Pseudagrion p. pruinosum (Burmeister, 1839)

Pseudagrion p. pruinosum View in CoL and P. p. elongatum

Since the synonymization of the name P. elongatum with P. pruinosum fraseri as suggested by Asahina (1988) was invalid, we had to discuss whether we consider P. elongatum a junior synonym of P. pruinosum , or if this taxon should be considered a subspecies of P. pruinosum , or perhaps even a species. The specimens and photos examined reveal differences between the mainland populations, for which the name P. elongatum is available, and the island populations are named P. pruinosum (including the jun. syn. P. p. fraseri and P. p. ranauense). Therefore, we provisionally suggest a subspecific status, but advise more intensive studies based on material from the whole range and molecular data.

Both taxa differ in a minute but seemingly constant difference in the male cercus, and in the colour pattern ( Table 3). The difference in the colour of the frons depends on age, because immature males of the nominate taxon also have a reddish frons as it is presented in mature P. p. elongatum ( Figs 74, 177)—however, we did not find a single male with black frons (as it is generally the case in mature P. pruinosum ; Figs 66, 69, 171) in the mainland populations. This character was already stated by Fraser (1933). Additionally, the postocular spots are in general still present in mature males of P. p. elongatum , although they may be hardly visible due to indistinct darkish red colour ( Figs 74, 176–178). In the nominate subspecies the postocular spots are distinctly present in younger individuals ( Figs 172, 173), but become completely obscurly blackish with maturity alike it is in the frons ( Figs 66, 69, 171). The dark dorsum of the pterothorax was found to be relatively constant in P. p. elongatum ( Figs 73, 175–178), whereas it is generally absent in this extent in P. p. pruinosum ( Figs 69, 71, 171). Thus it seems that the state of this character is not simply age-dependant, but until more in-depth analysis is available, it is preferably not recommended as a distinct character for separation. This obscurely blackish dorsum of the pterothorax is a character shared with P. obscurum , and in the latter taxon this may seemingly be a character to separate the adjacent P. simalurum . The records from the Andaman and Nicobar Islands, India published by Mitra (1995) and Sivaperuman (2014; but not listed in Sivaperuman 2015) are listed by the authors as P. pruinosum . We were not able to study specimens collected at these islands or even photos, thus the subspecific status needs to be verified.

Potential for new species

We studied specimens from most of the larger islands of Sundaland and Wallacea west of Lydekker’s line, and there are no larger areas where the occurrence of an undescribed species belonging to this group seems to be highly likely. At the same time, it should be added that there are extensive areas on large islands (e.g. Sumatra and Borneo), which are odonatologically very poorly researched. In this respect it is also striking that there are no published records of reddish Pseudagrion available from the main island Sumatra (except for P. pruinosum ). During this study a damaged female labelled as from Sumatra was found in RMNH collection, but we were not able to assign this specimen to a certain taxon because of its condition and sex. Beside this, there may be potential for the discovery of new species on some of the smaller islands. Laidlaw (1926) for instance, recorded P. pilidorsum from the Mentawai Archipelago. Based on current knowledge it is unlikely that this taxon indeed occurs in this island group. We were not able to study these specimens, but it seems possible that these may belong to an undescribed taxon. Also the status of P. ustum needs to be verified: the female holotype of P. ustum was likely collected in the Sula Archipelago, and photos reveal a possibly undescribed species on Taliabu Island which is part of the archipelago. These males from Taliabu ( Figs 169, 170) have a thin black humeral stripe similar to the adjacent P. acutidens ( Figs 5, 96, 150, 151) and P. coriaceum ( Figs 17, 18, 98, 152, 153), whereas it is usually lacking to this extent in P. ustum ( Figs 112, 168). In addition, S9–10 appear to be entirely red except for a black posterior margin on S9, whereas P. ustum ( Figs 129, 168), P. acutidens ( Figs 6, 7, 113a, b, 150, 151) and P. coriaceum ( Figs 115a, b, 152, 153) usually have black markings to varying degrees on S10 as well. Unfortunately there are no collected specimens from Taliabu available, moreover we were not able to study the holotype of P. ustum , and the water colour painting of Selys ( Fig. 78) is not sufficient for a certain determination. But if future studies reveal that the holotype of P. ustum represents the female of the Taliabu Island species and this species does not correspond with Sulawesian P. ustum , then the specimens from Sulawesi assigned to P. ustum will need to be erected as a new species.

Research recommendations and species conservation aspect

The present revision represents a solid foundation for further research, but should by no means be interpreted as final. Further targeted taxonomic and molecular studies, based on larger series of both males and females, are needed for this group as a whole. This could solve some of remaining taxonomic puzzles highlighted above, but may also provide more in-depth biogeographical and phylogenetic conclusions. In this respect, species of the Pseudagrion red-group occurring in Sundaland and Wallacea, by taking into account extreme insularity as well as diverse tectonic history and paleoclimatology of the whole region, would be excellent and extremely interesting model organisms.

Despite a well-justified reputation of being one of the most diverse biodiversity hotspots on the planet, also as regards the odonates ( Kalkman et al. 2008), the freshwater habitats in Sundaland and Wallacea region are also among the most threatened due to various anthropogenic pressures. Some among the species, e.g. P. enganoense , P. obscurum , and P. simalurum that are only known from small islands near Sumatra, have not been reported for nearly a century. Further odonatological faunistic fieldwork is urgently needed to obtain additional knowledge on species’ distribution, seasonal phenology, biology, and habitat requirements. This additional information will enable relevant threat status assessments, especially of narrowly endemic species which might already be on the brink of extinction or even extinct. For all of the above purposes, support with issuing relevant field work and collecting permits as well as funding is needed from national and international research and conservation agencies.