Mucronia enigmatica Fjellberg, 2010, sp. n.

Mucronia enigmatica sp. n.

(Figs. 93–109)

Type material. Holotype: Reproductive male (slide) from "Alaska: Juneau. Mts. between Mendenhall Glacier and Montana Creek Trail, 58o29'N, 134o40'W, 13.VII.1980. On melt-water ponds, 975 m. alt., A. Fjellberg leg." Paratype: Reproductive male (slide, without head) from the same sample. Both specimens are transparent due to long storage in diluted ethanol and details of chaetotaxy are hard to see (INHS).

Description. Body size 0.95 mm (in slide). Body slender, with long furca (Fig. 93). Eye-spots dark, otherwise almost white. Frons slightly swollen. Abd. 5–6 not fused. Eye-fields with 6+6 large ocelli. PAO about the size of an ocellus, roundish, with four blunt lobes and swollen mid section (Fig. 94). Details of antennal chaetotaxy not seen, apart from ant. 3 organ which has two large sensilla and a ventrolateral spinelike sensillum (Fig. 99). Labrum with 5,5,4 papillate stiff setae, 4 prelabrals, anterior edge with four low ridges followed by a median tubercle (Fig. 97). Ventroapical ciliation not seen (if present presumably very fine). Frontoclypeal field with 10–12 setae. Maxillary outer lobe with trifurcate palp; sublobal setae unclear, but at least two present (Fig. 98). Labial palps modified, with papillae A–B, C–D and E in three separate groups (Figs. 95, 103). Terminal setae of the papillae short, not longer than surrounding guards. All guards present, including e7. Guards b4 and d4 enlarged, recurved at apex. Four proximal setae present. Hypostomal papilla reduced, H, h1 and h2 subequal. Basal fields of labium with 4 median and 5 lateral setae (Fig. 95). Postlabial setae 3–4 on each side. Mandibles normal, with strong molar plate and 4–5 apical teeth. Maxilla highly modified with 6 elongated lamellae and a narrow long stylet representing the capitulum, stylet with a small hook in apical 1/3 (Fig. 96); Lam.1 with weak denticles in apical 1/3; Lam.2 with rows of small sharp teeth along the edges only, extending from base to tip; Lam.3 present as a small flap at base of Lam.2, with delicate serration; Lam.4–5 with fine denticles from base to tip, not clearly defined in rows; Lam.6 short, with few denticles. Quality of body cuticle and setal cover difficult to evaluate because of transparency of the specimens, but setal cover apparently short and uniform. Macrochaetae only differentiated at abd. 5–6. Longest macrochaetae on abd. 5–6 about twice as long as inner length of claw 3. Macrochaetae apparently pointed and smooth, without serrations/ciliations. Because of transparency body sensilla not seen. Ventral tube apparently with 2+2 lateral (distal) setae, no frontals and two caudals (posterior). Retinaculum with 4+4 teeth and a single seta on corpus. Manubrial chaetotaxy not clearly seen, but with many setae both dorsal and ventral (Fig. 104); apical thickening with blunt teeth, short ventroapical setae 1+1. Dens with many (at least 20) ventral setae, dorsal side with fewer than 10 setae (Fig. 105). Mucro elongated, with three visible teeth, one of the basal teeth absent or reduced; a long and distinct inner lateral lamella ending at subapical tooth; two lateral setae present (Fig. 102). Tibiotarsi with 11 apical setae (A1–7, T1–4). Tibiotarsi on first two pairs of legs shortened, each with three regular whorls of setae (Fig. 100). Last pair of tibiotarsi longer, with increased number of setae (more than 30, Fig. 101). Claws small, without visible teeth; unguiculus pointed, with high basal lamella (Fig. 109). Reproductive males with distinct genital papillae, central part cone-shaped and separated from rest of the papilla, surrounded by 6 setae (Figs. 106–107); four associated setae possibly situated outside the papilla. Genital tract with two bulbous swellings (Fig. 108).

Etymology. Enigmatic: Not clearly understood.

Discussion. The remarkably transformed maxillae and the modified labial palps may actually not be significant on generic level. Similarly transformed palps and maxillae are commonly seen in several genera of Isotomidae (Archisotoma, Parisotoma, Desoria) and probably reflects a specialisation to particular food or feeding habits. The simply lobed PAO and the bisetose mucro however, appears unique in the family. In the genera Micrisotoma Bellinger and Heteroisotoma Stach the PAO has a more irregular or complex fashion, different from the simple lobation observed in Mucronia. The complete ring of apical setae (11) on tibiotarsus probably represents a plesiomorphic condition in the family. The male genital papilla on the other hand, is very unlike the multisetosus type commonly found in Isotomidae and more resembles the types seen in the family Entomobryidae.

From the other side of the Bering Strait, in the alpine regions of the Aborigen mountains in Chukotka (Russian Far East), I have a sample (wet moss among rocks, 1,400 m alt., 23.VII.1979, V. Behan leg.) of what may be another species of the same genus. It has a similarly lobed PAO and bisetose elongated mucro, differing in its simple unmodified mouth parts, very long multiciliate macrochaetae and presence of dark body pigment. Also the mucro has two basal teeth and 1–2 secondary minute teeth between the subapical tooth and the basal teeth.

I have three specimens from Australia collected by P. Greenslade (NSW, 1.2 km NNE of S. Ramshead, 31o S, 148o E, subalpine pitfall trap, 1,800 m, July 1982, K. Green leg.) of another Isotomidae with elongate mucro bearing three outer setae and a complex set of teeth and spines, dens with many short spines on inner side in basal half, PAO roundish with a tendency to lobation.

These different species, all living in cold mountainous environments, may represent an old pre-Isotomidae line showing characters resembling both Isotomidae and Tomoceridae (mucro, dental spines).