Mortonagrion sp. cf. aborense (Laidlaw, 1914)

Figs 17–19

Selected references for Mortonagrion aborense:

Argiocnemis aborense Laidlaw, 1914;— Laidlaw 1914b: 347, pl xvi, fig. 7 (original description from NE India).

Mortonagrion aborense (Laidlaw, 1914);— Hämäläinen 1989: 1–3 (placed in Mortonagrion, list of synonyms, discussion);— Dow 2011: 35, 44, 45, Fig. 28 (discussion, illustration of genital ligula);— Dow et al. 2018: 11–12 (first record from Riau, two distinct forms found);— Dow et al. 2024: 35, Note 36 (summary of distribution, note on taxonomy).

Mortonagrion binocellata Fraser, 1922;— Asahina 1982: 456–458, Figs 1–10 (records from India and Thailand, description, illustrations).

Mortonagrion simile Ris, 1930;— Ris 1930: 7–12, Fig. 3 (original description of M. simile from North Sumatra);— Pinhey 1974: 271 (discussion of genital ligula).

Specimens collected. M1k: 1 ♂, 1 ♀ (RER23 _ COE315–316; RER), 2 x 2023; M3c: 1 ♀ (RER23 _ COE103; RER), 10 ii 2023; M3d: 1 ♂ (RER20 _ COE130; RER), 17 i 2020; M4e: 2 ♂♂ (RER23 _ COE104–105; RER), 13 ii 2023; M5a: 1 ♂ (RER20 _ COE131; RER), 20 i 2020; M11b: 2 ♂♂ (RER23 _ COE228–229; RER), 11 vi 2023; P1a: 2 ♂♂ (RER23 _ COE313–313; RER), 24 ix 2023; P2a: 1 ♂ (RER23 _ COE227; RER), 30 v 2023.

Taxonomic Remarks. Mortonagrion aborense certainly occurs in Sumatra but a second form (possibly a distinct species), at present only distinguishable by examination of the genital ligula, also occurs in Sumatra (Dow et al. 2018) and in Borneo (Dow 2016). On examination of the penis of the specimens from the Kampar Peninsula it was found that none are the true M. aborense but they also differ consistently and quite markedly from the second taxon and might actually be a third species. The genital ligula of M. aborense has a terminal segment that narrows markedly near its base then expands again into a broad, almost circular shape (called a ‘scoop’ by Pinhey 1974: 271), lacking flagella, see fig. 28 in Dow (2011) and Fig. 17 here. The other form reported from Borneo and Sumatra lacks the contraction and then expansion at the base of the terminal segment, instead maintaining an approximately even width then contracting to a relatively long, subrectangular apical tongue, shallowly bifurcated at the tip (Fig. 19), the two arms of the bifurcation can be considered as rudimentary flagella and, viewed laterally, slightly wrap around the shaft of the genital ligula. The Kampar form (the genital ligula was examined in all males except RER23_COE315 where the genital ligula was lost during extraction) contracts and expands as in M. aborense but the expansion is more gradual and the shape formed more angular, shares the apical tongue (but shorter) with the second form but with the tip of this better described as shallowly concave rather than bifurcated (Fig. 18). Note that Figs 17–19 are not drawn to scale or particularly accurately and are merely indications of the general shape of the genital ligula in the three forms so that the reader can more easily understand the differences.

Mortonagrion simile, currently considered a junior synonym of M. aborense, was described from Sumatra (Ris 1930) but the genital ligula was not described. Hämäläinen (1989) examined the holotype and allotype of M. simile but does not mention the genital ligula, which would probably require either extraction for examination or examination using a method that does not risk damaging the specimen such as non-destructive X-ray micro-computed tomography. As already noted by Dow (2016, considering the second form) the name M. simile is available and might ultimately prove to be valid for one of the two additional forms. Further work is required on this issue but if all three forms are actually of one species then that species apparently exhibits polymorphism (rather than simple variation) in the terminal segment of the genital ligula, not something that we are aware of ever having been reported before in Odonata and which would potentially raise wider questions about the use of penis structure for specieslevel taxonomy in the Coenagrionidae.