Revision of the key characters for the Thricops nigrifrons species-group (Diptera, Muscidae)

Abstract An analysis of key characters for the separation of Thricops nigrifrons and Thricops longipes (Diptera, Muscidae) is given. A revised key for Thricops nigrifrons and related species, including two species recently described from the Caucasus, is proposed.


Introduction
Th e Th ricops nigrifrons species-group is characterized by: long-plumose arista; dark body; holoptic head in male; absence of apical spur on t3 and of apical spurs on t1 in males; t2 without pv or v seta(e); lower margin of face not projecting; 4 postsutural dc; unmodifi ed fore tarsomeres in male; mid tarsomeres 3-4 each with a row of pale pv setulae in male (except T. dawkinsi); the male terminalia are similar (T. semicinereus -type).
Th e examined material is restricted to Eastern Europe, Transcaucasian region and Siberia, and do not include specimens collected in Western Europe. However, the proposed key characters are expected to be suitable for west-european specimens as well, because there is no gap in natural habitats of nigrifrons and longipes in Europe. Another reason is the fact that d' Assis-Fonseca (1968) came to the same main conclusions based on the investigation of the material from British Islands.

Material and methods
Th is analysis is based on the following material: Th ricops dawkinsi Vikhrev, 2009 - a, p, d, v = anterior, posterior, dorsal, ventral seta(e).

Notes on the identification of T. nigrifrons and T. longipes
On several occasions colleagues have expressed doubts that T. nigrifrons could be reliably separated from T. longipes. I have shared these doubts too, but currently I am convinced that a reliable (and rather easy) separation is possible. Let us fi rst consider the identifi cation characters proposed by Hennig (1962), d'Assis-Fonseca (1968), Gregor et al. (2002) and Savage (2003). 1 Th e longest aristal hairs distinctly longer than width of postpedicel ....nigrifrons -Th e longest aristal hairs slightly longer than width of postpedicel .... longipes Th is character was used as the main one by all the cited authors except for Gregor et al. (2002), although the estimation of length and wording diff er.
I agree that the aristal hairs are somewhat longer in nigrifrons, but I disagree with using this as the main character: the diff erence is very fi ne and the character is variable, with overlapping taking place. Gregor et al. (2002) gave the following measurements: nigrifrons -longest aristal hairs 1.05-1.30 times as long as width of postpedicel; longipes -0,85-1.15 times. Instead of the length of aristal hairs Gregor et al. (2002) proposed the following wording:  (1962), used by Gregor et al. (2002) as the only character for males, used as the main character by Savage (2003). I agree with this character, but these upper frontal setulae are fi ne and often partly or even completely broken, especially in specimens mounted from alcohol.  (1968), but the other authors excluded this character from their keys. According to the descriptions given by Savage (2003): in longipes "f3 … p and v surfaces covered with long hairs, longer than depth of femur"; in nigrifrons "f3 … p and v surfaces covered with setae of variable length, as long to much longer than depth of femur". I suppose that the source of the misunderstanding is that the fi ne hairs on the p and v surface of f3 are not homogeneous. In longipes, these hairs really are evenly long, at least twice as long as femoral width, but in nigrifrons the hairs on the p surface are rather long, usually about 1-1.5 times as long as femoral width, but on the v surface they are short, especially in basal half of femur where the hairs are 0.5-1 times as long as femoral width. Th us, the fi ne hairs on f3 are distinctly longer in longipes than in nigrifrons, but the diff erence is the most obvious in a comparison of the v hairs in the basal half of f3, for which the hind femur needs only to be observed in lateral view.  Savage (2003). In fact the notopleuron is setulose on the anterior part in both species. Th e notopleuron on the surface between the anterior and posterior setae is almost always bare in nigrifrons and usually setulose in longipes, but bare in a quarter of the examined specimens. Th is may be used as an additional character only.  Vikhrev and Sorokina (2009). Th e trace of a narrow and less dusted median vitta may be present in nigrifrons, but otherwise this character separates all examined specimens.

