What goes there in the twilight? A tale of Gomphids*

* This entry was adjusted on December 31, 2019, following the publication of a paper in Zootaxa by Oleg Kosterin on June 12, 2019. The paper addressed the considerable confusion surrounding a group of similar Macrogomphus species, showing, amongst others, that M. guilinensis from Central and northern Vietnam (and China) is a junior synonym of M. albardae, occurring in a variety of distinct forms.

During the last week of June and the first two weeks of July I travelled around the northern provinces of Vietnam. I ran into almost 40 species of gomphids, some unexpectedly common. An eye-opener was that so many species are active just before dusk. Sometimes I would visit a stream in the afternoon and see only the occasional gomphid, to return in the last 30 minutes of daylight and see scores of them. There are some species that I encountered both during the day and in the twilight hour, there were some that were very much more abundant at dusk and there were some that I only encountered at dusk. For some, this may explain the paucity of records. For others it was truly surprising, as I had seen them previously at other places during the day. An example of this category was Nihonogomphus thomassoni. I had seen it, for instance in Huu Lien, earlier in the year regularly during the day. At a stream in northern Bac Kan Province I did not see any during the day, but the first males would start appearing during the last 45 minutes of daylight, to become common (say 20 specimens in a 200 hundred meters of stream) during the last quarter, some still there when it as almost too dark to see them.
Species that were observed at dusk were for instance Lamelligomphus camelus, Melligomphus ardens, Phaenendrogomphus tonkinicus, Macrogomphus albardae, Orientogomphus naninus, Stylurus amicus, and Burmagomphus vermicularis. Of these O. naninus and S. amicus were only observed at dusk, O. naninus commonly and in many places. Keith Wilson already mentioned the crepuscular behavior of O. minor in Thailand and suggested due to this habit it to be overlooked. It comes as no surprise that the similar O. naninus displays similar behavior and is much more common than its two previous records suggested. I encountered it in Bac Kan Province, Ha Giang Province and Cao Bang Province. On the other hand, several species regularly encountered during the day disappeared towards dusk, like Megalogomphus sommeri, or Labrogomphus torvus, or species of the genera Leptogomphus, Merogomphus and Asiagomphus.

Macrogomphus albardae I saw both during the day and at dusk, a total of 4 times in Bac Kan Province, at 3 different sites. This is a brilliant species, if only because of the weird appendages. It was also a species I really wanted to verify, after seeing possibles last year (in Ba Be and Cuc Phuong). It is widespread.

This is the female of M. albardae

A male at midday.

Appendages in dorsal view

In lateral view

And in ventral view

Orientogomphus (Acrogomphus) naninus was only seen at dusk, when males appeared to hover over the streams. It was encountered at many streams in semi-forested surroundings. I did not encounter any females.

Male hovering above a stream in Bac Kan Province

The appendages and posterior segments in dorsal view

The appendages in lateral view

Close-up of appendages in dorsal view

Finally, a species caught at dusk along two widely (about 80km) removed streams was Stylurus amicus. This species easily identified because of the relative length of S9 to S8, with S9 about 1.5 times longer. Karube already found S. clathratus in Vietnam, and one of the mystery gomphids of Sebastien may be a third species. S. amicus apparently had not been verified inside Vietnam before.

The magnificent club of Stylurus species, in this case of S. amicus