Pleuroseta Richards, 1973

Kuwahara, Gregory K. & Marshall, Stephen A., 2023, A revision of the Australian genus Pleuroseta Richards (Diptera, Sphaeroceridae), European Journal of Taxonomy 910, pp. 69-97 : 70-74

Genus Pleuroseta Richards, 1973

Pleuroseta Richards, 1973: 355 (feminine). Type species: Leptocera (Pleuroseta) wentworthi Richards, 1973 , by monotypy.

Pleuroseta – Marshall 1989: 605 (as genus, Australasian/Oceanian catalog). — Roháček et al. 2001: 199 (world catalog). — Kuwahara & Marshall 2022: 17 (key).

Diagnosis

Pleuroseta species differ from all other genera of Sphaeroceridae in having a patch of setae on the posterodorsal corner of the anepisternum ( Fig. 1 ). The genus is further diagnosed by the combination of a relatively tall gena (eye height 1.5–2.4× genal height), a brown microtrichose notum, 3–5 pairs of dorsocentral bristles including at least one presutural pair, 2–4 pairs of dorsal bristles on the basal half of the mid tibia, an enlarged ventral seta on the mid basitarsus, male S5 with a posteromedial emargination surrounded by thick or long setae, and a surstylus divided into anterior and posterior parts (usually with long setae on the posterior section). The eastern Australian species form a monophyletic group further diagnosed below, but the single Western Australian species differs widely and is only provisionally included in the genus, as discussed below.

Redescription

BODY. Length 1.8–4.4 mm.

HEAD. Golden-brown to dark brown. Frontal width 1.9–2.3 × height. Interfrontal bristles in 4–5 pairs; two large lateroclinate orbital bristles, anterior orbital slightly smaller (0.8–0.9 ×) than posterior; ocellar and outer vertical bristles large and subequal; inner vertical bristle very large; paravertical bristle large, occipital bristle strong. Vibrissa large, subequal to genal length, vibrissal angle often with one distinct subvibrissal seta and 1–2 setulae; gena with large (0.6–0.7 × genal length) upcurved seta and 8–17 setulae. Lunule small, triangular; palpus elongate, clavate, narrow with 2–4 ventral setae. Antenna divergent; pedicel large, at least 0.7–1.0 × postpedicel; arista long pubescent. Eye with slight notch at about level of anterior edge of frons, eye height 1.5–2.4 × genal height.

THORAX. Dark brown to black, paler around margins, scutum and scutellum with fine brown microtrichia. Three to five pairs of dorsocentral bristles, middle pair(s) separated by 8–14 rows of fine acrostichal setulae. Two postpronotal bristles, inner bristle smaller; two notopleural bristles, anterior bristle larger; two supra-alar bristles, anterior bristle larger; postalar and intrapostalar bristles large. Anepisternum with posterodorsal patch of 3–8 setae and 0–13 setulae; katepisternum with distinct anterior seta, larger posterior seta, and several ventral setae. Scutellum large, length 0.4–0.6 × width, with four long marginal bristles, basal pair 0.8–0.9× length of apical pair.

LEGS. Brown to dark brown, with or without yellow bands on femora and/or tibiae; tarsi brown to yellow. Mid tibia with 4–8 dorsal bristles (2–4 pairs) in proximal half and 3–4 dorsal bristles (one anterodorsal, 1–2 dorsal, and one posterodorsal) in distal half. Ventral surface of mid tibia with strong apical seta in males of the P. wentworthi species group (see below) and row of thickened setae (but no enlarged apical) in male P. occidentalis sp. nov.; females with large apical seta. Mid basitarsus with enlarged ventral seta.

WING. Evenly infuscate or mostly infuscate with ill-defined pale spots and darker patches (mostly around the crossveins). Second costal sector 0.9–1.5 ×length of third costal sector. Vein R 2+3 sinuate; R 4+5 sinuate or gently upcurved; costa ending at or extending past apex of R 4+5. M 1 not tubular beyond dm-m (except slightly in P. occidentalis sp. nov.) but extending almost to wing margin as more-orless (terminally) coloured pseudovein; M 4 extending beyond dm-m, about halfway to wing margin; crossveins r-m and dm-m separated by 1.2–1.5× length of dm-m. CuA+CuP distinct but not tubular in the P. wentworthi species group, indistinct in P. occidentalis ; CuA slightly visible as a diffuselycoloured pseudovein in the P. wentworthi species group. Alula relatively large but narrow, length 2.8–4.5 × width, outer edge sinuate. Halter brown with pale apex.

