Darthus Webb & McCafferty, 2007

Webb, J. M. & Mccafferty, W. P., 2007, A new genus and species of Heptageniidae (Ephemeroptera) from Borneo, with revisions to the classification of the Ecdyonurinae, Zootaxa 1478 (1), pp. 41-48 : 41-45

Darthus Webb & McCafferty n. gen.

Larval description: Head: Subquadrate, anterior margin of head capsule slightly thickened (Fig. 2). Outer incisor of planate mandible with single terminal denticle. Hypopharynx with well developed lateral projections. Maxilla with scattered simple setae on ventral surface of galealacinia, distal dentiseta branched near base (Fig. 7). Labium with palps pointed apically, glossae subquadrate (Fig. 8).

Thorax: Pronotum without posterolateral extensions. Hind supracoxal sclerites rounded. Femora with regular row of strong setae (Fig. 9). Middle and hindtibiae with two rows of long, fine setae and row of long, clavate setae on lateral ridge (Fig. 9).

Abdomen: Tergum with median ridge on segments 2–9 (Figs. 1, 12). Lamellae of gills 1 absent; lamellae of gills 2–6 sickle-shaped (Fig. 10); lamellae of gills 7 oval, anal rib serrate (Fig. 11). Fibrils well developed on gills 1–6, absent on gills 7. Caudal filaments with short robust setae and sparse fine setae, intersegmental setae absent.

Adult: Unknown.

Etymology: The generic name is an allusion to the sir name of the fictional character Darth Vader. The gender is masculine.

Species included: Darthus vadorus Webb & McCafferty n.sp. (type species).

PLATE I. Darthus vadorus . Fig. 1, Dorsal habitus.

PLATE II. Darthus vadorus . Fig. 2, Head capsule, ventral view showing enlarged anterior margin (mouthparts removed). Fig. 3, Labrum: a. ventral b. dorsal. Fig. 4, Right mandible. Fig. 5, Left mandible. Fig. 6, Hypopharynx: a. dorsal b. ventral. Fig. 7, Maxilla: ventral. Fig. 8, Labium a. dorsal b. ventral.

Distribution: Darthus is known only from East Kalimantan, Indonesia on the island of Borneo.

Diagnosis: Larvae of Darthus can be differentiated from those of all other Heptageniidae by the following combination of characteristics: ventral surface of maxillae with scattered, simple setae; gills 1 without lamellae; gills 2–6 with sickle-shaped lamellae; abdominal terga 2–9 each with median ridge produced into sharp spine posteriorly; anterior margin of headcapsule slightly thickened; and hindtibiae with two rows of long, fine setae.

Discussion: Due to the presence of a slightly thickened anterior margin of the head capsule and two rows of long, fine setae on the hindtibiae, Darthus appears to be most closely related to Afronurus Lestage and Parafronurus Zhou & Braasch. Darthus lacks large, equatorial knob-terminated coiled threads (KCTs) on the eggs that are an autapomorphy of Afronurus as well as the whorls of fine setae at the articulations of the caudal filaments characteristic of Parafronurus . Because Darthus lacks the autapomorphies of each of the other two genera, and itself possesses many autapomorphies, a new genus is justified. Although the adult of Darthus is unknown, a male that may belong to this genus (see the species discussion, below) lacks titillators, indicating Darthus and Afronurus may be sister taxa.

PLATE III. Darthus vadorus . Fig. 9, Hindleg. Fig.10, Gill 4. Fig. 11, Gill 7. Fig. 12, Posterior abdominal segments, lateral view.

Afronurus is currently considered to be the basal genus in the Ecdyonurinae tribe Atopopini Wang & McCafferty ( Wang and McCafferty 2004). In order to maintain a strict phylogenetic classification ( Wiley 1981; Schuh 2000) of the Ecdyonurinae and to maintain the generic status of the Atopopini genera, a new tribe (= Afronurini n.trib.) must be established for the Darthus , Parafronurus , Afronurus clade. The new tribe is characterized in the larvae by the combination of a slightly thickened headcapsule and two rows of long setae on the mid and hindtibiae. Atopopini is restricted to include the genera Thalerosphyrus Eaton , Atopopus Eaton , and Asionurus Braasch & Soldán and is the sister tribe to Afronurini . The relationship of Asionurus to the other genera remains tenuous, however, because larvae share some apomorphies with Compsoneuria Eaton , such as narrowed glossae and enlarged lamellae on gills 1.

PLATE IV. Darthus vadorus . Fig. 13, Bipectinate setae on ventral side of glossa: scalebar = 10 µm. Fig. 14, Robust setae and small spines of hind femur: scalebar = 1 µm. Fig. 15, Egg: scalebar = 10 µm. Fig. 16, Micropyle of egg: scalebar = 1 µm.