Arthroceras Williston

Kerr, Peter H., 2010, 2592, Zootaxa 2592, pp. 1-133 : 73-75

Genus Arthroceras Williston

Figs. 5, 28, 44, 69, 90, 115–116, 127, 137, 159.

Arthroceras Williston 1886: 107 . Type species Arthroceras pollinosum Williston 1886 , by subsequent designation ( Coquillett 1910: 510).

Ussuriella Paramonov 1929: 181 . Type species Ussuriella gadi Paramonov 1929 , by monotypy.

Pseudocoenomyia Ôuchi 1943: 493 . Type species Pseudocoenomyia sinensis Ôuchi 1943 , by original designation.

Diagnosis. No single, obvious autapomorphic feature is known to define Arthroceras conclusively. However, it retains the following unique combination of primitively- and more recently-derived states: males are unique among orthorrhaphous flies in that their genitalia have both well-developed lateral ejaculatory processes and aedeagal tines. The spermathecal ducts of females are unique in that they are visibly inserted within the tubing that forms their common junction, near the genital chamber. The female terminalia also have spermathecal duct accessory glands.

Arthroceras species are mid-sized to large (4.5 to 13 mm), black, gray, or often yellowish-colored flies that have a fairly long, tapering antenna consisting of 5–8 flagellomeres, mandibles absent, laterotergite setose, tibial spur formula 0:2:1, M 3 present; hind tibial macrochaetae absent; female tergite 9 without ventrolateral arms; female spermathecal ducts with accessory glands. Arthroceras are restricted to the Holarctic Region. They are distinguished from all other Rhagionidae by the form of their antenna, which is composed of 5–8 similarly sized flagellomeres, tapering distally ( Fig. 5). They may be distinguished from Glutops Burgess (Pelecorhynchidae) by their setose laterotergite and parafacials not swollen and from Pseudoerinna Shiraki (Pelecorhynchidae) by the absence of conspicuously setulose eyes and fore tibial spurs.

Description. Head. Clypeus bulbous. Scape smaller than or subequal to pedicel. First flagellomere slightly enlarged, round in cross section. Antenna with 5–8 flagellomeres of similar shape, tapering distally; terminal flagellomere usually more elongate. Eyes inconspicuously setulose; in female, dichoptic; in male, holoptic, ommatidia split into dorsal and ventral areas and smaller ventrally, not strongly flattened dorsally. Labella with pseudotracheae, longer or shorter than palpus. Theca short and stout, divided into two separate, lateral sclerites. Palpus two-segmented; distal segment longer than proximal segment. Stipes convergent toward one another medially. Lacinia present, shorter than palpus, tip not serrated. Mandibles absent. Cibarial pump long, clearly not as wide as long. Cornu nearly as long as or longer than cibarial pump. Pharyngeal pump moderately broad anteriorly, mostly flat along its length, approximately same length as cibarial pump (excluding cornu).

Thorax. Mesonotum with or without vittae. Dorsocentral bristles absent; all dorsal setae of equal length. Anepisternum setulose on dorsal and posterior margins. Laterotergite setose. Proscutellum absent. Subscutellum mostly flat or slightly bulbous. Wing hyaline, without markings, or membrane lightly infuscate. Lower calypter reduced. Upper calypter well developed, with broad curvature, lobe-like, width twice length or less. Costa extends to wing tip or just past wing tip, to R 5. Humeral crossvein well developed. Sc-r crossvein present, very weakly developed, positioned distal to h by approximate length of h. Dorsal side of R 1 setulose, ventral side bare. All other wing veins without setulae. R 1 and R 2+3 separated at wing margin. R 2+3 sinuous, apical third of R 2+3 ultimately bends anteriorly slightly, toward wing tip. Length of R 2+3 longer than R 5, but less than twice as long. R 4+5 aligned with R 5. Base of R 4 –R 5 fork proximal or directly above distal end of cell dm. R 4 nearly straight apically. R 4 and R 5 contain wing tip. R 5 aligned with R 4+5. R 5 clearly longer than R 4+5 (r-m to R 4 origin). R-m crossvein at proximal side of central one-third of discal cell (or more centrally). CuA 1 origin at bm. CuA 2 greater than 1/2 length of posterior vein of cell bm, less than 2/3 length of posterior vein of cell bm. M 3 present. Alula full, rounded, with broad curvature. Anal lobe well developed. Cell cu p open. Halter knob between 1/2–2/3 length of stem. Tibial spur formula 0:2:1. Hind coxal tubercle absent. Hind tibial macrochaetae absent. Postmetacoxal bridge absent.

Abdomen. Abdominal segments evenly tapered. In female, last 3 abdominal segments telescoping; tergite 7 much wider than long; intersegmental membrane between segments 7 and 8 especially long; sternite 8 as wide as long or wider than long. Male terminalia with epandrium simple, not containing hypandrium ventrally. Epandrium wider than long, strongly notched anteriorly. Tergite 10 absent. Hypoproct triangular (rounded posteriorly), setose. Cercus base held underneath epandrium. Cerci directly adjacent, separation distance one quarter width of cercus or less. Cerci, in posterior view cupped, forming circular outline medially. Hypandrium separated partially from gonocoxites by incomplete suture. Gonocoxite with sinuous dorsal ridge, leading to gonocoxal apodeme. Gonocoxal apodemes short or long enough to reach anterior margin of hypandrium. Sperm sac forming separate, distinct lobes ventrally. Lateral ejaculatory processes present, not part of sperm sac posteriorly. Ejaculatory apodeme moderately long, reaching anterior margin of hypandrium, rod-shaped or laterally compressed. Aedeagal tines present. Endoaedeagal process present. Female terminalia with three spermathecae, spherical, moderately to well sclerotized. Spermathecal ducts longer than five times length of sternite 9, but not so long as to be difficult to measure; not inflated at base of spermathecae. Spermathecal duct accessory glands arise at approximately halfway along the length of the spermathecal ducts. Ejection apparatus of spermathecal ducts thickened, sclerotized, with furrows. Common spermathecal duct thickened, subequal in length to longest diameter of genital chamber. Genital chamber oval, moderately sized. Accessory gland posterior to genital chamber prominent, retains dye easily; with paired extensions posteriorly. Sternite 9 anterior end pointed, posterior end with broad extensions posteriorly that are held in horizontal plane. Tergite 10 partially split, short (length less than half width). Sternite 10 roughly pentagonal, pointed posteriorly, posterior half below first cercal segment. Cercus two-segmented. First segment of cercus not elongate, with or without ventral process. Ventral lobes of first segment of cercus not curving ventrally towards one another to form ring. Basal cercal segment adjacent dorsally. Second cercal segment not elongated with or without apical sensory pits.

Larva. Unknown.

Biology. Members of this genus are found in the Russian Far East, Canada, and the USA. Arthroceras and Spania exhibit similar distributions ( Nagatomi 1966). Their biology is not known.

Literature. Key to Arthroceras of the world in Nagatomi (1966). Key to Nearctic species in Webb (1987b).

Notes. In the Palearctic catalogue ( Majer 1988), Arthroceras pollinosum is given as the type species for the genus, by original designation. Webb (1987b) states that the type species is Arthroceras pollinosum by ‘original description.’ Both are incorrect. Williston (1886) created Arthroceras for two species, Arthroceras pollinosum and Arthroceras leptis (Osten Sacken) and did not explicitly designate the type species for the genus. James (1965) correctly gives credit to Coquillett (1910) for the type species designation.

The earliest family group name available for Arthroceras is Arthrocerinae , even though the correct stem for this genus is Arthrocerat- ( Sabrosky, 1999).