MALE ABDOMEN. Preabdomen brown. T2–5 and S2–4 broad rectangular, desclerotized marginally, uniformly long setose in posterior half; T4 and T5 with long posterolateral setae. S5 broad, uniformly setose laterally with emarginate or desclerotized posteromedial area. Synsternite 6+7 (S6+7) highly modified in species other than P. occidentalis sp. nov.; S6 highly modified in association with posteromedial part of S5, S7 with elongate dextral extension sometimes reaching right side of abdomen; ring sclerite usually present. Epandrium often large (0.5–0.9× head height), rounded, open posteroventrally (i.e, subanal plate absent) in P. wentworthi species group; cercus strongly fused to and usually indistinct from epandrium (prominent only in P. occidentalis ), with several long setae; subepandrial sclerite well-developed and X- or Y-shaped. Hypandrium Y-shaped (as in Fig. 5C ) or V-shaped, with a broad sclerotized sheet between the lateral arms (as in Fig. 5D ), anteromedial apodeme sometimes weakly developed beyond articulation point with lateral arms; lateral arms strongly connected to anteroventral corners of epandrium and anteromedial apodeme of hypandrium (except in P. occidentalis , in which lateral arms are not strongly fused to anteromedial apodeme); no additional posterodorsal or ventral lobes. Surstylus various, bi- or trilobed and narrowly fused posterodorsally to ventral corner of subepandrial sclerite. Postgonite various. Distiphallus composed of several sclerites and divided into basal and apical sections.

FEMALE ABDOMEN. Preabdomen dark brown. T2–5 and S2–5 broad rectangular, desclerotized marginally, uniformly long-setose along entire surface; T4 and T5 with long posterolateral setae. Postabdomen telescoping, fitting fully or partially into segment 5 when at rest: elongate and largely membranous in P. occidentalis sp. nov., stout and well-sclerotized in the P. wentworthi species group. Spermathecae (2+1) elongate and pear-shaped with an elongate, subconical stem, or stout and spherical with a short stem; ducts elongate and membranous.

Similar genera

As noted by Richards (1973), Pleuroseta and Howickia Richards, 1951 (= Biroina Richards, 1973 ) have similar mid tibial chaetotaxy, and some characters of the male genitalia also suggest that the genera are closely related. Species of both genera often have multi-lobed surstyli, blunt setae on S5 and surstyli, and a distiphallus composed of several sclerites. Pleuroseta , however, does not have the ventral and posterodorsal hypandrial lobes that defines Howickia and further differs from Howickia in having at least three pairs of large dorsocentral bristles, a large globose epandrium, large phallic structures that often bulge out from between the surstyli, and a bipartite (as opposed to tripartite) female T8. Species in the P. wentworthi species group also differ from Howickia in having a sinuate R 4+5, male tibia lacking ventral bristles other than an apicoventral bristle, a large alula, and conspicuous modifications to S6+7. Some highly distinctive characters of S6+7, especially the relatively complex structures on the ventral part of S6, are unique to the P. wentworthi group.

Richards (1973) also noted the maculated legs and paired tibial bristles of Pleuroseta as superficial similarities to Poecilosomella Duda, 1920 , but Pleuroseta has an unmaculated thorax, less distinctly patterned wings, and a straighter R 2+3. The male and female genitalia of Pleuroseta and Poecilosomella are dissimilar, offering no evidence for a close relationship between these two genera.

Pleuroseta wentworthi group species superficially resemble many species of Leptocera Olivier, 1813 , Rachispoda Lioy, 1864 , and Pseudocollinella Duda, 1924 in having a large body, multiple large dorsocentral bristles, paired (or almost paired) dorsal bristles on the basal half of the mid tibia, and an enlarged ventral mid basitarsal seta. However, Pleuroseta differs from those genera in having an apicoventral bristle on the mid tibia, and a patch of posterodorsal setae on the anepisternum; the former two genera also differ from Pleuroseta in having more than four scutellar bristles. No genitalic characters suggest that the above genera are related to Pleuroseta . Other genera with an enlarged ventral mid basitarsal seta (such as Chaetopodella Duda, 1920 , Opacifrons Duda, 1918 , Paralimosina Papp, 1973 , and Spinilimosina Roháček, 1983 ) do not seem to have other characters that suggest a close relationship with Pleuroseta and differ widely in characters of the male terminalia.

Distribution

Pleuroseta is thus far known only from Australia, where three species occur in New South Wales and/or Queensland, and one occurs in Western Australia.

