Tuesday, 7 April 2015

Trigomphus carus - a new species for Vietnam

Saturday April 4 it was an absolutely lovely day and I woke up very early for birdwatching in Pia Oac. When the sun became hot I went to a lower area on the mountain and checked also some streams and a small pool by the roadside. It was here around noon that I noticed a small gomphid perched on a blade of grass over the pool. I was taken aback by it. From a distance its pattern reminded me of Davidius species, but after a quick photo I was able to catch it and in hand quickly concluded it was a Trigomphus species. Only a few minutes after I had taken it another landed on exactly the same blade of grass. Because these are difficult and variable species and because it was a new species to Vietnam for sure, I took this male as well.

There are 14 known species of Trigomphus, although the true number may be smaller, with several synonyms in use. (The 14th is the new species from Vietnam I found last year, which will be described by Karube in an article to be published in May this year*). The particular thorax pattern, shape of appendages and of the genitalia all fit very well Trigomphus carus, a species only known from Fujian in China, as far as I can tell.

This species has the dorsum of the thorax marked by thin "7" shaped stripes and a small antehumeral spot. In fact as is often the case, this character is somewhat variable. One specimen has it, the other does not. Also there is some variation in the lateral thorax pattern. Shape of posterior hamulus is rather distinctive for T. carus and also the somewhat triangular outline of the anterior hamulus is indicative. The inferior appendages, interestingly, curve outward slightly, whereas in most species they curve inward. The match in appendages, anterior and posterior hamulus and penile organ, plus the close match in pattern of the thorax and size (one specimen has abdomen 37 and HM 30mm, the other 36 and HW 29mm), makes identification as Trigomphus carus possible.

* Described as Trigomphus kompieri in the May 2015 issue of Tombo

Trigomphus carus male as found, perched on a leaf 40cm above the water of a small pons
The same male, in hand. Note the anal triangle with 5 cells.
Both males scanned in dorsal view. Note the absence of the antehumeral spots in the top individual, which shows the sometimes limited use as an identification feature.
Both males in lateral view. The bottom individual with slightly less black on the sides of the thorax.
The face of Trigomphus carus, with all-black labrum.
Appendages in dorsal view. Note the lateral process at two-fifths.
Close-up of the right superior appendage in dorsal view.
Lateral view of the appendages. Left side and right side not exactly the same.
Inferior appendages in ventral view. Characteristically slightly curved outwards. The orange bit is poop.

The ventral view of the hamuli. Note the bulging swollen posterior hamuli adorned with a crest-like inward-facing hook.
And lateral view of the hamuli. Here the "hollowed out" triangular anterior hamulus is essential.


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