Xiaomyia Saether & Wang, 1993

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 : 685-691

Xiaomyia Saether & Wang, 1993

( Figs. 1–4 )

Type species. Xiaomyia aequipedes Saether & Wang, 1993 , by original designation.

Material examined. Pe / ♂, THAILAND, Phang Nga, Sri Phang Nga NP, 8°59′N 98°27′E, 16.vi.2004, Cranston, MV ‘ XIAO’ (molecular voucher) ( ANIC). ♂, Tam Nam Lod, 23.12.1989, LF [light trap], H. Malicky ( ZSM); ♂, Sob Pong , 23.12.1989, H. Malicky ( ZSM) .

CHINA (all collected H.Q. Tang, deposited EJNU unless stated otherwise): Southeast, 5♂, 34 Pe , Anhui Pr., Huangshan C., Tangkou T., Fu stream, 30°04′N 118°09′E, 05.viii.2014; 3 Pe , Zhejiang Pr., Yongjia C., Shangtang T., 28°12′N 120°40′E, alt. 150 m asl ., 04.v.2019; 2♀, 4Pe, Fujian Pr., Nanping C., Wuyishan NNR, Tongmu Ctr. , 27°45′N 117°40′E, 09.viii.2014; 8♂, 3♀, 1 Pe / ♂, 12 Pe , Fujian Pr., Zhangzhou C., Nanjing Co., Huboliao NNR, 24°31′N 117°14′E, 20.xi.2012; 2 Pe, Taiwan Pr., Pingdong Co. , Mudan T., 22°09′N 120°49′E, 19.iii. 2013, 233 m asl .

South, 10♂, 2 Pe / ♂, 5P/ ♀, 26 Pe, 3 Le, 2L/ Pe, Guangdong Pr., Guangzhou C., Zengcheng Distr. , Shuimei Vill., Lan Stream, 23°21′N 113°58′E, 20.i.2017 (2♂, 2♀, 2Pe, 2Le, ZRC) ; 3Le, L/P, as previous, except 13.xii.2018; Le, L, as previous, except 2.xi.2017 ( ANIC) ; 3 Pe, Guangdong Pr., Fengkai Co., Heishiding NNR, 23°28′N 111°54′E, 320 m asl, 26.viii.2011 ; 2 Pe, Guangdong Pr., Yangchun C., Ehuangzhang NNR, 21°50′N 111°29′E, 60 m asl, 14.xi.2013 , Y.D. Lei; 2 Pe, Guangxi Pr., Guilin C., Gongcheng Co., Longhu Ctr. , 25°05′N 110°57′E, 195 m asl, 28.vii.2013 , Y.D. Lei; 4 Pe, Guangxi Pr., Guilin C., Lipu Co., Magangqiao Ctr. , 24°40′N 110°20′E, 171 m asl, 29.vii.2013 , Y.D. Lei; Pe, Guangxi Pr., Guilin C., Lipu Co., Balu Ctr. , 24°26′N 110°28′E, 182 m asl, 30.vii.2013 , Y.D. Lei; 20 Pe, Guangxi Pr., Hezhou C., Gupo Mt. NNR., 24°36′N 111°34′E, 473 m asl, 25.viii.2011 ; Pe, Guangxi Pr .., Hezhou C., Fuchuan Co , Xiling Mt. , 24°57′N 111°06′E, 527 m asl, 28.vii.2013 , Y.D. Lei; Pe, Guangxi Pr., Laibin C., Jinxiu Co., Lahe , 23°58′N 110°07′E, 466 m asl, 30.xi.2013 , Y.D. Lei; 3 Pe, Guangxi Pr., Laibin C., Jinxiu Co., Dishui Ctr. , 24°01′N 110°07′E, 370 m asl, 31.vii.2017 , Y.D. Lei; 4 Pe, Guangxi Pr., Baise C., Napo Co., Nazhen Ctr. , 23°00′N 105°53′E, 463 m asl, 23.ii.2012 , W. Xia & C.B. Duan; 6 Pe, Hainan Pr., Changjiang Co., Bawangling NNR, 19°05′N 109°08′E, 30.xi.2011 ; 4 Pe, Hainan Pr., Lingshui Co., Diaoluo T., Diaoluo Mt. NNR., 18°40′N 109°56′E, 31.viii.2013 ; 2 Pe, Hainan Pr., Wuzhishan C., Wuzhi Mt. NNR., site 1, 18°45′N 109°36′E, 03.xii.2011 .

