III and IV fused and Ant. III with small globular sensillum in proximal position. Anterior labral setae R₁ and R₂ thick, curved and smooth. Oral fold with 1+1 sword-like macrosetae. Basomedian field of labium furnished with 6+6 setae. Presence of 3 setae around abdominal sensory fields, no E3 spine/setae on dd. Mid abdomen with swollen sensilla s3 and s3´. Base of Abd. IV sternite with 1+1 neosminthuroid setae, smooth and with pointed tip.

Spinaethorax spinotricosus (Palacios-Vargas & Sánchez, 1999), comb. n.

Figs 1–15.

Figures 1–5. 

Spinaethorax spinotricosus. 1 dorsal chaetotaxy of head 2 anterior part of head with labrum, of—oral fold 3 mandible 4 maxilla 5 labium with ventral head back, bfl—basolateral field of labium.

Figures 6–10. 

Spinaethorax spinotricosus. 6 thoracic and abdominal chaetotaxy with abdominal sensory field enlarged 7 ventral tube in lateral view 8 tenaculum 9 male genital plate in lateral view, arrow shows anterior direction 10 female genital plate frontal view, av—anal valve setae, nch—neosminthuroid setae.

Figures 11–12. 

Spinaethorax spinotricosus. 11 dorsal Ant. I–IV, with Ant. IV organ enlarged 12 furcula, posterior view with anterior seta enlarged.

Figures 13–15. 

Spinaethorax spinotricosus. 13 leg I 14 leg II 15 leg III, with detail of each feet complex.

Holotype: female mounted on slide. Original label: 23/00/1991, Mexico, Campeche, Cueva Xtancumbilxunaan (cave), 29.xii.1996, A. Ruíz and S. Aguilar col., ext. soil. Paratypes: 1 female on slide, 23.viii. 1991, J. G. Palacios col., direct collection and 2 juveniles on slides, the same data as the holotype. Type material deposited at Facultad de Ciencias, UNAM.

Mexico, Campeche, Cueva Actún Guachapil (cave), 1 male, 1 female and 2 juveniles on slides. Original label: 14/iv/2012, 22.iii.1997, A. Ruiz and S. Aguilar col., ext. guano; 1 female on slide, 29.v. 1997, J. G. Palacios col., ext. guano. Material deposited at Facultad de Ciencias, UNAM.

Unpaired seta a0 between antennal basis. Labral chaetae R₁ and R₂ thick, curved and smooth. Oral fold with 1+1 sword-like macrosetae. Basomedian field of labium with 6+6 setae. Ant. III with small globular sensillum. Manubrium with 4+4 posterior setae. Mucro with both lamellae serrated and with rounded tip.

For lengths of different character see Table 1. Body length 0.5–0.7 mm. Habitus globular, as other members of Neelidae. No pigmentation, cuticle finely granulated, linea ventralis without crossing with integumentary channels on ventral head back. Mid and hind Abd. with numerous spine-like microsetae, stouter spines around all sensory fields on body and apex of head.

Head. Head length and width 215 and 145 µm, respectively. No eyes. Head with smooth, pointed ordinary setae and spines of different width and length (Fig. 1). Frontal part with ordinary setae (lateral ones longer than axial, 26–30 µm, respectively 10–16 µm), seta a0 present; medial part between posterior s.f. ordinary or slightly spine-like setae (20 µm); posterior part with 3+3 stouter spines IL₁–IL₃ of different lengths (28–42 µm) and 1+1 smaller axial spines IL₄ (17–21 µm), others similar to smaller setae (10–14 µm). Labrum with 5,5,4 setae, 4 prelabrals. Pattern of labral setae (Fig. 2) after Massoud and Vannier (1967): a-row: 2R₁ + 2R₂, m-row: m + 2r₁ + 2r₂ and p-row with 5 ordinary setae (11 µm). Anterior R₁ and R₂ slightly thick, smooth and curved, R₂ (11 µm) longer than R₁ (9 µm). Medial setae (m-row) equal (11 µm), smooth median setae in one line with others. Maxillary palp simple, with 1 enlarged terminal seta (18 µm), 1 basal seta (14 µm) and 1 sublobal hair (Fig. 2). Basomedian field of labium with 6+6 setae (Fig. 5), median ones slightly longer (12 µm) than others (10 µm); basolateral field with 1+1 setae (10 µm), oral fold with 1+1 basal setae (8 µm) and 1+1 terminal sword-like macrosetae (25–30 µm). Head with 3+3 smooth postmedian setae ventrally (Fig. 5); 2+2 anterior setae equal (16 µm); posterior 1+1 seta slightly curved at tip (18 µm). Mandible with 4 apical teeth, medial ones longer (Fig. 3). Maxilla as in Fig. 4.

