What species A and B and species like A. xanthenatus (from Thailand) and A. acco from Vietnam have in common is that the abdomen has transverse bands on most segments. This is completely different in the species from mainland China, from Hainan and Taiwan or from Japan, which invariably have longitudinal markings on the dorsum of the abdomen. This trait is also visible on that other species recorded in the region, A. auricolor.
Something else intriguing is the amount of yellow on S1-2. This is very extensive in A. acco, rather prominent in A. xanthenatus and also very extensive in the description of A. auricolor. In species A at least there is a lot of variability, with some females, as we will see, with almost entirely yellow S1-2. But I am getting ahead of myself. For now, suffice it to say there are several species in northern Vietnam characterized by transverse yellow bands on the dorsum of the abdomen: Asiagomphus acco, Asiagomphus auricolor, Asiagomphus sp. A and Asiagomphus sp. B. Moreover, at least A. acco and A. sp. A are widespread and common. This latter fact is rather surprising. Somehow this species has eluded the sampling of visiting entomologists, even if it is widespread and common.
Let's turn to the enigmatic A. auricolor. Fraser (1926) describes it as follows:
"Gomphus auricolor sp. nov., (Fig. 4, a). Female. Abdomen 45mm. Hindwing 40mm. Head glossy black, the bases of the mandibles and a broad transverse stripe over the upper surface of frons bright citron yellow. Prothorax black with an anterior collar, a geminate spot on dorsum and a large spot on each side bright citron yellow. Thorax black marked with bright citron yellow as follows: - a short, broadly interrupted mesothoracic collar, a broad, parallel, antehumeral strips broadly confluent with the collar below. Laterally entirely yellow save for two narrow black lines, one traversing the spiracle, the other on the posterolateral suture, confluent below and cutting off a spot of yellow at lower border of thorax. Legs entirely black; hind femora with a row of gradually lengthening, closely set, short spines. This femur extends to middle of segment 2. Abdomen black marked with yellow as follows: - segments 1 and 2 almost entirely yellow, segment 1 has only a small sub dorsal tuft of short black hairs on each side. whilst 2 has a small semilunar, middorsal apical black spot, segment 3 with only a fine linear later-basal yellow stripe, 4 with a tiny basal point, 5 to 7 with large bass-lateral bright citron yellow spots nearly confluent over the dorsum, 8 unmarked, 9 with its apical half yellow, 10 unmarked. Anal appendages pale, small, conical. Vulvar scale not visible. Wings hyaline; a basal subcostal nervure in the right forewing of one specimen only: 5 rows of postanal cells; pterostigma pale brown, over 5-6 cells, braced; nodal index 15-18/14-11 | 17-15/12-14; remainder of venation typical for Gomphus."
The few females caught for species A did not have all yellow S1-2, as described for A. auricolor. They also did not have the yellow spot on the dorsum of the head Asahina described for A. xanthenatus females. However, as it turns out, the females are also variable, just like the males. On June 26 we found a nearly dead (due to age) female specimen with almost completely yellow S1-2. Almost simultaneously Sebastien published photos on his blog of another female, likewise with almost completely yellow S1-2. Both shared the three horns also seen in less extensively yellow females associated with Asiagomphus sp. A males. It occurred to me that this all made sense. It would be rather strange that there is a common Asiagomphus species all over northern Vietnam, but that Fraser would describe a different species that just so happens to be a one off and never to be seen again different species. Checking the specifics of this old female against the description, it seems to close a match to ignore.
Its abdomen measures 47mm and the hindwing 40mm. It has an all black face, with unmarked labrum, but with pale bases to the mandibles and with traverse line over the top of the frons. The prothorax is marked as described for A. auricolor, but this matches with many species, just as the thoracic markings (no humeral spot or stripe, extensively yellow on metepisternum). But the markings on the abdomen are an exact match with the description. S1-2 are yellow but for a patch of black hairs sub dorsally on either side of S1 and S2 has apically a black semi-lunar mark. Likewise, the markings of S3-10 match. The wing formula is close enough (15-18/17-12 | 19-16/13-16). There are also some differences. The vulvar scale, although small, is visible, the spines on the legs seems not as densely spaced as the description seems to suggest and the appendages are blackish, not pale. Nevertheless, Asiagomphus species with such extensive yellow S1-2 (shared only with A. acco) are rare and it is rather unlikely that this is not A. auricolor. The differences can be explained away due to the small sample size of the description (one or very few (Fraser speaks of the right forewing of one specimen only, as if there were several. Another female collected near the first had less extensive yellow on S1-2, but more on S3 and also had pale appendages).
If these specimens can be recognized as A. auricolor, then so should the less clearly marked yellow specimens and of course all the males. The females can then be recognized by the combination of thoracic and abdominal markings combined with a small horn on the occipital ridge and in most cases two prominent additional horns over the lateral ocelli. The males can be recognized by the shape of the hamule and inferior appendage in ventral view (wide U-shape) in combination with all black labrum. Nevertheless structurally they seem very close to A. xanthenatus as illustrated by Asahina and future research may well point out that xanthenatus and auricolor are closely related or even co-specific.
|One of several males caught close to the Asiagomphus auricolor female. This male has particularly extensive yellow on S1-2, an intermediate amount of yellow on the metepisternum, and humeral spots.|
|Two males caught together with the Asiagomphus auricolor female. They have the typical lraverse abdominal markings and structural characteristics of Asiagomphus sp. A. Bottom right is the Asiagomphus auricolor female.|
|Normal female of Asiagomphus sp. A, aka Asiagomphus auricolor. Seperable from A. acco by the humeral spot, small amount of yellow on metepisternum, black dorsum to S1-2 and larger traverse markings on abdomen.|
|But all females share a small horn on the occipital ridge and variable horns over the lateral ocelli.|