Southwest, Pe, Yunnan Pr., Xishuangbanna Pref., Jinghong C., Menglun T., Luosuo R., 21°55′N 101°16′E, 18.ii.2019; Pe, Yunnan, 7 km E. Mung Lun, 28.v.1980, E.J. Fittkau [Label 1, added in red ink ‘cold water, upper part of Mekong’ and in pencil ‘?Telmatogetoninae’. Label 2 in pencil “? Xiaomyia , see email Cranston of 15.iv.2014 ”] ( ZSM); Pe, Guizhou Pr., Chishui C., Lushi T., 28°29′N 105°56′E, 270 m asl, 13.viii.2018, W. Han & T. G. Gou; 3♂, 2♀, 4 Pe, Chongqing Mun., Wuxi Co , Chengxiang T., Daning R., 31°24′N 109°37′E, 1002 m asl, 03.ix.2018, W. Han & T. G. Gou; 2 Pe , Chongqing Mun., Chengkou Co., Gaoguan T., Re R., 31°5′N 108°55′E, 860 m asl, 06.ix.2018, W. Han; 4 Pe , Hubei Pr., Enshi Pref., Xianfeng Co. , Erxianyan , 29°43′N 108°47′E, 1620 m asl, C.M. Zhang .

Diagnosis. Male ( Fig. 1A–C ). Wing smoky dark except distal margins ( Fig. 1A ). Proctiger present ( Fig. 1B ), shallowly concave in the middle; pseudovolsella well-developed, extending clearly beyond a weak inferior volsella (basal lobe); depressed oval area (possibly remnant of pars ventralis or virga) present on ventral side of hypopygium ( Fig. 1C ).

Female ( Fig. 1D ). Gonocoxapodeme straight, weakly sclerotised. Dorsomesal lobe well-developed, with clear outer crystalline ‘ribs’ and inner microtrichiose part. Ventrolateral lobe spherical, with microtrichia covered whole surface, with some elongate setae occurring on only inner margin. Apodeme lobe indistinct. Vagina modest, semi-enclosed apically by fusedcoxosternapodemes. Spermatheca two, pale, oval, with thickened wall near a distinct neck, spermathecal ducts sclerotised in apical 1/4, relative straight, without loop, connected with each other just before the common opening. Labium small, with weak microtrichia apically. Cerci with well-developed, and usually dark sclerotised inner lobe, bearing 8–12 long setae.

Pupa ( Fig. 2 ). T IV with posterior transverse band consisting of solitary spine ( Fig. 2A, B ), one pair ( Fig. 2C–E ) or two pairs ( Fig. 2F ); T V with anterior transverse toothed band contiguous ( Fig. 2A–C ) or interrupted medially ( Fig. 2D–F ). Number of hooks on T VI ranging from one to three pairs ( Fig. 2B ).

Larva ( Figs. 3 , 4 ). Medium-sized, ca. 3–4 mm, body purple in life, blue when preserved. Head capsule ventrally with dark sclerotised mentum and submentum, separated medially by triangular membranous area ( Figs. 3A , 4A, D ). Occipital margin dark brown, broadest mediolaterally, weaker middorsally. Eye-spots separate, larger dorsal above smaller ventral.

Dorsal surface of head ( Figs. 3B , 4F ). Frons and clypeus separate; S3 located on flexible membranous area, with clypeus comprising of three separate, small sclerites, the middle clypeal sclerite ‘tear’-shaped; labral sclerite divided into two small fragments ( Fig. 4F ).

Antenna ( Fig. 3C ). Five-segmented, with three short apical segments; basal segment with basal ring organ; style distinctly longer than the minute Lauterborn organs, which are subequal to length of 3rd segment; blade extending far beyond antennal apex, more than three times as long as the flagellum.

Labrum ( Fig. 3D ). SI bifid, SII and SIII simple, thin, and SIVa, b conventionally developed. Labral lamellae absent. Pecten epipharyngis comprising three broad rounded scales, with three chaetulae laterales, weakly serrate apically. Premandible with apical tooth, broadened near midpoint but without additional tooth, lacking a brush.

