Chrysotus dot, Grootaert & Velde & Isabella Van de, 2024

Grootaert, Patrick, Velde, & Isabella Van de, 2024, New long-legged flies from mangroves on Pulau Ubin (Singapore) (Insecta: Diptera: Dolichopodidae), Raffles Bulletin of Zoology 72, pp. 303-323 : 305-307

Chrysotus dot , new species

( Figs. 2 , 3 )

Type material. Holotype male. Singapore, Pulau Ubin , Sungei [= River] Tock Kew basin ( OBS), 19 March 2019, Malaise trap ( PU22 ) in back mangroves ( ZRCBDP0317760 , leg. P. Grootaert; in ethanol, LKCNHM). GenBank accession code: PP893289.

Paratypes. Singapore, Pulau Ubin , Sungei Tock Kew basin ( OBS) , 9 males, 22 females, PU22 , 19 March 2019 (Ma10640) ; 2 females, PU22 , 26 March 2019 (MA10955) ; 6 males, 9 females, PU22 , 2 April 2019 (Ma10718) ; 6 males, 2 females, PU22 , 16 April 2019 (MA10986) ; 2 females, PU22 , 24 April 2019 (MA10972) ; 3 males, PU22 , 15 May 2019 (MA11506) ; 1 male, 2 females, PU22 , 28 May 2019 (MA11189) ; 1 male, 1 females, PU22 , 3 June 2019 (MA10982) ; 5 males, 4 females, PU22 , 11 June 2019 (MA11180) ; 1 male, 2 females, PU22 , 19 June 2019 (MA10976) ; 1 female, PU22 , 25 June 2019, (MA10967) . Barcodes of 58 specimens from PU22 are available.

Other material examined. 2,357 barcodes of C. dot , new species, are available from Pulau Ubin, Pulau [= Island] Semakau, and Coney Island.

Etymology. The species is named after the brown spot or dot that is present on the fore femora of the males and females. The English word “dot ” is used as a noun.

Diagnosis. A small species (1.1–1.3 mm) with a black spot located dorsally on the middle of the fore femur in both sexes. Four dorsocentrals, 5 uniserial acrostichals. Haltere yellowish. Legs yellow, with apical 2 tarsomeres of all legs brown. Male: Antenna yellowish brown, pedicel contrastingly darker brown than the yellowish brown postpedicel. Frons very wide, eyes touching near middle of face, with at least 4 rows of enlarged eye facets on each side of face. Palpus yellow, large, oval, hiding proboscis and shorter in females, lacking black bristling. Apical 2 tarsomeres of all legs brown, slightly dorsoventrally flattened. Pulvilli present on all legs, though minute and narrow. Sternite 8 lacking macrosetae. Cerci yellow, ventral surstylus dark brown. Tip of aedeagus widened with complex structure. Female: palpus brown, not hiding the proboscis, covered with black bristles and a distinct apical. Legs coloured like in males. Apical tarsomeres of all legs brownish but not widened.

Male. Length: body 1.1–1.3 mm; wing 1.1–1.2 mm.

Grootaert & Van de Velde: New long-legged flies from mangroves on Pulau Ubin

Head. Frons very wide, metallic green. Eyes touching near middle of face, with at least 4 rows of enlarged eye facets at each side of face. All bristles black. A pair of long retroclinate, diverging ocellar bristles, a pair of proclinate vertical bristles as long as ocellars. Postoculars, short, fine, uniserial above, not distinct below.

Antennae. Yellowish brown, pedicel contrastingly darker brown than postpedicel which is more yellowish. Pedicel with dorsal apical bristle nearly as long as pedicel is high. Postpedicel kidney-shaped, arista apical near middle, set with minute hairs. Palpus yellow, very large, oval, hiding proboscis, lacking distinct bristles.

Thorax. Shiny metallic green. All bristles black. Four dorsocentrals almost equally long. Five uniserial acrostichals. A pair of widely separated marginal scutellars 2.5 times as long as scutellum is long.

Legs. Yellow, fore femur with a blackish brown spot located dorsally in middle. Mid coxa brown, hind coxae paler brown. Apical 2 tarsomeres of all legs brown.

Fore legs. Coxa with indistinct pale apical bristles. Femur and tibia lacking distinct bristles. Length femur, tibia, tarsomeres (in mm): 0.33: 0.34: 0.12: 0.047: 0.039: 0.032: 0.032.

Mid legs. Coxa lacking an exterior bristle, but a long bristle on middle of margin between anterior and dorsal part of the coxa. Femur with a brown posterior preapical bristle. Tibia with a short anterodorsal in basal quarter. Length femur, tibia, tarsomeres (in mm): 0.39: 0.39: 0.19: 0.09: 0.06: 0.032: 0.032.

Hind legs. A brown exterior bristle near base of coxa, longer than coxa is high and a slightly shorter apical bristle. Trochanter with a long dorsal bristle longer than trochanter. Femur with 3 preapical anteroventral bristles, middle bristle longest, longer than width of femur. Tibia lacking distinct bristles except for a few short apical bristles.