. longipes
In spite of rare cases of overlapping, this character is at least as reliable as, and much easier to use than the width of the aristal hairs. Th e body size difference was also mentioned by Hennig (1962) and Savage (2003). It should be noted that this character works for the forest zone where both species are present, but not for the extreme northern populations of longipes from the tundra zone, where nigrifons has not been recorded. Specimens collected near Vorkuta (67.5°N) have a body size 6-8 mm only. Proposed by d'Assis-Fonseca (1968), but with a misprint, so the indistinct vitta was wrongly ascribed to longipes and the distinct one to nigrifrons. Probably because of this, no one else has drawn attention to this reliable character, which separates all the females I have examined (Fig. 1).  Savage (2003). I agree that usually nigrifrons has more yellowish dusting while longipes is more greyish, but the reverse situation may also occur. Personally I fi nd this character diffi cult to use and prefer not to include it in the key.
Distribution. Th e distribution of these species in mountain areas seems sporadic and there aren't enough reliable records. In the Austrian Alps, in the Oetz Valley, both species overlap at about 1500 m asl. Below this level, in the coniferous and broad-leaf for- est zone, nigrifrons is found; above it, in the upper forest zone and above the tree-line, only longipes is found (A.C. Pont, pers. comm.). In the Russian Caucasus (Krasnodarsky Kray and Karachay-Cherkessia) longipes is found at the altitudes 1800-2500 m asl., while nigrifrons is not found. In the mountain area in Turkey, Bolu prov., 40.6N 31.8E nigrifrons is found at the altitudes 1450-1950 m asl., while longipes is not found. In European Russia nigrifrons is common in the area between Moscow and St. Petersburg (55-60°N), but absent or at least rare in Karelia at 65°N. Th e southern border of distribution of longipes seems to be the northern part of Moscow region (56°N), it is common in the northern coniferous forest zone (taiga) and is still the dominant species in the tundra around Vorkuta (67.5°N). In Abisko National Park (North Sweden, 68°N), with birch forest and mountain tundra (A.C. Pont, pers. comm.) and in birch-willow forest in Murmansk (69°N) only longipes was found.

Key for the Th ricops nigrifrons species-group
Males 1 f2 with a comb of 3-4 long and strong setae on p-pv surface at base and the fi ne setae in av and pv rows 1.5-2 times as long as f2 width. Legs at least partly yellowish (tibiae) or both tibiae and femora yellow. In lateral view, f3 on basal half with fi ne hairs on v surface at most as long as femoral width, much shorter than the strong av setae (the hairs on p surface at most 1.5 times as long as femoral width). In posterior view postsutural part of scutum densely yellowish-grey dusted without a median vitta between acrostichals (a pair of vittae present laterad to dc rows). Abdomen with the median vitta on tergite 3 inconspicuous, at most a narrow trace of a vitta present. Frons with both pro-and reclinate setulae on upper half. Secondary characters: body length usually 7-7.5 mm, rarely 6-8 mm; ground setulae absent between the two notopleural bristles, longest aristal hairs 1.05-1.30 times as long as width of postpedicel ......... nigrifrons (Robineau-Desvoidy) - In lateral view, f3 on basal half with fi ne hairs on v surface about twice as long as femoral width, about as long as the strong av setae (these hairs on p surface at least twice as long as femoral width). In posterior view, postsutural part of scutum subshining black with only thin greyish dusting, consisting of two vittae restricted to areas between ac rows and slightly beyond dc rows, median vitta between ac rows always present. Abdomen with a black subshining median vitta on tergite 3 wide and distinct on at least anterior 2/3 of tergite. Frons with all setulae on upper half reclinate. Secondary characters: body length usually 8.5-9 mm, rarely 7.5-9.5 mm, ground setulae usually (in 75% specimens) present between the two notopleural bristles, longest aristal hairs 0. Postsutural part of scutum in posterior view with the median vitta indistinct (Fig. 1a), or if more or less distinct then narrow, sometimes slightly widened posteriorly (Fig. 1b). t3 with only 2 ad setae. Longest aristal hairs 1.05-1.30 times as long as width of postpedicel. Body length usually 6.5-7