Xiaomyiini, Tang & Cranston, 2019

Tang, Hongqu & Cranston, Peter S., 2019, A new tribe in the Chironominae (Diptera: Chironomidae) validated by first immature stages of Xiaomyia Saether & Wang and a phylogenetic review, Raffles Bulletin of Zoology 67, pp. 684-693 : 691-692

Xiaomyiini , new tribe

Diagnosis. Male. With unique spinose scale and long spur on apex of mid- and hind-tibia; eye rounded-ovoid without dorsomedial extension; strong costal extension on wing, Cu 1 ending in shallow fork near wing margin. All leg ratios (LR) greater than 1.5. Hypopygium with reduced inferior volsella; gonostylus flexible at junction with gonocoxite, with apical tooth.

Female ( Fig. 1D, E ). Gonocoxapodeme VIII straight. Dorsomesal lobe microtrichiose on inner surface, with outer crystalline rib. Ventrolateral lobe well-developed, with dense microtrichia and apical setae. Apodeme lobe indistinct. Seminal capsules two, ovoid, spermathecal ducts sclerotised in anterior 1/4. Labium small, with weak microtrichia apically. Cercus with extended inner lobe, sclerotised, bearing 8–10 long setae on apical 1/3.

Pupa. With heavily sclerotised cuticle. Frontal setae and thoracic horn absent. Thorax with anterior dorsocentral seta (Dc) displaced ventrally and lateral antepronotal seta (LAps) on low protuberance. Abdomen with no taeniate lateral seta and anal lobe fringe; only one (lateroventral) seta located on segment VIII; seven tergal setae present. Anal lobe disc with dorsal and ventral.

Larva. Body purple when live, patchy indigo when preserved in alcohol. Head capsule deep dark brown; posterior occipital margin dark brown laterally, weaker medio-ventrally ( Fig. 4D, E ), and less so dorsally. Eye-spots separate, large dorsal spot always anterolateral to ( Shangomyia ) or above ( Xiaomyia ) the ventral small spot. Labrum with SI simple or bifid, never pectinate or plumose; premandible simple, with one prominent apical tooth and one broad node, without brush. Antenna five-segmented, AR less than 1.0, blade more than three times length of flagellum. Mandible with four inner teeth, innermost tooth truncated; seta interna simple, strong, curved; inner margin of mola with feathered, bifid or simple broad setae perhaps representing displaced seta interna. Mentum with prominent median tooth, 4–5 laterals; outermost tooth squat. Ventromentum well-developed, extending margin of dorsomentum, with ( Shangomyia ) or without ( Xiaomyia ) internal (dorsal surface) spinules. All major cephalic setae simple or apically bifid/finely feathered, e.g., seta submenti simple or bifid. Maxilla with unique hyaline, ribbed and spinulose plate-like lobe, extending posteriorly from ventral maxillary surface ( Fig. 4B, C ), located immediately dorsal to the mentum, and ventral to salivary gland outlet on the prementum.

Anterior parapods partially fused basally; posterior parapods short, with 14 dark-brown claws. Procercus somewhat sausage-shaped, bearing six anal setae apically and one pair lateral setae basally. Anal tubules present.

Ecology. The search for the larva of Xiaomyia to complete the life history has been as challenging as it was for Shangomyia . The latter eventually was found mining in immersed wood in a shaded (i.e., cool), fast-flowing tropical river ( Cranston, 2003). That larva has not been found by any routine sampling method — even where drift netting showed that pupal exuviae were frequent, as in Sungai Belalong ( Brunei). Larvae remain exceedingly difficult to locate even with knowledge of the in-stream immersed wood habitat. Extrapolating this habit to the larva of Xiaomyia and searching in immersed wood in the rivers that provided pupal exuviae failed, but by good fortune a pupal exuviae remaining attached to a pharate male (Fujian Prov.) and subsequently many pharate adults (Guangdong Prov.) were found; what’s more, two pharate larvae with developing pupal spinulation were obtained at the same locality, and is the basis for description here.

However, the larval habitat remains unknown, although pupal exuvial collections are from associated small streams (of 2nd or 3rd order), usually in open channels with water depth no more than 0.5 m, and dissolved oxygen greater than 8.0 mg/l., with exposed moss-covering cobbles. The assemblage at four sites with many pupal exuviae shows Xiaomyia usually co-occurs with species of Sublettea Roback, 1975 , which may indicate a shared in-stream habitat, perhaps with an active hyporheic zone. The challenge of rearing the egg mass from the fertilised female using in-situ water fails at the transition from I–II phase, suggesting some special microhabitat or perhaps even symbiotic lifestyle. An intact larva showed gut contents comprising exclusively fine organic and mineral fragments of maximum size 10 microns, lacking complete cells of periphyton or microarthopods/crustaceans or wood fibres, suggesting that the larva had been grazing aufwuchs (biofilm).

Figures

Fig. 1. Adult. A, habitus of male Xiaomyia Saether & Wang, 1993 (Anhui); B, hypopygium dorsal view Xiaomyia (Anhui); C, hypopygium, ventral view, Xiaomyia; arrow indicates remnant of virga (Fujian); D, female genitalia, ventral, Xiaomyia (Chongqing); E, female genitalia, ventral, Shangomyia (Yunnan). Scale bars = 400 µm (A); 50 µm (B, C); 100 µm (D, E).

Fig. 4. Larval colour photographs, Xiaomyia Saether & Wang (A, B, D, F, G) and Shangomyia Saether & Wang (C, E). A, head capsule, ventral view; B, C, maxilla and premento-hypopharyngeal complex, arrows showing ribbed lobes; D, E, posterior occipital region; F, dorsal sclerite of head; G, larval abdomen, showing pupal abdominal spinulation beneath larval skin. Scale bars = 200 μm (A, G); 100 µm (F); 50 µm (B, C, D, E).