Length femur, tibia, tarsomeres (in mm): 0.39: 0.39: 0.17: 0.08: 0.05: 0.32: 0.05.

Wings. Brownish tinge with pale brown veins. Squama brown with 2 pale brown bristles. Haltere yellowish (not clear white).

Abdomen. Brown, indistinctly shiny. All bristles black and short. Sternite 8 lacking macrosetae though a row of bristles is present ( Fig. 3 ).

Male terminalia. Fig. 3 . Cercus yellow, surstylus brown. Epandrial lobe with 3 bristles, the apical bristle as long as lobe is wide, other 2 bristles minute. Ventral surstylus with a strong subapical bristle and a smaller bristle below. Apex of aedeagus widened with a complex structure ( Fig. 3A–C ). Postgonite short, densely set with short hairs ( Fig. 3C ).

Female. Length: body 1.2–1.3 mm; wing 1.10–1.11 mm.

Similar to males in colouration of legs. Palpus brown and much smaller than in males, with longer black bristles and a distinct apical. Proboscis yellowish, not hidden by palpus, long, about half as long as eye is high. Face as wide as postpedicel, apically protruding and pointed. Postpedicel reniform as in males, but more uniformly brown. Eye facets not enlarged and eyes not as wide as in males so that the row of lower postoculars is visible. Postoculars black above, longer and yellow below. Marginal bristles on tergites longer than in males.

Comments. This small new species is easily recognised by the yellow legs with a black spot located dorsally in the middle of the fore femora and this is in both sexes. The postpedicel is yellowish brown and somewhat kidney-shaped with an apical arista. The male has a very large oval yellowish palpus, while the female has a smaller brown palpus. It is the most common Chrysotus species in back mangroves. It hardly visits front mangroves and was never observed in non-mangrove biotopes where many species of the entirely black Chrysotus species are dominant. The genetic variation measured by the number of haplotypes is low, especially in comparison with the terrestrial black Chrysotus species.

The tip of the aedeagus in Chrysotus dot , new species, has a complex structure with folds and a spine-like projection ( Fig. 3A–C ), similar to species of the C. leigongshanus species group ( Wei, 2018). However, this species group has black femora, no pulvilli on the hind tarsus and sternite 8 with a pair of strong setae. In Chrysotus dot , new species, the femora are yellow, there are small pulvilli present on all legs, and sternite 8 does not bear strong setae.

Several other, but less common and yet to be described Chrysotus species with yellow legs occur in back mangroves. They do not bear the typical black spot on the fore femora and have a brown triangular or trapezoid postpedicel with a short ventral protuberance. One of these species, C. ubinensis , new species, is described hereafter.

National distribution. The new species is one of the most common Chrysotus and dolichopodid species, in Singapore in the back mangroves of Pulau Ubin. It was found all over the island in all 26 back mangrove stations that were sampled with Malaise traps ( Fig. 1 ). In total, 2,784 specimens were collected in the back mangroves and 2,357 specimens were barcoded, showing very little variation in haplotypes (less than 0.7%).

In the front mangroves of the replanted mangroves of Pulau Semakau, only a single specimen was found in the three Malaise traps that were operational for two full years ( SMN3 ), while 239 specimens were found in the adjacent old mangroves ( SMO), thus underlining that C. dot , new species, is really a back mangrove species. It was further found in the back mangroves of Coney Island (17 specimens in 5 sampled stations), Berlayer Creek (1 specimen in 2 sampled stations), and Sungei Buloh Wetland Reserve (227 specimens in 14 sampled stations). Remarkably , it was not found in the mangroves at Sarimbun , Lim Chu Kang, and Mandai .

Distribution in Southeast Asia. The new species was found in all three mangrove sites in Brunei we investigated (Tutong, Berambang, and Labu). They differ by 2.3% in haplotype structure from the Singapore populations and a deeper morphological study is needed to show if they represent the same species. Only a single female was found in a mangrove site in southern Thailand (Surat Thani, leg. A. Samoh) and genetically it falls entirely in the population of Pulau Ubin.

Phenology. The new species is active throughout the year, but with its peak activity at the start of the rainy season in April and the lowest activity in the dryer periods of the year from end June to end September, with a second peak of activity from October to December.

Figures

Fig. 2. Chrysotus dot, new species, habitus paratype male (ZRCBDP0083923). Photograph by Miteshravin S/O Ravindran.

Fig. 3. Chrysotus dot, new species, male terminalia. A, Hypopygium lateral; B, hypopygium ventral; C, Detail postgonite and tip phallus. Scale bar = 0.1 mm. Drawings by P. Grootaert.

Fig. 1. Distribution of the sampling stations with Malaise traps on Pulau Ubin. A, all sites from 2005 to 2019; B, detailed position of traps in the OBS area in 2019.