Psectrosciara crassieton Amorim and Greenwalt, 2022

Greenwalt, Dale E., Amorim, Dalton De Souza, Hauser, Martin, Kerr, Peter H., Fitzgerald, Scott J., Winterton, Shaun L., Cumming, Jeffrey M., Evenhuis, Neal L. & Sinclair, Bradley J., 2022, Diptera of the Middle Eocene Kishenehn Formation II, Palaeontologia Electronica (a 22) 25 (2), pp. 1-52 : 4-6

Psectrosciara crassieton Amorim and Greenwalt , sp. n.

Figure 1G

zoobank.org/ 379D327C-CA5D-4EDF-867A-DCBE30BB84E6

Holotype. Female, USNM 625035 , deposited in the Paleobiology collections of the National Museum of Natural History in Washington, D.C.

Locality and horizon. Spring site, Middle Fork of the Flathead River (Pinnacle, Montana, USA). Middle Eocene Coal Creek Member, Kishenehn Formation.

Etymology. The species epithet crassieton is derived from the Greek words crassus (fat, stout) and eton (abdomen, belly) in reference to the more stout habitus of this specimen relative to P. makrochaites .

Diagnosis. Psectrosciara crassieton is differentiated from extant or the two other extinct species of the genus, P. makrochaites and P. fossilis , by its stout size and M 1 incomplete, interrupted at its base, respectively.

Description. Female. Body length, 2.65 mm ( Figure 1G ). Head dark brown, 0.28 mm long, 0.27 mm high; antenna uniformly dark brown, 0.28 mm long, terminal flagellomere elongated, bulbous, 87 μm long, 72 μm wide, number of flagellomeres not discernible; palpus brown, narrow, elongated and setose; labellum brown. Thorax dark brown, longer than wide, 0.65 mm long; scutum bare. Legs dark reddish brown, fore coxa large. Wing 1.06 mm long, 0.5 mm wide, Sc not visible, ratio wing length/ section costal 1, 3.32; WL/C2, 2.58; WL/C3, 3.19; C1+C2/WL = 0.69. Abdomen 1.85 mm long, 0.54 mm high, tergites reddish brown with light intersegmental areas, lateral aspects of tergites covered with long, thin hair, 0.1 mm long, perhaps twice that long; spermatheca sclerotized, 0.09 mm in diameter, tergite 8+9 dark, sclerotized.

Synimpressions. Impression of a coprolite

Remarks. The superfamily Scatopsoidea is comprised of the small families Scatopsidae , Canthyloscelidae and Valeseguyidae , the latter with a single extant species. There is a controversy about the position of the superfamily, with authors assigning it either to the Psychodomorpha ( Wood and Borkent, 1989; Amorim, 1994) or to the Bibionomorpha (e.g., Wiegmann et al., 2011). The largest of these families, Scatopsidae , contains nearly 400 extant species in 36 genera, the vast majority of which (31) are in Scatopsinae ( Haenni and Amorim, 2017) . The subfamily Psectrosciarinae is composed of the genera Psectrosciara and Anapausis Enderlein, 1912 , with 25 and 43 described extant species, respectively ( Haenni and Amorim, 2017). Psectrosciarinae has a single described fossil, Psectrosciara fossilis , from 16 Ma Chiapas amber ( Nel and Coty, 2016).

Psectrosciara makrochaites and P. crassieton both clearly fit in the genus Psectrosciara . The scutum without an elevated U-shaped anterior ridge, scutellum without long setae, the fore tibia not produced beyond the base of the tarsi, the apex of hind tibia not club-shaped and strongly swollen apically, C not swollen at junction with R 4+5, r-m present, M not fused with R 4+5, base of M 1 absent and wing with macrosetae (not observed in P. crassieton ) are Psectrosciarinae features observed in both species described here. The specimens can be recognized as belonging to the genus (i.e., not to Anapausis ) based on R 4+5 long, gradually approaching C and body elongate ( Cook, 1958, 1981).

Both P. makrochaites and P. crassieton belong to the brunnescens group of Psectrosciara , as indicated by the sinuose CuA, the absence of modification of the fore and hind tarsomeres and the modified setae at the tip of the tibiae; as such, both species fit well in the brunnescens group. Females of the group- scatopsiformis have an elongate spermatheca; the males have stout, short spine-like setae on the tibiae and the tarsi ( Amorim, 1982). Ten species of Psectrosciara are known from North America, six of which belong to the brunnescens group ( Amorim and Brown, 2020). In Cook’s (1958) study of the genus, the females of P. forcipata and P. stonei belong to the brunnescens group. Psectrosciara makrochaites is clearly more similar to P. stonei based on the shape of the female terminalia sclerites.

The holotype of P. makrochaites and P. crassieton , both females, differ from P. fossilis (known from the male holotype), also a species of the brunnescens group ( Nel and Coty, 2016) in that in this latter species M 1 is complete, not interrupted at the base. Psectrosciara makrochaites and P. crassieton differ from one another in that the latter is more stout. The ratios of the length of the scutum to the length of the abdomen is 4.4 in P. makrochaites vs. 2.9 in P. crassieton ; the ratio of the length of the abdomen to the height of the abdomen is 5.3 in P. makrochaites vs. 3.2 in P. crassieton .

Fossils of ectaetiine scatopsids are known from late Albian/early Cenomanian mid Cretaceous amber fossils of France ( Fate et al., 2013). The Ectaetiinae are assumed to be sister to all remaining scatopsids except the Aspistinae ( Amorim, 1982, 1994), meaning that the Psectrosciarinae should not be considered a young offshoot in the evolution of the family, but rather a group that may have originated at the mid or late Cretaceous. Nevertheless, Psectrosciara makrochaites and P. crassieton are the oldest known representatives of their subfamily, already corresponding to differentiated members of the brunnescens group.

Figures

FIGURE 1. Psectrosciara makrochaites Amorim and Greenwalt sp. n., female, USNM 625934 and P. crassieton Amorim and Greenwalt sp. n., female, USNM 625035. A, P. makrochaites, habitus; B, Head and antennae; C, Wing, veins M1, M2 and CuA; black arrows denote long abdominal hairs; D, Microsetae on wing surface; E, Legs. Short arrow denotes fore tibia not produced beyond base of tarsus, long arrow denotes apex of hind tibia not club-shaped, swollen at apex; F, Genitalia, spermatheca; G, P. crassieton, habitus. Scale bars equal 1.0 mm (A, G), 0.5 mm (B), 0.3 mm (C), and 0.2 mm (D-F).