Sunday, November 09, 2008

Vestalis amoena; Vestalis amethystina


Vestalis amoena

Among Singapore’s damselflies, species from the genus Vestalis probably have the most attractive wings. Depending on the viewing angle and reflecting sunlight, the clear wings of these damselflies can appear to sparkle with purplish iridescence. We have two species in Singapore, Vestalis amoena and Vestalis amethystina. Both species look very much alike with the same metallic green colouration. In fact, the nine species known to exist in Sundaland are so similar in appearance that even differences in wings venation, body colouration and male penis structure are too ambiguous for species recognition. The only sure way to distinguish the species is by examining the male’s anal appendages.

Vestalis amethystina

Although it seems difficult but with a good digital camera giving a close-up shot of the male’s anal appendages, it is actually rather easy to identify our two Vestalis species in the field without having to capture them. For Vestalis amoena, the appendages are less curved towards each other and appear shaped like a horseshoe.

On the other hand, Vestalis amethystina has their appendages more curved towards each other. Females can be separated by the yellow labium of V. amoena and black labium of V. amethystina.

The geographical distribution of these two species is interesting. Both of them occur together from southern Thailand, Malay Peninsula to Sumatra. But V. amoena extends into Borneo while V. amethystina does not. In Borneo a close relative V. amaryllis, not found in the Malay Peninsula or Sumatra, exists. So it seems V. amaryllis became isolated in Borneo, differentiate into an independent species and occupies an ecological niche before V. amethystina could establish itself.

In Singapore, both V. amoena and V. amethystina occupy similar habitat: flowing streams in dense forest vegetation. However V. amoena is reportedly less common than V. amethystina. This seems to be very true as I encounter V. amoena less often during regular trips into the nature reserves. One very possible reason is that V. amoena prefers larger and faster flowing streams.
As forests degradation occurs, silt and sediments become accumulated causing streams to flow slower thus making it a more suitable habitat for V. amethystina. Historical records show that V. amoena was once found in Ulu Pandan. This is a perfect case in point of how much forests we have lost and how vulnerable forest damselflies are.



Pictures taken at: Central Catchment Nature Reserve, June 2008.



References:

Laidlaw, F. F., 1931. ‘A list of the dragonflies (Odonata) of the Malay Peninsula with descriptions of new species’, Journal of the Federated Malay States Museums, Singapore, vol 16, pp. 175-233.

Lieftinck, M A., 1965. ‘The species-group of Vestalis amoena Selys, 1853, in Sundaland (Odonata, Calopterygidae)’, Tijdschrift voor Entomologie, vol 108, pp. 325-364.

Orr, A. G., 2003. A Guide to the Dragonflies of Borneo. Their Identification and Biology, Natural History Publications (Borneo) Sdn. Bhd, Malaysia.

Orr, A. G., 2005. Dragonflies of Peninsular Malaysia and Singapore, Natural History Publications (Borneo) Sdn. Bhd, Malaysia

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