Mandible ( Fig. 3E ). Without dorsal tooth, with apical tooth and three conventional inner teeth, decreasing in size from outer to inner, then ‘diastema’ (gap) distal to broad flat innermost lobe/tooth; inner surface with evidence of one or two additional ‘teeth’; seta subdentalis strong, curved, blade-like, extending to 2nd/3rd inner tooth. Inner margin of mola with a series of 4–5 setae, some or all partially feathered/branched, almost aligned and contiguous with seta subdentalis ( Fig. 3E ).

Mentum ( Figs. 3A, F, G , 4A ). With somewhat paler, prominent, rounded to domed median tooth (ventromentum), with four or five pairs of retracted lateral teeth and outermost broad and protruding tooth. Ventromental plate prominent, broad, covering much of dorsomentum, without beard or striae; head setae (maxillary, frontal, clypeal and setae submenti) strong, simple ( Fig. 3A ) or apically bifid to finely branched ( Fig. 3G ).

Maxilla ( Figs. 3H , 4B, C ). Maxilla squat with many short sensilla and setae, lacking elongate lacinial chaeta (chaetae). Hyaline ovate lobe (plate) c. 50 μm long, ribbed on ventral surface, spinose on dorsal, arises medio-posterior to palp, lying dorsal to mentum, directed posteriorly.

Body ( Fig. 4G ). Body setae short. Procercus strong, darkened laterally, slightly longer than width, with two short lateral setae and six strong apical setae. Anterior parapods elongated, partially fused basally, nearly all claws pectinate. Posterior parapods reduced, with c. 14 large curved, simple, dark yellow claws. Anal papillae without constriction(s).

Remarks. We supplement the male generic diagnosis of Saether & Wang (1993) with newly observed characters. Diagnoses for the female, pupa, and larva are given here for the first time.

Wide sampling of pupae showed variation in the tergal armament, which provides evidence for species diversity in this genus. For example, in Chongqing populations ( Fig. 2D, F ), in addition to the already mentioned variation, the S II has dense soft spines and associated males have a trifid superior volsella. Ongoing molecular studies indicate minimally four species occur in China (H. Tang, unpublished data). We cannot unequivocally assign our specimens to the sole described species, X. aequipedes Saether & Wang and thus the generic diagnosis covers all variation without allocating to named species.

Distribution ( Fig. 5 ). Detailed data for all records of Xiaomyia from across the Oriental region are presented. The genus is widespread in subtropical and tropical zones, with the most northerly records from around Mt. Qingling, China (31°23.5′N), which is the traditional boundary between the eastern Palaearctic and Oriental regions ( Heiser & Schmitt, 2013). We expect that the genus extends from the South Indian first records across all suitable intervening habitat to Thailand and east Asia. The most southerly record is from southern Thailand. Although sampling further south is inadequate, we presume the genus occurs in Borneo/ Indonesia but is unlikely to exceed Wallace’s Line.

Sampling in China has been most intensive in mountain streams of Guangdong-Guangxi border and Hainan Island. Fewer records come from Yunnan Province, although sharing climate and altitude, and despite similar sampling effort. The distribution of Xiaomyia closely parallels that of its sister genus Shangomyia (H. Tang, pers. data, unpublished), which also has an extensive distribution in Oriental Asia, also including suspected hidden diversity.

Phylogeny, dating and ranking. Analysis of molecular data shows a monophyletic clade of Xiaomyia and Shangomyia is supported by maximum bayesian posterior probability (PP) and bootstrap (BS) values (node C1, fig. 2, Cranston et al., 2011). This pair in turn was sister (at node M, fig. 2) to all other sampled Chironominae , with maximum PP, and BS of 77%. BEAST analysis provided median ages, in Ma, and ranges (HPD, highest posterior density intervals) for a mid-Cretaceous split between Xiaomyia and Shangomyia of 88 Ma (62–111) and between this clade and the remaining Chironominae of 148 Ma (132–166) ( Cranston et al., 2011; fig. 3, derived from unpublished node age data).

Recent new and existing molecular data, additional calibration fossils, and more intensive sampling of Tanytarsini and Pseudochironomini have provided another estimate of the tempo of diversification in the Chironominae (Krosch et al., in press). The analysis specifically estimated dates for a phylogeny for Riethia Kieffer, 1917 and lacked Xiaomyia but included the well-validated sister group Shangomyia . This dated phylogeny showed Shangomyia as sister to all other Chironominae and provided a nodal date of 134.5 Ma (HPD 101–197) (Krosch et al., in press).

