This spring I noticed in early May that Philoganga were quite common on Mau Son mountain in Lang Son Province. The thing was that they were consistently pale bluish-green on the abdomen and none where orange. This triggered me in collecting more Philoganga this year for comparison. Although sample size is still small, it supports the impression in the field from larger numbers of specimens that Mau Son specimens were larger and more robust on average than P. vetusta.
The issue is complicated, because older males seem to sometimes develop pruinosity that makes them appear whitish. It is possible that this is also only the case in P. robusta, but I am not entirely sure. There is also some variability in size of specimens, especially, it would seem, in P. vestusta.
Wilson & Reels (2001) reviewed the literature on Chinese records of Philoganga. There is no need to repeat here their excellent overview of mistakes and inconsistencies in the records of these very similar species. What they did conclude was that some characteristics mentioned in the literature to separate the species were clearly erroneous. The number of cells in between the wing margin and 1A is such a character. Indeed, I could find no consistent difference between what I consider P. vetusta and P. robusta in this character.
Wilson & Reels do suggest that the presence or absence of an antehumeral stripe could be of help. I found that the antehumeral may be somewhat more prominent in P. robusta, but that it can also be prominent in P. vetusta. Prominence seems to be related to age, with older specimens becoming darker on the dorsum of the synthorax with reduced or faded antehumeral.
The number of antenodal cross veins has also been mentioned as possible identification character. This was already pointed out by Wilson & Reels as invalid. Indeed, I counted 12-14 antenodals in definite P. vetusta (N=4) and 13-15 in P. robusta (N=3). Likewise postnodals range from 30-31 in P. vetusta to 29-34 in P. robusta. I have found no differences either for the few females I collected, but it is hard to draw conclusions, as the specific identity is difficult to prove, although one was caught in tandem with P. vetusta male and thus presumably that species
Wilson & Reels mentions that abdomen size of specimens from Hainan for P. robusta was 61.0 mm. My specimens are slightly smaller, 56-58.5 mm. They are however clearly larger than the bright orange typical P. vetusta that I was able to collect, which ranged from 51-54mm. I plotted the sizes in a graph replicated below.
It thus seems that the hypothesis that P. robusta can be identified by colour alone from P. vetusta is supported by the measurements of the specimens involved. Nevertheless more specimens are needed and from different areas to substantiate this. After all, local populations may be genetically homogeneous, which may influence the size distribution.
In the graph a single Philoganga species incertae is represented by one of the green squares. This specimen was caught in Cao Bang Province close to Cao Bang city on May 20. From its size it seemed to fit P. vetusta. However, the dorsum of S1-4 is black, not even whitish. It has a cream bar over the cranium of the head, which is absent in either P. vetusta and P. robusta when mature, although this may be an indication that the present specimen is still immature. In fact at first I dismissed it as just that, an immature P. vetusta. However, as can be seen from the pictures below, it has two strange characteristics in its wing I have not seen in any other Philoganga. The most striking is that vein 1A seems to run on further than in other species, where it curves back to the wing margin. Therefore even at the wing margin there is only one row of cells between 1A and CuP, whereas in P. robusta and P. vetusta there is a whole field of easily 10 cells. Furthermore the quadrangle is much more square than the elongated quadrangles of either P. robusta and P. vetusta. These aspects indicate the possibility of this being in fact an unknown species of Philoganga.
|Two males Philoganga robusta from Mau Son and what is presumably a female of the same species. Note the blueish-green colour of S1-4, although the smallest specimen also has a hint of orange.|
|The same three specimens in lateral view. They are not pruinose, but the males lack orange on the thorax or first abdominal segments.|
|Typical P. vetusta male caught in tandem with female. Pattern of dorsum of S9 of female is different from possible female P. robusta. However, this female is the larger of the two, 72mm total body length against 67mm for the Mau Son female.|
|The same specimens in lateral view, the male displaying bright orange both on the thorax and on the first abdominal segments.|
|Philoganga species incertae. Note the wing pattern with absence of a field between 1A and CuP, which is very different from the wing pattern of specimens illustrated above. Note also bar over cranium.|
|Hind wing detail of Philoganga vetusta, showing 10 cells between location where 1A reaches the wing margin and location where CuP reaches wing margin and field at widest location 3 cells wide.|
|Hind wing detail of Philoganga species incertae, displaying three cells only between 1A and CuP, and field, if you can call it that, 1 cell wide. Apologies for the legend (A1 versus 1A)|