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

Chrysotus ubinensis , new species

( Figs. 4 View Fig , 5 View Fig )

Type material. Holotype male. Singapore, Pulau Ubin , Sungei Tock Kew, 1 male, 3 females, 16 April 2019, PU22 ( LKCNHM).

Paratypes. Singapore, Pulau Ubin , Sungei Tock Kew, 3 females, 16 April 2019, PU22 ; male, 19 March 2018, ZRCBDP0317766 : GenBank accession code: PP893290 .

Other material. Pulau Ubin, Chek Jawa, 1 male, 1 female, 16 June 2012 ( PU1 ); Chek Jawa, 1 male, 4 females, 20 April 2012 ( PU2 ). More than 200 specimens were barcoded from Pulau Ubin: PU1-2013 , PU01-2016 , PU01-2019 , PU02-2016 , PU09 , PU10 , PU13 , PU19 , PU20 , PU21 , PU22 , PU23 , PU25 , and PU26 . Female, ZRCBDP0084405 _YuTing_ IDOM13 _ PU01 _ 23Jun2016: GenBank accession code: PP893291; ZRCBDP0257018 _Nabilah-Kareen_ MIPD19 _ PU20 _ 29Mar2018. GenBank accession code: PP893292; Pulau Semakau, old mangroves, 1 male, 7 females, 4 July 2013 ( SMO2 ) (Ma6107; male terminalia illustrated). 15 from SM02 , 1 from SM01 , 5 from SM03 , 1 from SMN2; Sarimbun , ZRCBDP0041305 _RP161_WTH24_SR03_ 28May2014; GenBank accession code: PP893294; Sungei Buloh Wetland Reserve , ZRCBDP0084993 Minh _ IDOM20 _ SB02_ 14Mar2016; GenBank accession code: PP893295; Kranji Marshes , ZRCBDP0329050 _ Jayanthi _MTJ024_ KM04 _ 08May2019. GenBank accession code: PP893296 .

In total, 225 specimens were barcoded and all specimens had less than 1% difference in barcode.

Diagnosis. A small species (1.8–2.2 mm). Male postpedicel trapezoid, a little longer than high, with a distinct ventral projection. Females with a shorter trapezoid postpedicel, a little shorter than high, also with a short ventral projection. Frons and face broad in both sexes, parallel-sided, twice as wide as postpedicel. Legs yellow, but mid and hind coxae brown, apical 2–3 tarsomeres of all legs brownish (somewhat variable in males, in females only apical tarsomere brown). Sternite 8 with a pair of weak macrosetae.

Etymology. The species is named after the island, Pulau Ubin, where it was found for the first time.

Male. Length: body 1.8 mm; wing 1.75 mm.

Head. Frons and face grey dusted, both parallel-sided, wide, two times as wide as postpedicel.

Antennae. Black. Scape as long as pedicel. Pedicel lacking a long dorsal bristle. Postpedicel trapezoid, a little longer than high, with a distinct ventral projection. Vertical bristle short, ¼ length of ocellar bristle (verticals much longer in female). Palpus yellow, apical bristle not distinct from the other small black bristles.

Thorax. Mesoscutum metallic green, grey dusted. All bristles pale brownish. 4–5 pairs of short acrostichal bristles; 6 dorsocentral bristles, anterior most very short, the following dc becoming longer toward scutellum, prescutellar longest and outside the row. A pair of very long marginal scutellar bristles, no bristle at the outside.

Legs. Yellow, except for brown mid and hind coxae and apical 2–3 tarsomeres of all legs brownish (not black).

Wings. With a greyish tinge, veins yellowish brown. Squama yellowish with a row of 5 pale brown bristles. Haltere white.

Abdomen. Brown in ground-colour with metallic green shine. Sternites with a few long, fine bristles. Male terminalia ( Fig. 5 View Fig ). Eighth sternite with a pair of weak macrosetae. Cercus yellowish, ventral surstylus brown with a black apical spine ( Fig. 5A–C View Fig ). Tip of phallus simple ( Fig. 5A View Fig ). Postgonite conical in lateral view ( Fig. 5C View Fig ).

Female. Length: body 1.76–2.18 mm; wing 1.28–1.95 mm.

Face as wide as frons, both parallel-sided and two times as wide as postpedicel. Clypeus protruding. Proboscis much stronger than in males, palpus also larger than in males, yellow with black bristles with a distinct apical bristle. Postpedicel in females also trapezoid, a little shorter than high, with a shorter ventral projection. Vertical bristle longer than in males, proclinate and half as long the brown retroclinate ocellar. A postvertical bristle, upper postoculars uniserial brown and becoming pale and longer below. Apical tarsomere of all legs brownish in contrast to the other tarsomeres.

Comments. Chrysotus ubinensis , new species, is part of a group of six cryptic species in Singapore that are recognised thanks to the barcoding differing from at least 6.3% to more than 10%. They all have yellow legs, with only the mid and hind coxae brownish, as well as the apical tarsomere that is brownish and, in some species, contrastingly black. All species of this group have a more or less triangular third antennal segment that in the new species is rather large, almost trapezoidal in males with a distinct ventral projection. The third antennal segment is smaller in females and the ventral projection is shorter. In the other species, the third antennal segment is smaller and a ventral projection is indistinct. This means that identification on morphological grounds is difficult and barcoding is necessary, especially because females are more similar than males. On Pulau Ubin, three other species—in addition to the new species—were also found and are very rare. All were females and two species were only found in the secondary forests on Pulau Ubin, Nee Soon, Bukit Timah Nature Reserve, and Kranji Marshes.

National distribution. Confirmed by the 225 available barcodes, C. ubinensis , new species, is very common in back mangroves on Pulau Ubin (nearly 200 specimens barcoded from 14 of the 26 stations ever sampled with Malaise traps all over the island). The species is also common in the old back mangroves of Pulau Semakau ( SMO) with more than 20 barcoded specimens, while there was only a single record from the replanted mangroves ( SMN), which did not have established back mangroves during the two-year study (2012–2014). Although many other mangroves were surveyed in Singapore during the last 15 years, the new species is very rare in other sites; there is a single record from Sarimbun , a single record from Sungei Buloh Wetland Reserve , and a single record from Kranji Marshes , which were formerly mangroves but has turned into a freshwater swamp forest. It was also the only record of the species that is not a true back mangrove species. A noteworthy observation is that the haplotypes in the new species do not vary much (less than 0.3%) over all the mangroves in Singapore. The new species was not yet recognised outside of Singapore as morpho-species nor as haplotype .

General comments on Chrysotus . The barcoding showed that there are several cryptic species of Chrysotus present in the same samples. There were at least 10 cryptic species ‘hidden’ in a Chrysotus with black legs, of which the males are morphologically difficult to distinguish. A thorough study of the male terminalia of this group is needed to see whether they belong to species already described ( Wei, 2018). Association of females, representing 60–80% of the population, to males is generally very difficult because firstly, females were generally not described and, secondly, they are very similar in morphology. Genetic barcoding is mandatory to provide reliable identifications so that accurate data will become available for ecological studies.