Chersodromia Walker, 1849

Grootaert, Patrick & Shamshev, Igor V., 2012, The fast-running flies (Diptera, Hybotidae, Tachydromiinae) of Singapore and adjacent regions, European Journal of Taxonomy 5, pp. 1-162 : 22-24

Genus Chersodromia Walker, 1849

Chersodromia Walker, 1849: 1157 . Type-species: Tachypeza brevipennis Zetterstedt, [1838] , by subsequent designation of Rondani (1856: 147) [= Chersodromia arenaria (Haliday, 1833) ].

Coloboneura Melander, 1902: 229 . Type-species: C. inusitata Melander, 1902 , by monotypy.

Halsanalotes Becker, 1902: 41 . Type-species: H. amaurus Becker, 1902 , by monotypy.

Thinodromia Melander, 1906: 370 . Type-species: T. inchoata Melander, 1906 , by monotypy.

Recognition

Chersodromia is recognised from other drapetine genera that occur in the Oriental Region by the following combination of features: eyes widely separated or contiguous on face, gena distinctly extended below eye, cells br and bm subequal in length, vein A 1 very weak or absent, and legs (at least hind tibia) bristly.

Diagnosis

Very small to quite large (1.0-4.0 mm) usually black and densely greyish pollinose, rarely pale yellow ( C. flavicaput group) flies. Frons broad, with sides divergent above. Eyes usually widely separated or contiguous ( C. flavicaput group) on face. Vertical bristles usually very prominent, 1 or 2 pairs, but sometimes practically absent. Ocellar tubercle with 2 pairs of bristles. Antenna with pedicel bearing long ventral subapical seta; postpedicel varying in shape, often with dorsoapical extension; stylus arising apically or dorsoapically, arista-like, long. Gena distinctly extended below eye. Palpus small, usually with short subapical seta and sometimes with sensory pit. Thorax usually largely densely pollinose. Postpronotal bristle prominent. Mesonotal bristles prominent but sometimes acrostichal and dorsocentral bristles little differentiated or greatly reduced in number; 2 long setae sometimes present between mid and hind coxae. Legs short, robust, with femora of subequal width; mid leg lacking secondary sexual characters in male; hind tibia with distinct bristles. Wings normally developed or sometimes reduced (non Oriental species); lacking colour pattern (in Oriental species), hyaline, finely infuscate or milkywhite; Rs very long, originating close to humeral crossvein or near middle of R 1; R 1 usually long and meeting costa at least near middle of wing, sometimes short ( C. flavicaput group); R 2+3 usually long and meeting costa beyond wing midway, sometimes very short and meeting costa before wing midway ( C. flavicaput group); veins R 4+5 and M 1+2 parallel; A 1 present as fold or absent; crossvein CuA 2 absent; crossvein bm-cu oblique or nearly transverse; cells br and bm of subequal length. Abdomen usually with tergites 1-7 unmodified, rarely tergite 2 modified ( C. glandula sp. nov.); no squamiform setae; glandlike intersegmental structures absent. Terminalia with epandrium completely divided; left epandrial lamella fused to hypandrium; left surstylus differentiated from epandrial lamella, undivided or divided; right surstylus differentiated or barely differentiated from epandrial lamella; cerci separated; hypoproct unmodified; hypandrium setose subapically; phallus very short or sometimes elongate ( C. flavicaput group); usually two rod-shaped apodemes (i.e. ejaculatory and ventral apodemes) present, rarely only one (some species of C. flavicaput group). Female usually similar to male, except some slight differences in setation of legs; terminalia short or elongate; tergite 8 separated from sternite 8; cercus rather short, slender or ovate.

