Rheotanytarsus alliciens 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 : 579-581

Rheotanytarsus alliciens Giłka et Zakrzewska , sp. nov.

Type material. Holotype. Adult male (distal flagellomeres, palpi and tarsi missing or broken and separated) preserved in 12 x 7 x 2 mm piece of amber (SIZ 155, Fig. 6 A; syninclusion: Acari, 1 specimen).

Derivatio nominis. From Latin, meaning ‘interesting, engaging’. This is the oldest Rheotanytarsus , known from male with a fully-developed hypopygial digitus.

Diagnosis. Tibial combs of mid and hind legs with short spurs at most twice the length of the comb. Gonostylus distinctly narrowed in distal half, much longer than gonocoxite. Anal point long, triangular, tapering to slender pointed apex, bearing two long crests; spinulae absent. Superior volsella roundish, with slightly projecting posterior part. Digitus long, distinctly extending beyond superior volsella. Stem of median volsella stout and short, with several pectinate or leaf-shaped lamellae fused at bases.

Description. Adult male (n = 1). Total length 2.3 mm; wing length 1315 Μm.

Head ( Fig. 6 B, C). Eyes bare, with dorsomedian extensions. Frontal tubercles not observed. Antenna with 13 flagellomeres. Clypeals unobservable.

Thorax ( Fig. 6 B). At least several acrostichals present; Dc at least 7 on each side, Pa 2 on each side, Scts at least 5.

Wing ( Fig. 6 D). Slender, with anal lobe weak, broadest at 2/3 length, width: 380 Μm, length/width ratio 3.46. Venation observable only in part. FCu placed distally of RM; VR Cu non-measurable. Visible veins ending as follows (in order from base to tip): An, Cu1, M3+4, R4+5, M1+2. Wing macrotrichia unobservable.

Legs. Apices of fore leg tibiae poorly preserved. Tibial combs of mid and hind legs with short spurs at most twice the length of the comb. Sensilla chaetica on ta1 of p2 not observed. Pulvilli not observed (distal tarsomeres missing or poorly preserved). For length of leg segments and leg ratios see Table 5 .

Hypopygium ( Fig. 6 E–H). Gonostylus cylindrical, slightly curved, distinctly narrowed in distal half, c. 90 Μm long, much longer than gonocoxite. Anal point long, triangular, wide at base, tapering to slender pointed apex, bearing two long crests, spinulae absent, posterolateral setae not observed ( Fig. 6 E, F). Superior volsella roundish, with slightly projecting posterior part; digitus long, distinctly extending beyond superior volsella ( Fig. 6 E–G). Stem of median volsella stout but short, c. 5 Μm, with several pectinate or leaf-shaped lamellae fused at bases ( Fig. 6 G, H). Inferior volsella reaching 1/3 length of gonostylus, somewhat S-shaped, with slightly swollen head-like apex ( Fig. 6 E, F).

Remarks. Adult males of Rheotanytarsus are distinguishable by the long and usually distally narrowed gonostylus and by the elongate anal tergite point and its crests. Their lamellae are more or less fused into a plate on the stem of the median volsella. They are also known to lack the digitus. In rare cases, the lamellae are separate or fused only at their bases, and the digitus is vestigial and only exceptionally fully developed (Kyerematen & Saether 2000, Giłka 2011a, Moubayed-Breil et al. 2012). We recognize that the fusion of the lamellae and the formation of the plate, as well as the reduction of the digitus are distinct trends in Rheotanytarsus ; in contrast, the lack of the plate and the presence of the digitus indicate plesiomorphy as displayed by R. alliciens ( Fig. 6 E–H) and few extant Rheotanytarsus (op. cit.). The adult male of R. alliciens is peculiar in the shape of its superior volsella which is roundish, but has the slight posterior projection typical of Rheotanytarsus ( Fig. 6 F, G).

Figures

FIGURE 6. Rheotanytarsus alliciens sp. nov., male. A— holotype (inclusion); B— habitus; C— head with antenna; D – wing; E – H— hypopygium: E, F— dorsal aspect; G— superior volsella, digitus and median volsella; H— median volsella (magnified c. three times relative to hypopygium drawn).

Tables

TABLE 5. Leg segment lengths (µm) and leg ratios of male Rheotanytarsus alliciens sp. nov.

fe	ti ta1 ta2	ta3	ta4	ta5	LR
P1 -	305–310 - -	-	-	-	-
P2 -	- - -	-	-	-	-
P3 670	595–605 380–400 255	195	110	-	0.64–0.66