A remarkable new genus of leafhoppers (Hemiptera, Cicadellidae, Iassinae) from Southeast Asia

Abstract Tardrabassus pakneunensis, n. gen. & sp. is described and illustrated. The new genus shows morphological affinities to three leafhopper subfamilies, Tartessinae, Deltocephalinae, and Iassinae, but is tentatively placed in Iassinae based on the position of the ocelli, the reduced lateral frontal sutures, the leg chaetotaxy, and the structure of the male genitalia.


Introduction
The higher classification of leafhoppers, family Cicadellidae, has long been controversial, partly due to the tremendous diversity of the family (>20,000 described species in >2,600 genera) and the fact that there are numerous genera that have combinations of the morphological features normally diagnostic for more than one subfamily or tribe (e.g., Wei et al. 2007Wei et al. , 2010Dietrich 2011a,b;Viraktamath and Dietrich 2011). Such taxa are important because they may bridge morphological gaps between major leafhopper lineages and shed light on phylogenetic relationships. The new leafhopper species described herein falls into this category. Here we describe and illustrate the new species, place it in a new genus and include it tentatively in the cicadellid subfamily Iassinae.

Materials and methods
Specimens examined are deposited in The Natural History Museum, London (BMNH). Morphological terminology follows Dietrich (2005).
Description. Robust, depressed leafhoppers. Color mostly dark brown to black with few symmetrical yellow markings.
Head in dorsal view ( Fig. 1A) wider than pronotum; crown short, more than four times wider than long, anterior and posterior margins parallel; texture longitudinally rugulose and with numerous minute pits; with indistinct transverse depression preapically; ocelli small, on crown just posterad of anterior margin, mesad of antennal pits and well separated from eyes; anterior margin of head depressed, forming distinct shelf in lateral view (Fig. 1B), transition from crown to face narrow but smooth, without transverse carina; frontoclypeus in anterior view ( Fig. 2A) evenly broadened from anteclypeus to dorsal margin, convex and rugulose ventrally, concave and transversely striate dorsally; antennal ledge closer to anterodorsal than to anteroventral corner of eye, represented by prominent carina, slightly oblique, nearly horizontal, not concealing antennal base; antenna length one third width of head; lateral frontal sutures extended dorsomesad from antennal pits to margin of crown, not reaching ocelli; gena broad, obtusely incised ventrad of eye, with obtuse ventrolateral projection almost completely concealing proepisternum; lorum large, flat, dorsal 2/3 bordering frontoclypeus, ventral 1/3 bordering anteclypeus, extended nearly to ventral margin of gena; maxillary sensillum near mid-height of lorum, closer to lorum than to lateral margin of gena; anteclypeus small slightly convex, ovoid, with transverse preapical concavity, apical margin carinate, rounded and slightly upturned, extended slightly beyond ventral margin of gena; rostrum with distal segment greatly expanded and depressed, lamelliform.
Distribution. Southeast Asia (Laos). Etymology. The genus name, a masculine noun, was formed by combining the names of three genera that resemble the new genus in certain respects: Tartessus, Drabescus, and Iassus, reflecting the apparently mixed morphological affinities of the genus.
Notes. Tardrabassus is difficult to place in the present subfamily classification of leafhoppers. The structure of the head somewhat resembles that of Drabescus (Deltocephalinae: Paraboloponini) in coloration and in the form of the antennal ledges and shape of the facial sclerites. It also resembles Tartessus (Tartessinae) in having cucullate intercalary setae on hind tibial row AD. It resembles both of these genera in the form of the pronotum (produced medially with prominent transverse rugae) and forewing (broad appendix extended around wing apex, venation complete and well delimited). The placement of the ocelli on the crown, the dorsally obsolete lateral frontal sutures, and the poorly differentiated chaetotaxy of the front femur distinguish the new genus from Drabescus and most Tartessinae (except some Thymbrini--recently transferred from Ledrinae by Jones and Deitz (2009)), but are consistent with both Iassinae and Ledrinae. Ledrinae have the ocelli on the crown, but distant from the anterior margin and have the ocellocular area of the face much wider. In the structure of the male genitalia, however, Tardrabassus is most similar to Iassinae. The pregenital sternite is enlarged with the posterior margin produced, concealing the basal half of the subgenital plates when the genital capsule is retracted, a synapomorphy of Iassinae. Other aspects of the male genitalia, including the ligulate subgenital plates, the sigmoid style, and the simple aedeagus are all plesiomorphic features shared with various other leafhopper groups. The unusual combination of plesiomorphic (e.g., poorly differentiated front femoral chaetotaxy, presence of an r-m crossvein in the hind wing) and apomorphic features (e.g., retracted male genital capsule, broad gena) suggests that Tardrabassus represents either an early diverging lineage of Iassinae or a lineage distinct from previously recognized cicadellid subfamilies. Among previously described genera of Iassinae it is perhaps most similar to the Neotropical genus Bythonia (Bythoniini), but differs in having the ocelli on the crown, the antennal ledges and lateral frontal sutures reduced, and the dorsum without conspicuous setae.
The distal segment of the rostrum of Tardrabassus is more strongly expanded than in any other known leafhopper. Similar, although less extreme, conditions occur in Coriojassus Evans (Iassinae) and in various Idiocerini. The shape of the distal segment varies among the three pinned males available for study, possibly because the expanded part is partially membranous and became distorted as a result of drying. In other leafhoppers that exhibit a similar condition, it occurs only in males. We cannot confirm that the condition is sexually dimorphic in Tardrabassus because females remain unknown. The function of this modification is also unknown. Notes. The species name is based on that of the type locality, a village on the Mekong River in northern Laos. "Pang Ngeon" could not be located in available gazetteers and databases but may represent an alternative spelling of the type locality. Additional specimens have not been found, despite extensive recent field work in Thailand and Vietnam.

Figure 2.
Tardrabassus pakneunensis sp. n. A head, anteroventral view B front femur, anterior view C front tibia and tarsus, anterior view D hind leg (except coxa and trochanter), anterior view e male pygofer and subgenital plate, lateral view (chaetotaxy of plate not shown) F male genitalia and anal tube, lateral view: anal tube (X), membranous internal lobes of genital capsule (ml), aedeagus (a), connective (c) and style (s) G apex of aedeagus, caudal view H aedeagus, lateral view I connective, dorsal view J style, lateral view K left subgenital plate, ventral view.