Remarks

The genus Chersodromia belongs to the unique complex of the empidoids inhabiting the narrow intertidal and supralittoral zones of the sea shores ( Grootaert 1992, 1994; Shamshev & Grootaert 2004a, 2005a, 2005c; Ulrich 2004). The representatives of this ecological group usually occupy very specialised niches, e.g. reef-flats, sandy beaches, crab holes, wet rocks, the mangrove directly exposed to the sea, etc., and only rarely penetrate inland ( Chvála 1995; Grootaert et al. 2007). Currently, 65 species of Chersodromia are known worldwide, including 6 species described here. 10 species are recorded now from the Oriental Region. Chvála (1977) subdivided Chersodromia into four species groups. However this classification is based on Palaearctic species and it does not work for the Oriental representatives of the genus.

There are four 'yellow' Chersodromia known in the Oriental Region and they probably form a natural species group. Provisionally we refer to them as the C. flavicaput group. They do not occur on sandy beaches as most Chersodromia , but are found in forests and one species in mangroves. Within that group, C. bulohensis sp. nov., C. flavicaput and C. obscura form a distinct subgroup characterised in having two rod-shaped apodemes, the left surstylus with a strong apical bristle and a set of spinules. Chersodromia sylvicola sp. nov. does not have these characters and therefore this species appears to belong to a different branch that is closer to the Chersodromia with dark body ground-colour. For a further discussion on this group please refer to Grootaert et al. (2007).

The remaining species of Chersodromia belong to various groups sensu Chvála (1977), but some species are intermediate. For example, C. malaysiana sp. nov. belongs to the C. cursitans group; C. nigripennis Shamshev & Grootaert appears to have an intermediate position between the C. incana and C. cursitans groups; C. singaporensis Shamshev & Grootaert belongs to the C. speculifera group and probably is closely related to three species of Chersodromia described from Papua New Guinea ( Grootaert 1992).

Key to species of Chersodromia from the Oriental Region

1. Body largely yellow to pale yellow; wing with vein R 2+3 very short, meeting costa before middle of wing; not on sandy beaches..............................................................................................................2

– Body entirely brown to black; wing with vein R 2+3 long, meeting costa beyond middle of wing; on sandy beaches..................................................................................................................................5

2. One pair of strong vertical bristles.....................................................................................................3

– Verticals minute, not distinct from postoculars..................................................................................4

3. Scutum in anterior third with brown median vitta broadly connected near middle with broad brown area on each side, leaving sides of notopleural and prescutellar depressions yellow. Hind tibia with 3 long bristles on apical half ( Thailand)........ C. obscura Grootaert, Cumming & Shamshev, 2007

– Mesonotum uniformly yellowish. Hind tibia with 4 long bristles on apical half ( Singapore)............ ........................................................................................................................ C. sylvicola sp. nov.

4. Hind tibia with 5 long bristles on apical half ( Papua New Guinea; Thailand)....................................... ....................................................................... C. flavicaput Grootaert, Cumming & Shamshev, 2007

– Hind tibia with 3 bristles on apical half ( Singapore)........................................ C. bulohensis sp. nov.

5. Two pairs of vertical bristles................................................................................................................6

– One pair of vertical bristles...................................................................................................................8

6. Halter pale. Abdominal tergite 2 thickened posteriorly ( Singapore; Malaysia)........ C. glandula sp. nov.

– Halter at least with brownish yellow knob.Abdominal tergite 2 unmodified..........................................7

7. Palpus brownish. Postpedicel conical, without dorsoapical extension ( Singapore)........................... ....................................................................................... C. nigripennis Shamshev & Grootaert, 2005

– Palpus white. Postpedicel with distinct dorsoapical extension ( Malaysia)............ C. pasir sp. nov.

[The Palaearctic C. luanchuanensis Yang, An & Gao, 2002 from China (Henan) will also key out here.]

8. Halter with darkened knob ( Singapore; Malaysia)............................................ C. malaysiana sp. nov.

– Halter pale.............................................................................................................................................9

9. Palpus pale yellow ( Malaysia).......................................................................... C. tiomanensis sp. nov.

– Palpus brownish ( Singapore)....................................... C. singaporensis Shamshev & Grootaert, 2005