Rank. All publications that concern at least one life stage of Shangomyia and/or Xiaomyia have commented on the need for a high rank for the clade ( Coffman et al., 1988; Saether & Wang, 1993; Cranston, 2003; Cranston et al., 2011). However, all refrained from taking formal action, in some cases due to uncertainty about the correct relationships, citing ‘differences’ and ‘uniqueness’, and in all cases, the lack of the larva. Despite robust molecular phylogenies providing strong justification, Cranston et al. (2011) cited the erratic, unstable position of Beardius Reiss & Sublette, 1985 , that sometimes was sister to all Chironominae including Shangomyia + Xiaomyia , thus preventing facile ranking. Since then no additional Beardius have become available for verification of the placement based on DNA. However, the revision of Beardius by Pinho et al. (2013) and morphology-based phylogeny suggest that Oukuriella Epler, 1986 is sister group to Beardius in a cluster of genera identified as a ‘ Microtendipes group’ in Cranston et al. (2011). With Beardius removed from consideration in relation to Shangomyia + Xiaomyia , recognition of the clade as being of tribal or subfamily rank is justified. The question is ‘what rank’? There are no hard and fast rules governing this — as stated recently: “.... taxonomic rank does not of itself confer natural comparability: Any rank above species is a function of convention and discretion as well as actual data, and as long as monophyletic groups are recognised the fact that families or tribes are not uniformly or evolutionarily equivalent does not hamper studies” ( DeSalle & Goldstein, 2019: p. 2). Tribal status for Shangomyia + Xiaomyia does not impact the stability of named ranked taxa around this grouping — as sister to the three already recognised tribes in the subfamily — and does not impinge on equivalence at nodes deeper in the tree (specifically Orthocladiinae and Prodiamesinae ). An alternative, expansion of taxa ranked at subfamily level, would do nothing for information content: depiction of a resolved phylogeny offers details of relationships without needing destabilisation of ranks. For this reason we support tribal rank, as Xiaomyiini , which takes our authorship according to the rules governing nomenclature, yet ought to be credited to Saether & Wang (1993) for their explicit discussion of the taxonomic issues and recognition that their formally-named genera “eventually may deserve status as a separate tribe” (loco citato, p. 194). Regarding the pupae, Coffman et al. (1988) had suggested them to “likely represent a new higher taxon as well, perhaps a new subfamily” (opere citato, p. 164) but with reasoning based on a combination of character states that were inconsistent with any described subfamily, and without named taxa included.

We note that using the age of a clade as an absolute gauge for rank is simply meaningless for taxa that originated in the Cretaceous. This would imply ordinal rank for a genus of insect were it a mammal or bird. Another misunderstanding concerns the significance of ‘disparity’ in assessing rank. Only when disparate morphologies are placed in a robust phylogenetic framework that autapomorphies (that give rise to disparity) can interpretation be correct. In our case, the suite of unusual features of Xiaomyia and Shangomyia do indeed indicate morphological ‘disparity’ associated with high rank, as stated by Coffman et al. (1988) and Saether & Wang (1993). This contrasts with past unjustified elevations in rank of apomorphic orthoclads, both terrestrial and marine intertidal in habit, such as Eretmopteridae Kellogg, 1900, Clunioninae Kieffer, 1906 , and Oreadomyiinae Kevan & Cutten-Ali-Khan, 1975.

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. 2. Xiaomyia Saether & Wang, 1993, T IV–VI of pupa. A, Fujian (Nanjing); B, Hubei (Enshi); C, Yunnan (Luosuo R.); D, Chongqing (Wuxi); E, Guangxi (Napo); F, Chongqing (Chengkou). Scale bars = 100 µm (A–F).

Fig. 3. Xiaomyia Saether & Wang, 1993, larva. A, head capsule, ventral view; B, dorsal surface of head; C, antenna; D, labrum; E, mandible; F, mentum, ventral view; G, ventromentum, detail, indicating variation of lateral teeth; H, ribbed lobe. Scale bars = 50 µm (A, B); 25 µm (C–H).

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).

Fig. 5. Distribution of Xiaomyia spp. from this study.