Thorax and abdomen (Fig. 6). Dorsally with ordinary setae, swollen sensilla s1, s2, s3, s3´, spines of different size (4–6 and 25–45 μm), 6+6 wax rods (wrc1, 2, 4, 6–8) with straight setae (3–4 µm) and 2+2 (wrc 3 and 5) with thicker and longer straight setae (6–7 µm). All wrc placed in small cuticular depressions. Trichobothria or their sockets not observed. Th. II with 3+3 ordinary setae (12–14 µm) and 6+6 stouter spines (25–36 µm) around thoracic sensory fields, axial spines smaller (25 µm); sensory fields at leg II base with 2+2 ordinary setae (20 µm), 3+3 stouter spines, anterior ones longer (45 µm) than posterior (26–28 µm), 1+1 lateral sensillum s1 (8 µm) broadened at tip and 1+1 swollen transparent rod with bifid tip and no base (8 µm) above s.f. of leg II. Th. III with 5+5 ordinary setae (6–8 µm), 4+4 wrc (wrc 1–4) and several small spine-like microsetae, whose overall number is not seen clearly; at leg III base with 1+1 ordinary seta (16 µm), 3+3 stouter spines, medial ones longer (42 µm) than lateral (22 µm) and 2+2 wrc (5, 6). Anterior Abd. medially with 1+1 wrc 7 and 2+2 swollen sensilla s3 and s3´; hind Abd. with abdominal sensory fields, which are surrounded by 2+2 ordinary setae (12 and 18 μm), 1+1 stouter spines (30 µm), 1+1 swollen sensilla s2 and 1+1 wrc 8 above Abd. s.f. Dorso and dorso-lateral anterior and hind abdomen covered with numerous spine-like microsetae (4–6 μm) arranged as in Figs 6 and 10. Their overall number is not seen clearly. Abd. tergum VI with 3+3 setae (17 µm) and 1 unpaired axial seta (14 µm). Anal complex with three anal valves, each with on seta (7 µm). Abd. VI sternum with 4+4 setae (18–20 µm) and 2+2 very small globular structures (1–2 µm) next to anal valve setae. Female genital plate (Abd. V sternum, Fig. 10) with 4+4 setae (8–11 µm) and 1+1 axial microsetae (4 µm). Male genital plate with 5+5 ordinary setae (12 µm) arranged in circle and with 2+2 spine-like setae (7 µm) difficult to observe (Fig. 9), laterally surrounded with 4+4 setae (18 µm). Abd. IV sternum with 3+3 setae in one row (18–20 µm), one seta more laterally and 1+1 distal setae (8 µm). Lateral part of Abd. IV sternum with 1+1 short and pointed neosminthuroid setae (6 µm) (Fig. 10).

Appendages. Ant. III and IV not separated (Fig. 11). Length of antennae 140 µm, ratio antenna/head = 0.65; length of antennal segments I, II, III–IV as 11, 26 and 103 µm. Ant. I furnished with 3 short setae (8–10 µm). Ant. II with 1 medial seta and 5 apical setae arranged in a whorl. Ant. III organ consists of 2 transparent rods (7 µm), 1 leaf-like transparent sensillum Sg (12 µm) and spine-like seta (7 µm). Proximal part of Ant. III bears 1 globular sensillum Sg (4 µm). Ant. IV with 13 curved macrosensilla S finely blunt at tip (24–32 µm); subapically with 1 long and thick subapical sensillum Sy (16–18 µm) and with 1 thick shorter apical sensillum Sx broadened at tip (12–14 µm); Ant. IV organ like a tiny, hardly visible spine (5 µm); apically with curved setae a and sa apical in position (10 µm). Complete chaetotaxy of antennae provided in Table 2. Setae numbers of legs I–III (Figs 13–15): scx I: 1, 1, 3 (one spine); scx II: 1, 1, 1; coxae: 1, 1, 2; trochantera: 4, 3, 3; femora: 8, 8, 8 and tibiotarsi: 13, 14, 13. Thin meso- or microsetae as in following numbers on leg I: femur with 2; on leg II: femur with 1; on leg III: trochanter with 1, femur with 1. For complete setation of legs see Table 3. Tibiotarsal tenent hairs ordinary, straight and pointed (16–18 µm). Unguis narrow, in distal part bended; both unguis and unguiculus unequally long in leg I, II and III: unguis 31, 27 and 26 µm, respectively, unguiculus 14, 15 and 16 µm, respectively. Length ratio unguis I, II, III / ti. I, II, III width (31, 27, 26/ 17, 16, 17 µm) = 1.8, 1.7, 1.5. Unguis I and III with three auxiliary lamellae la, lp, Bp, unguis II with only lp and Bp (Fig. 14); unguiculus I with small internal tooth on distal part, II and III untoothed and III with bended tip; unguiculi without apical filament and basal lamellae. Ventral tube with 2+2 distal setae and without posterior lobe (Fig. 7). Retinaculum with 3+3 teeth, no setae on corpus (Fig. 8). Furcula well developed (Fig. 12), length of manubrium, dens (dp and dd) and mucro: 56, 31, 71 and 74 µm, respectively. Manubrium with 4+4 setae posteriorly, lateral ones shorter (10–15 µm) than axial (16–20 µm). Dens in proximal part (dp) with 2+2 posterior setae, lateral ones (14 µm) shorter than those axial (22 µm); distal part (dd) apically with 2+2 broad, blunt lateral spines (9 µm) and 1 medial sharp spine (7 µm) on anterior side; with 2 external (E1–E2) posteriorly and 2 internal (J1–J2) spines (6–7 µm each, distal with short apical filament), and 1 medial, subapical seta (18 µm). Mucro with serrated lamellae and rounded tip. Base of mucro furnished with small scale without base. Middle mucro width 7 µm.

