Saturday, 19 July 2014

The Scalmogomphus mystery

This spring, on May 10 and 11, I saw many Scalmogomphus guizhouensis in the Yen Bai area. After that upon other visits to the same area I did not see them anymore. This made me think it was a species that only flies early in the season. To my surprise this proved incorrect. On July 3 and 4 I passed through Yen Bai Province again and this time found fully mature males sitting on rocks in the middle of streams or on bushes nearby. I also saw a female. Nothing was different in their structural appearance from the May specimens.

Afterwards I visited Pia Oac Nature Reserve in Cao Bang Province and posted for a long time at a spot where a larger stream exits from the woods into the open, because I had observed a Sieboldius species there that I hoped to check. On 7, 8 and 9 July time and again female gomphids came to that exact spot and hovered in the middle of the stream while dropping eggs. After I was able to catch one two very interesting things became apparent. The first was that this female had very long vulvar lamina, not just long, but also extremely pointy. The second that the thorax pattern was slightly different, but that otherwise they seemed exact copies of female Scalmogomphus guizhouensis. Subsequent females that I caught all had the same lamina. It was thus not a freak occurrence. I did not see any males, but on the third day I finally was able to first take some pictures and then also catch a male. To my disappointment I could not find any structural differences between these males and the Yen Bai males of Scalmogomphus guizhouensis. It is very unlikely that these males, caught at the exact spot where the females came to oviposit and with one male observed to chase one of the females, and with all females present consistently showing the extremely long lamina, belonged to one species and the females to another. But how then to explain the consistent structural difference in the females. In fact I took photos of one female with the thorax pattern almost the same as those from Yen Bai, so the only difference consisted of these long vulvar lamina. What does this signify? With the males exactly the same it is hard to claim it as a different species. But what then? A different subspecies? And if such obvious structural differences have no value to separate different species, what does that mean for other species that have been described based only on females that are structurally different from other females.

4 species of Scalmogomphus have been described, but the differentiating characters are not always clear. In none of the descriptions I have found reference to extremely long and thin lamina of the female. Below the strange and obvious difference is illustrated, together with the otherwise great similarities.

Scalmogomphus sp from Pia Oac. This is the specimen closest in thorax pattern to the specimens from Yen Bai. Clearly the extremely long lamina can be seen, reaching past S10.
Another Scalmogomphus sp from Pia Oac, with typical reduced black line over metepisternum. The extremely long lamina can again clearly be seen.

Female Scalmogomphus guizhouensis from Yen Bai. Extremely similar, apart from lamina.

Because this is the lamina found on females in Yen Bai. Rather robust and falling short of the distal margin of S10.

Whereas this is the lamina on the females from Pia Oac, thinner, and especially longer, reaching past distal margin of S10.

In lateral view, this is especially evident

This is a male Scalmogomphus sp caught in Pia Oac

And this is a male Scalmogomphus guizhouensis from Yen Bai. Under the microscope appendages and outward appearance of organs on S2 are identical.

Another male from Yen Bai
Another male from Pia Oac
And yet another male from Yen Bai

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