Systematics

Of the four known species of Pleuroseta , three ( P. wentworthi , P. ingens sp. nov., P. monteithi sp. nov.) are externally similar, with a relatively large body, striped legs, patterned wings, eight proximodorsal bristles on the mid tibia, at least two pairs of distinct presutural dorsocentral bristles, distinctively enlarged male genitalia with modifications to the male S6+7, short female terminalia, and pear-shaped spermathecae with elongate, subconical stems; these three species form the P. wentworthi species group. The remaining species, P. occidentalis sp. nov., is much smaller and uniform in colour, with reduced overall chaetotaxy, smaller male genitalia with a simple male S6+7, and female terminalia with segments 6–9 elongated to form a largely membranous, telescopic ovipositor similar to that of most species of Howickia . Other characters such as the posterodorsal patch of setae on the anepisternum, enlarged presutural dorsocentral bristle, mid basitarsus with an enlarged ventral bristle, posteromedial emargination of the male S5, and two-part surstyli support the placement of P. occidentalis in the genus.

The placement of P. occidentalis sp. nov. in Pleuroseta is provisional, given the marked differences between this species and the more homogeneous P. wentworthi species group, and the relatively weak putative synapomorphies linking P. occidentalis to the P. wentworthi group. However, there are no characters suggesting that Pleuroseta including P. occidentalis is a paraphyletic group (i.e, there are no synapomorphies linking either the P. wentworthi group or P. occidentalis to another genus). In the absence of such evidence, it is better to treat P. occidentalis as a provisional member of Pleuroseta , to which it is easily keyed on the basis of the patch of anepisternal setae, than to place it in a monobasic new genus. Some characters of P. occidentalis (unmaculated wings and legs, gently curved R 4+5, elongate telescopic female terminalia) superficially resemble some species in the large, heterogeneous genus Howickia . Pleuroseta occidentalis , however, lacks the tripart hypandrium that defines Howickia .

Key to species of Pleuroseta

1. Small species (1.8–2.3 mm). Legs entirely dark brown; wing uniformly infuscate and not patterned. Proximal half of mid tibia with four dorsal bristles. Male S5 with stout, tooth-like setae flanking posteromedial emargination; ventral part of S6 unmodified ( Fig. 3B ). Female terminalia elongate, telescoped, largely membranous ( Fig. 4A–B ) ....................................... P. occidentalis sp. nov. (WA)

– Large species (2.8–4.4 mm). Legs dark brown with yellow bands; wing infuscate with pattern of light spots. Proximal half of mid tibia with eight dorsal bristles. Male S5 bare or with unmodified setae flanking posteromedial emargination; S6 with additional lobes situated beneath posteromedial edge of S5 (as in Fig. 7B ). Female terminalia short, well-sclerotized ( Figs 9A–B , 14A–B )............ 2

2. Fore tibia entirely brown; fore tarsomeres 2–5 white. Interfrontal bristles in two larger pairs and two smaller pairs. First postsutural pair of dorsocentral bristles separated by 9–14 acrostichal setulae. Male terminalia extremely large, epandrium almost as large as head ( Figs 6A , 10A )...................... 3

– Fore tibia with basal two-thirds yellow; all fore tarsomeres yellow. Interfrontal bristles in three large pairs and one small pair. First postsutural pair of dorsocentral bristles separated by 7–9 acrostichal setulae. Male terminalia relatively small, epandrium much smaller than head ( Fig. 12A )................. ..................................................................................... P. wentworthi ( Richards, 1973) (NSW, QLD)

3. First postsutural pair of dorsocentral bristles separated by 12–14 rows of acrostichal setulae. Male S5 with deep, linear posteromedial emargination extending almost to anterior margin of sternite ( Fig. 7B ). S6 with three distinct, parallel, posteriorly directed lobes ( Fig. 8A ). Anterior section of surstylus with closely approximated dorsal and ventral lobes, dorsal lobe rectangular and ventral lobe short. Basal section of distiphallus short, stout and boot-shaped, apical section longer ............. ..................................................................................................................... P. ingens sp. nov. (QLD)

– First postsutural pair of dorsocentral bristles separated by 9–10 rows of acrostichal setulae. Male S5 with large, triangular posteromedial emargination reaching at most to anterior fifth of sternite ( Fig. 11B ). S6 with a dark T-shaped sclerotization, only the middle lobe of which extends posteriorly. Anterior section of surstylus with well-separated dorsal and ventral lobes, dorsal lobe semicircular and ventral lobe elongate. Basal section of distiphallus elongate, curved, apical section shorter ...................................................................................................... P. monteithi sp. nov. (QLD)

Figures

Fig. 1. Pleuroseta ingens sp. nov., anepisternum showing posterodorsal patch of setae and setulae (debu00161215). Scale bar =0.50 mm.

