Riethia hamodivisa Cranston

Cranston, Peter S., 2019, Riethia (Kieffer 1917) (Diptera: Chironomidae) revised for the Austro-Pacific region, Zootaxa 4646 (3), pp. 461-500: 477-478

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Riethia hamodivisa Cranston


Riethia hamodivisa Cranston   sp.n.

( Figs. 1B View FIGURE 1 , 2D View FIGURE 2 , 3G, 3L View FIGURE 3 , 6G View FIGURE 6 )


Rietha ‘divided hookrow’ Cranston 1966, 2000

Type material. Holotype Le / Pe / ♂, AUSTRALIA, Australian Capital Territory ( ACT), Brindabellas, Blundells Ck., 26.iii.1988, 35°22’S 148°50’E,. ix.1998 (Cranston). GoogleMaps  

Paratypes (collected Cranston, deposited ANIC, unless otherwise stated): P ♀, as Holotype except 26.iii.1988; Le / Pe / ♀, 8.xi.1995, ex-wood (McKie); Pe/ ♂, Black Mt. , pool, 13.i.1988; P ♂, Victoria, Tambo R., ‘USWW’ (Hortle).  

Other material examined. New South Wales: Pe, Micalong Ck., 35°17’29”S 148°31’56”E, 4.i.2001 GoogleMaps   ; 2Pe, Kosciusko N.P., Yarrangobilly, Yarrangobilly R., 35°39’S 148°28’E, 14–15.i.1991. GoogleMaps  

ACT: numerous Pe, Brindabellas, Blundells Ck., and Lees Ck., 35°22’S 148°50’E, GoogleMaps  

Victoria, Tambo , 2P ♀, Currawong Ck., 36°48’S 147°54’E, 11.xii.1990 (Hortle) GoogleMaps   ; numerous Pe & P ♂, Tambo R., several sites, 1990–1991 (Hortle)   ; Pe, Bindi Ck., 37°08’S 147°51’E, 29–30.xii.2000 (Cranston & Gullan). GoogleMaps  

Tasmania: Pe, 41°50’S 146°00’E, Cradle Mt. - L. St. Clair N.P., Frog Flat, 25.i.1990. GoogleMaps  

Unreared pupae and larva, excluded from paratype series. Pe, Queensland, Mt Lewis, Windmill Ck., 16°34’S 145°16’E, 8–9.ix.1997 (McKie) GoogleMaps   ; Pe, nr Mareeba, Davies Ck., 17°01'S 145°35'E, 19–20.vi.1997 GoogleMaps   . L., ACT, Corin , Gibraltar Falls, 35°28'S 148°55'E, 30.i.2001. GoogleMaps  

Description. Male. Thorax pale yellow, with deeper yellow vittae; legs pale without banding. Wing membrane unmarked, veins yellow. Setae of TIX fine, in median cluster. Hypopygium ( Fig. 1B View FIGURE 1 ) with gonocoxite slightly narrowed at junction with gonostylus, gonostylus apically tapered but without distinct tooth, bearing only simple setae. Superior volsella ( Fig. 2D View FIGURE 2 ) predominantly without microtrichia, with 3 long setae anterolateral setae, posterior setose projection weak and continuous with inferior volsella, with medially-directed broad, apically rounded digitus with band of setae across base, otherwise devoid of microtrichia, with 3–4 simple setae. Pseudovolsella 2 long approximated setae, without tubercle ( Fig. 2D View FIGURE 2 ). Inferior volsella well developed, basally appressed to inner contour of gonocoxite and linked to superior volsella, with 6–7 pectinate scales and many long simple setae. Mensural features as in Table 1 View TABLE 1 .

Female. Pigment as male. AR 0.38, LR not measureable.

Pupa. Very pale with darker yellow dorsal thorax, anterior wing sheaths and lateral apophyses on abdominal segments V–VIII; comb yellow. Frons with weak warts, cephalothorax weakly rugulose with several rows of dorsal tubercles. Hook row on II with wide medial interruption; with continuous conjunctival spinule band on III and IV. Pedes spurii B weak, vortex weak. Abdomen ( Fig. 3G View FIGURE 3 ) with tergite II with rectangular-shaped armament pattern, quite dense, with anterior transverse band not delimited either by size or density, TIII–V well filled with spinules, TVI with anterior-median elongate/oval area, separated by microspinules median area from posterior transverse band of stronger spinules; TVII and VIII bare. Taeniate setae 3, 4, 4, 5, on VIII evenly distributed along segment. Comb ( Fig. 3L View FIGURE 3 ) with 1 strong posteriorly-directed spine and 2 weaker inner spines. Anal lobe with 20–32 uniserial taeniae.

Larva. Head capsule pale yellow with dark occipital margin, mandible golden yellow, mentum and inner 4 distinct mandibular teeth golden. Clypeus ( Fig. 6G View FIGURE 6 ) dilate anteriorly, squared off posteriorly with straight posterior margin; clypeal setae inserted towards centre. Mandible with broad spine beside insertion of seta subdentalis, with 2 serrations on mola.Antennal ratio 1.4, with each segment shorter than preceding. The mentum usually is 10% longer than the width of a ventromental plate. Mensural features as in Table 2 View TABLE 2 .

Etymology. Based on Latin hamus —hook and divisa —divided, referring to the unusual median division of the transverse row of hooklets on the posterior of tergite II.

Diagnosis. The adult male of R. hamodivisa   is identifiable solely on minor differences in the hypopygium, notably the inferior volsella with digitus lacking any pectinate scale and the distal area bare, with 3 pectinate scales. The pupa has the hookrow on the posterior of tergite II widely divided medially into two parts (see remarks below), with dense armament on TII and III. The larva has an AR> 1.4, the mentum and all mandibular teeth golden-yellow (as head capsule) and the clypeus c. 55 x 32µm, with clypeal setae positioned medially.

Remarks. This taxon was recognised as distinct from ‘R. zeylandica’ first from a pupa with a hookrow on the posterior of tergite II widely divided medially into two parts. Adult males (including examined pharates) differed from ‘R. zeylandica’ in the volsellae of the male genitalia. Subsequently in associated larvae the dorsal head sclerites ( Fig. 6G View FIGURE 6 ) differentiate. Only teneral or pharate (non-emerged) adults are available for description.

A second species with a divided hookrow is R. paluma   sp. n. ( Fig. 4B, F View FIGURE 4 ), known from a single reared female only, but it is clearly differentiated in the pupa by the divided conjunctival bands on II and IV, and the absence of armament on TII, and very reduced armament on TIII–VI.

A divided hookrow occurs also in two exuviae from far north Queensland, one from Windmill Creek, another from Davies Creek. Both exuviae have complete but narrow conjunctival spinule bands, and have well developed armament on TVII unlike the nearly bare TVII of R. hamodivisa   ( Fig. 3G View FIGURE 3 ), and are excluded from the type series of R. hamodivisa   .

DIstribution and Ecology. R. hamodivisa   seems to be restricted to south-eastern Australia, from southern NSW and ACT, through eastern Victoria and with a solitary specimen from Tasmania. Records range from near pristine submontane creeks to more polluted creeks in dairy-farming country. A rearing from Black Mountain, Canberra, is the only record from standing water. Two exuviae from streams in north Queensland tablelands, if belonging to R. hamodivisa   , would be a significant extension to the range.