Tanytarsus congregabilis Giłka et Zakrzewska, 2013

Giłka, Wojciech, Zakrzewska, Marta, Dominiak, Patrycja & Urbanek, Aleksandra, 2013, Non-biting midges of the tribe Tanytarsini in Eocene amber from the Rovno region (Ukraine): a pioneer systematic study with notes on the phylogeny (Diptera: Chironomidae), Zootaxa 3736 (5), pp. 569-586 : 581-583

Tanytarsus congregabilis Giłka et Zakrzewska , sp. nov.

Type material. Holotype. Adult male (tarsus of left fore leg missing, tarsi of hind legs broken and separated). Paratypes. Adult male (left hind leg and wings poorly observable); adult male (tarsi of all legs missing and/or unobservable, distal abdominal segments missing). Three syninclusions preserved in 15 x 10 x 4 mm piece of amber (SIZ K-8855, Fig. 7 A).

Derivatio nominis. From Latin, meaning ‘gregarious’. We presume that the specimens examined were trapped in resin during swarming.

Diagnosis. Femur of mid leg longer than femur of hind leg. Gonostylus stout, slightly curved at mid length, as long as gonocoxite, in distal half with several setae placed at median margin and directed anteriorly. Median setae on anal tergite absent. Anal point triangular, tapering to blunt apex; in lateral aspect visible as straight prolongation of anal tergite, with tip bent down. Superior volsella stout, bean-shaped, with broadly rounded apex, directed posteromedially. Stem of median volsella bulb-shaped, with slender leaf-shaped lamellae. Inferior volsella robust, club-shaped.

Description. Adult male (n = 1–3). Total length 2.7–3.2 mm; wing length 1620–1680 Μm.

Head ( Fig. 7 B, C). Eyes bare, with dorsomedian extensions. Frontal tubercles not observed. Antenna with 13 well discernible flagellomeres, AR 1.05–1.07, plume fully developed ( Fig. 7 C). Length of palpomeres 2–5 (Μm): 45–48, 112–141, 97–141, 179–193. Clypeals present but uncountable in lateral position.

Thorax ( Fig. 7 B). Ac at least 8, Dc 5 on each side, Pa 3 on each side, Scts unobservable.

Wing ( Fig. 7 D). Ellipse-shaped, with weak anal lobe, broadest at 2/3 length, width: 410–430 Μm, length/width ratio 3.77–4.10. FCu placed distally of RM; VR Cu 1.16–1.18. Veins ending as in most extant Tanytarsus (from base to tip): An, Sc, Cu1, R1, R2+3, M3+4, R4+5, M1+2; ends of R1, R2+3, R4+5 equidistant (VR C 1.0). Wing membrane densely covered with macrotrichia in distal part of r4+5, m1+2, and at posterior margin of wing. Proximal section of Cu1 slightly deflected.

Legs. Tibia of fore leg with single, 32–37 Μm long spur (single comb composed of 6 teeth up to 15 Μm long observed in one specimen is recognized as artefact). Tibial combs of mid and hind legs separated, fan-shaped, teeth up to 15 Μm long (mid leg) and c. 15–20 Μm long (hind leg); each comb with slender spur, 20–30 Μm long (mid leg) and 40–48 Μm long (hind leg). Sensilla chaetica on ta1 of p2 not observed. For lengths of leg segments and leg ratios see Table 6 .

Hypopygium ( Fig. 7 E–I). Gonostylus stout, slightly curved at mid length, boomerang-like, c. 115–125 Μm, as long as gonocoxite, in distal half with several setae placed at median margin and directed anteriorly ( Fig. 7 F–H). Median setae on anal tergite absent. Anal point triangular, tapering to blunt apex; in lateral aspect visible as straight prolongation of anal tergite, tip bent down ( Fig. 7 E); spinulae, crests and posterolateral setae unobservable from ventral/lateral side. Superior volsella stout, bean-shaped, with broadly rounded apex, directed posteromedially. Digitus not observed ( Fig. 7 F, G). Stem of median volsella c. 25 Μm long, bulb-shaped, with 5–6 slender leafshaped lamellae ( Fig. 7 I). Inferior volsella reaching 1/3 length of gonostylus at most, robust, club-shaped, armed with several stout curved setae ( Fig. 7 F, G).

Remarks. The structure of male hypopygium of Tanytarsus congregabilis suggests inclusion in the lugens species group. T. congregabilis fits the group diagnosis (Reiss & Fittkau 1971, Giłka & Paasivirta 2007) and displays the following character combination: gonostylus stout; median setae on anal tergite absent; superior volsella bean-shaped, with broadly rounded apex, directed posteromedially; digitus not observed; stem of median volsella bulb-shaped, bearing leaf-shaped lamellae ( Fig. 7 E–I). Unfortunately, the specimens examined are positioned such as to render observation of the dorsal aspect of hypopygium impossible, but the shape of the anal point in lateral view is similar to that known from Tanytarsus bathophilus Kieffer (Reiss & Fittkau 1971, fig. 36a). The peculiar, boomerang-shaped gonostylus is the best character for rapid identification of the species ( Fig. 7 F–H).

The large size (the largest among the known Eocene Tanytarsini ) of adult Tanytarsus congregabilis males are also consistent with the lugens group diagnosis, although T. congregabilis is one of the smallest representatives of the group. We observed a similar pattern in other families of dipterans of the suborder Nematocera which, in the Baltic amber, are represented by specimens smaller than those belonging to related extant taxa (cf. Szadziewski 1988). The nature of the resin trap, enclosing primarily small insects, does not seem to particularly affect the Tanytarsini , as neither the fossil nor extant representatives of the tribe are larger than 6 mm. Our hypothesis gains an additional support from the fact that the three T. congregabilis specimens examined in this study are syninclusions in an amber chunk slightly larger than 0.5 cm 3.

Figures

FIGURE 7. Tanytarsus congregabilis sp. nov., male. A— type series: holotype and two paratypes (syninclusions); B— habitus (holotype); C— antenna; D— wing; E – I— hypopygium: E— lateral aspect; F, G— ventral aspect; H— distal part of gonostylus; I— median volsella (H, I— magnified three times relative to hypopygium drawn).

Tables

TABLE 6. Leg segment lengths (µm) and leg ratios of male Tanytarsus congregabilis sp. nov.

fe	ti	ta1	ta2	ta3	ta4	ta5	LR
P1 715–805	590–660	775–795	460–510	365–415	280–285	115–125	1.28–1.34
P2 765–780	720–780	425–460	245–290	180–215	110–140	75–90	0.55–0.60
P3 700–715	910–915	515–555	335–365	260–290	150–170	90–110	0.57–0.61