Fig. 5. A. Pleuroseta occidentalis sp. nov. paratype, left wing, ventral view (debu00244165). B. Pleuroseta occidentalis paratype, hypandrium, dorsal view (debu00245797). C. Pleuroseta wentworthi (Richards, 1973), hypandrium, dorsal view (debu00264733). D. Pleuroseta ingens sp. nov. paratype, hypandrium, dorsal view (debu00160984). E. Pleuroseta monteithi sp. nov. paratype, hypandrium, dorsal view (debu00160892). Scale bars: A=0.50 mm, B–E=0.10 mm.

Fig. 3. Pleuroseta occidentalis sp. nov. paratype (debu00245797). A. Aedeagus and associated structures, lateral view. B. Male S5–8, ventral view.Abbreviations: bp =basiphallus; dp =distiphallus; ea=ejaculatory apodeme; phal=phallapodeme; pg =postgonite. Scale bars: A=0.13 mm; B=0.25 mm.

Fig. 4. Pleuroseta occidentalis sp. nov. paratype (debu00245809). A. Female terminalia, dorsal view. B. Female terminalia, ventral view. C. Spermathecae. Abbreviations: cer =cercus; epi =epiproct; hyp =hypoproct. Scale bars: A–B= 0.40 mm; C= 0.04 mm.

Fig. 7. Pleuroseta ingens sp. nov.paratype (debu00160984).A.Aedeagus and associated structures, lateral view. B. Male S5–8, ventral view. Abbreviations: bp =basiphallus; dp =distiphallus; pg =postgonite; phal=phallapodeme; rs =ring sclerite. Scale bars: A=0.40 mm; B= 0.70 mm.

Fig. 9. Pleuroseta ingens sp. nov. paratype (debu00162621).A. Female terminalia, dorsal view.B. Female terminalia, ventral view. C. Spermathecae. Abbreviations: cer =cercus; epi =epiproct; hyp =hypoproct. Scale bars: A–B =0.80 mm; C=0.20 mm.

Fig. 14. Pleuroseta wentworthi (Richards, 1973) (debu00264635). A. Female terminalia, dorsal view. B. Female terminalia, ventral view. C. Spermathecae. Abbreviations: cer =cercus; epi =epiproct; hyp =hypoproct. Scale bars: A–B = 0.40 mm; C = 0.06 mm.

Fig. 6. Pleuroseta ingens sp. nov.A. Male paratype habitus, lateral view (debu00162612). B. Male paratype terminalia, lateral view (debu00160984).C. Maleparatype terminalia, posterolateral view (debu00160984). Abbreviations: bp=basiphallus; cer=cercus; dp=distiphallus; epa=epandrium; hyp=hypandrium; se=subepandrial sclerite; ss=surstylus. Scale bars: A =1.0 mm; B–C=0.40 mm.

Fig. 10. Pleuroseta monteithi sp. nov. A. Male holotype habitus, lateral view (debu00160870). B. Male paratype terminalia, lateral view (debu00160892). C. Male paratype terminalia, posterolateral view (debu00160892). Abbreviations: epa =epandrium; hyp =hypandrium; se =subepandrial sclerite; ss =surstylus. Scale bars: A = 1.0 mm; B–C =0.40 mm.

Fig. 12. Pleuroseta wentworthi (Richards, 1973).A. Male habitus, lateral view (debu00403539).B. Male terminalia, lateral view (debu00264733). C. Male terminalia, posterolateral view (debu00264733). Abbreviations: epa =epandrium; hyp=hypandrium; se = subepandrial sclerite; ss =surstylus. Scale bars: A =1.0 mm; B–C =0.30 mm.

Fig. 8. Pleuroseta ingens sp. nov. paratype (debu00160984).A. Male S6+7 and ring sclerite, ventrolateral view. B. Male surstylus, anterolateral view. Abbreviations: rs= ring sclerite. Scale bar: A=0.50 mm; B=0.30 mm.

Fig. 11. Pleuroseta monteithi sp. nov. paratype (debu00160892). A. Aedeagus and associated structures, lateral view. B. Male S5–8, ventral view. Abbreviations: bp = basiphallus; dp =distiphallus; pg =postgonite; phal=phallapodeme; rs =ring sclerite. Scale bars: A=0.40 mm; B= 0.60 mm.