Dixella curvistyla Greenwalt and Moulton

Greenwalt, D, 2016, The first fossil New World Dixidae with a critical discussion of generic definitions, Palaeontologia Electronica 19 (3), pp. 1-32 : 8-10

Dixella curvistyla Greenwalt and Moulton , new species

Figures 3.3. 4.2 , 8 , 9

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Etymology. The specific epithet is derived from a combination of the terms curvi (L) meaning curved and stylus (L) meaning stalk or pike, and denotes the sharply curved gonostylus of this species.

Holotype. Dixella curvistyla Greenwalt and Moulton , male; NMNH, USNM 546860 .

Type Horizon. Middle Eocene Coal Creek member, Kishenehn Formation.

Type Locality. Disbrow Creek site, Middle Fork of the Flathead River (Pinnacle, Montana) .

Differential Diagnosis. Distinguished from the three previously described Eocene fossil Dixella based on wing width (1.32 mm vs. 0.56 in D. distans Hennig, 1966 ), length of flagellomere F1 (0.234 mm vs. 0.415 mm in D. succinea Meunier, 1906 ) and form of the gonostylus (very long and narrow in D. filiforceps Hennig, 1966 ).

Description. Adult male ( Figure 8 ), body length 3.36 mm.

Head. Black. Eyes dichoptic, large, oblong, width 0.186 mm, length 0.284 mm. Three prominent setae at lateral margin of eye. Maxillary palp length 0.34 mm, two basal segments wider than long (31 x 41 μm); segments three and four 42 and 54 μm long, respectively, both 35 μm wide; terminal segment length 0.169 mm, width 21 μm, terminus with several short setae ( Figure 9.1 ). Most distal flagellomeres missing ( Figure 9.1 ). Scape black, large, subspherical, 72 μm in diameter. Pedicel black, globular, diameter 0.1 mm. Flagellomeres setose, light brown. F1 cylindrical, diameter 32μm, length 0.23 mm. F2–F4 gradually decreasing in diameter.

Thorax. Lateral view. Black, length 1.08 mm, no setae apparent.

Wings. Length 2.89 mm (measured from base of the humeral crossvein), 1.17 mm wide. L/W ratio = 2.47. Vein R 2+3 arched ( Figure 3.3 ). Vein Sc apex even with Rs origin. Crossvein r-m distal to Rs bifurcation. Crossvein m-cu even with r-m apex, less heavily pigmented than r-m. A1 apex slightly basal to Cu bifurcation, distal to Sc apex, Rs origin. R 2+3 bifurcation even with M bifurcation. Ratios R 2 / M 2, M 2 /M 1, M 2 /M 1+2, R 2+3 /R 3 = 2.03, 0.70, 0.75, 0.52, respectively. Wing surface infuscate; veins, except M, Cu, margin of wing covered with setae. Haltere dark brown, length 0.48 mm; knob oblong, diameter 119 μm.

Legs. All legs setose, brown. At least two legs, including left hind leg, with tarsal claws with four observable teeth ( Figure 4.2 ). All legs mostly disarticulated; mid and hind tarsi identification based on swollen apical portion of metatibia, combined fragment lengths. The foreleg femur, tibia lengths 1.08 and 1.0 mm, respectively. Midleg femur, tibia, tarsus lengths 1.50, 1.13 mm, 1.70 mm, respectively. Hind leg femur, tibia, and tarsus lengths 1.30, 1.26, 1.9 mm, respectively.

Abdomen and genitalia. Abdomen brown, setose, length 2.35 mm (including genitalia). Anterior half of abdomen with light bands of varying widths wherein setae are absent or reduced. Sternite 9 posterior margin with numerous setae. Gonocoxite length 220 μm, apical lobe length 79 μm, width 24 μm ( Figure 9.2 ), distal margin with several prominent setae, each about 10 μm in length. Gonostylus length 158 μm, very broad, base 79 μm, midpoint 54 μm, bent approximately 75 degrees at midpoint, tapering to very fine point.

Female unknown.

GREENWALT & MOULTON: NEW WORLD FOSSIL DIXIDAE

Figures

FIGURE 3. Line drawings of wings. 1, Dixella intacta; 2, D. eomarginata; 3, D. curvistyla. Wing venation is from McAlpine (1981). Scale bar equals 1 mm.

FIGURE 4. Tarsal claw teeth of the Kishenehn Dixella. 1, Dixella intacta left metatarsal claw; 2, D. curvistyla left metatarsal claw; 3, D. spinilobata right metatarsal claw. Arrows point to teeth. Scale bars equals 50 μm.

FIGURE 8. Habitus of Dixella curvistyla, holotype. Scale bar equals 3 mm.

FIGURE 9. Morphological features of Dixella curvistyla. 1, Head and antennae; 2, Terminalia. The arrow points to the apical lobe of the gonocoxite. Scale bars equal 0.5 mm (1) and 0.2 mm (2).