What goes there in the shade - Idionyx and Macromidia

May 17 and 18 at Tam Dao I was lucky enough to run into several of those interesting Cordulids that lurk mostly in shady places, dart out over shaded clearings in erratic flight and otherwise hang somewhere in the dark. Only under cloudy conditions or towards the end of the day do they really venture out in the open, but they are sometimes quite common. These are the Idionyx species, with their imposing appendages, and the Macromidia species. They look superficially quite similar, although the males of Macromidia miss the weird appendages of Idionyx. The females look even more similar. I saw a few Idionyx carinata females on the Tam Dao 2 trail, so at around 1000 m asl. But in the lowland stretches of the streams in the Tay Thien area I saw several handful. The problem of course that they are difficult to catch and I only managed females. Checking in the field they had no horns, so I thought they would all be Macromidia. One was decidedly larger but the four others were all the same small type with large yellow basal markings to the wings. Under the microscope there are interesting differences in the shape of the vulvar lamina. The one female had the lamina shaped into a deep V as is normal in many species. But the smaller specimens had the two halves firmly closed together. This did not look like the same genus after all. The larger species is Macromidia rapida, a common species in the Tam Dao area, although not easy to find. Although M. genialis shanensis could also be present, that species has some different markings on the thorax and face. It also, apparently, misses the small brown markings at the base of the wing. The smaller species in fact belong to a group of small Idionyx that miss horns on the vertex. This could be either I. yolande, I. victor or I. thailandica. I. yolande and I. victor have a slightly different thorax pattern (although that of the female published by Karube (2011) looks very similar). I. victor also has apparently less yellow at the base of the wing. The present species is therefore I. thailandica. In fact Sebastien Delonglee also reported having caught this species in the Tay Thien area (including males). I too will be keeping my eyes open for the male in the next few weeks. Coincidentally, what also separates these small Idionyx species from Macromidia are the yellow shins and the different ventral pattern.

Macromidia rapida, female, and Idionyx thailandica, female

Macromidia rapida, female. Note brown on dorsum of thorax, brownish-yellow face and dark brown bases to the wings.

Idionyx thailandica, female. Note yellow shins (not just this species), large ochre bases to the wings and lack of obvious lamina in lateral view (again, indicative of the genus, not the species). The straight yellow line over the spiracle is indicative of the species.

Idionyx type of vulvar lamina

Macromidia type of vulvar lamina

In lateral view, no obvious lamina visible for Idionyx

But obvious for Macromidia

Belly pattern of Macromidia rapida, female

Very different belly pattern of Idionyx thailandica/yolande group of females