Sensory fields (Figs 1, 6, 14 and 15). 6+6 s.f. placed in depressions each with secretory rod (10–12 µm), i.e. blunt seta with basal part inserted on cuticle and placed in upper margin of the field. Following arrangement: (a) anterior and posterior field on head (s.f. 1, 15 × 10 and s.f. 2, 20 × 10 µm) each with secretory rod and 1 seta on margin (14–18 µm); (b) thoracic field (s.f. 3; 35 × 20 µm) with secretory rod, 3 internal spines (4 µm) arranged in triangle, 2 external marginal setae (12–14 µm) and 6 sword-like spines of different lengths (25–36 µm), 3 spines are in anterior position and 3 spines above s.f. in axial position; (c) fields at base of legs II and III (s.f. 4, 5; 20 × 15 µm) each with secretory rod, 2 internal spines (4 µm) in s.f. 4 and 1 internal spine (4 µm) in s.f. 5. S.f 4 with 2 marginal external setae (20 µm), 3 stouter sword-like spines, medial ones longer (45 µm) than lateral (26–28 µm), 1 lateral sensillum s1 broadened at tip (8 µm) and 1 swollen rod with split tip and without base (8 µm) above s.f. 4. S.f. 5 with 1 marginal external seta (16 µm), 3 stouter sword-like spines, medial ones longer (42 µm) than lateral (22 µm); (d) abdominal field (s.f. 6; 30 × 25 µm) with secretory rod, 1 internal spine (4 µm), 2 marginal ordinary setae (12 and 18 μm), 1 sword-like spine above s.f. (30 µm) and 1 swollen sensillum s2 on the margin of s.f. (6 μm). Wrc 8 is located above Abd. s.f.

Both sexes known.

The genus is named after the spine-like setae on thorax and abdomen.

Spinaethorax spinotricosus is currently known from two caves and is putatively spread in the cave systems of Yucatán Peninsula, mainly in places with accumulation of bat guano or other type of rich organic material.

The young have only one proximal seta on dens.

This species, described from a cave in State of Guerrero, shares with new type species of the genus similar generic characters like presence of sword-like macrosetae on oral fold, globular sensillum on Ant. III., stouter spines on Th., fusion of Ant. III and IV, 3+3 setae around Abd. s.f., 2+2 setae on proximal part of dens and absence of E3 spine on distal part of dens. The main differences consist in setation of hind Abd. (numerous thickened macrosetae in S. tonoius comb. n. vs. numerous spine-like microsetae in S. spinotricosus comb. n.), in chaetotaxy of the apex of head (only one spine IL ₁ in S. tonoius comb. n. vs. three spines IL ₁–IL ₃ in S. spinotricosus comb. n.), in the structure of tenent hairs on Ti. (more developed in S. tonoius comb. n. vs. shorter in S. spinotricosus comb. n.) and in setation around Abds.f. (absence of axial spine in S. tonoius comb. n. vs. presence of spine in S. spinotricosus comb. n.). Species description is based only on one adult specimen (holotype) and does not allow us to describe it in an appropriate way, along with drawings and measurements.

The family Neelidae was for a long time an overlooked group of Collembola, mainly due to its small size and lack of diagnostic characters. In spite of the foregoing facts recent years molecular phylogenetic analysis have revealed unexpected diversity within this family (Schneider et al. 2011). Recently Neelus has been revised (Kováč and Papáč 2010) as well as Megalothorax with a redescription of the nominal species Megalothorax minimus (Schneider & D´Haese, 2013). Soon after additional new taxonomical characters in genus Megalothorax were defined (Papáč and Kováč 2013). S. spinotrichosus comb. n. and S. tonoius comb. n. share many specific characters. These species were included in the genus Megalothorax (Palacios-Vargas & Sánchez, 1999), because some features (fusion of Ant. segments III and IV, absence of E3 spine on distal dens) indicated that those specimens belonged to this genus. On the other hand, some characters link those specimens to Neelus (dp with 2+2 setae - except N. fimbriatus, 1+1 neosminthuroid setae at the base of Abd. IV sternite). Schneider and D´Haese (2013) stated that chaetotaxy of M. spinotricosus clearly differs from the other Megalothorax by the presence of great number of microsetae on Abd. and pointed out that M. spinotricosus deserved its own genus. On the basis of recent diagnostic features, material of these two species was re-examined arriving to the same conclusion as Schneider and D´Haese (2013) that they represent a new genus in the Neelidae. Spinaethorax gen. n. differs from other genera by striking morphological features and combinations, which clearly separate them, e.g. sword-like macrosetae on oral fold, six setae on basomedian field of labium, Ant. III and IV fused, presence of small globular sensillum Sg on Ant. III, three marginal setae around abdominal sensory field, absence of spine E3 on dd and dp with two seate. For comparison with other genera see Table 4.