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 Xanthogramma stackelbergi?
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| Carnifex |
Posted on 02-06-2020 23:22
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 Member Location: Vienna, Austria Posts: 1867 Joined: 23.06.15 |
Found mid June in Wienerwald, near Vienna.
Cheers, Lorin Font Color All requests are from the urban area of Vienna, if not otherwise stated. My Diptera observations (and other living forms) can be found here, and corrections or comments over there would also be appreciated. |
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| Sundew |
Posted on 03-06-2020 02:39
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 Member Location: Berlin and Baden-Württemberg, Germany Posts: 3890 Joined: 28.07.07 |
That is a difficult question. Here we have to discriminate between X. stackelbergi and X. dives. Unfortunately we do not have a lateral, or even better, ventral picture of this male. So what can be said? There was an English key by Speight & Sommaggio (2010) that unfortunately is no longer online (I could send you a PDF), and there is a key in Dutch by van Steenis et al. (2014, https://www.repos...ent/645906). The former authors said concerning the yellow marks on the tergites: X. dives - inner (medial) extremity of yellow marks on tergite 2 usually pointed (but may be rather rounded), the pale marks nearly always reaching their greatest length on the lateral margin of the tergite (i. e. pale marks hardly, if at all, cut away postero-laterally); pale marks on tergite 3 nearly always reaching lateral margins of tergite at their maximum length X. stackelbergi - inner extremity of yellow marks on tergite 2 very rounded; pale marks on tergite 2 often cut away postero-laterally, so that usually they are closest to the posterior margin of the tergite at some distance from its lateral margins; pale marks on tergite 3 cut away antero-laterally, so that they occupy a greater part of the length of the tergite at some distance from its lateral margins So the shape of the uppermost pale marks (on tergite 2) fits X. stackelbergi but can occasionally be seen in X. dives, whereas the marks on tergites 3 and 4 better go with X. dives (compare the figures in the Dutch paper). X. dives males usually have infuscated wing tips that seem to lack in your fly, but Speight & Sommaggio state: "The darkened wing membrane occurring at the wing tip of males of X. dives can be very useful, when present. It is well developed in the lectotype, but may be hardly discernable in some males of X. dives." So ambiguity again! I had a similar discussion with an expert (https://diptera.i...d_id=54819) who formerly called himself "Piet Nord", later "Fred Fly". He wrote explicitly: "specimens of the pedissequum-group with hind legs nearly black from distal end of femur to tibia and tarsi dorsally and with no clear dark rings visible on femura 3 and tibia 3 are belonging to X. dives". For this reason I tend to X. dives, but a residual uncertainty remains... Regards, Sundew |
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| Carnifex |
Posted on 03-06-2020 22:31
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Member Location: Vienna, Austria Posts: 1867 Joined: 23.06.15 |
Thank you Sundew for this comprehensive answer. I do have both your recommended papers, as I am interested in the genus Xanthogramma for quite a while now, but all of my personal encounters have only resulted in X. dives so far, so I do not have any personal experience with other species. In addition to the shape of the yellow spot on tergite 2, I usually look for the infuscated wing tip (but as you said, males often do not show this feature) and then for the extent of the dark spot below the pterostigma - it should extend beyond r2+3 in dives but remain restricted in stackelbergi. Going back to the original image, I made a crop of the wing and it really looks confinded to the border of r2+3:  On Steven Falks Flickr album there is clearly a X. stackelbergi-male shown (https://www.flick...548622900/) and comparing the wing here with his X. pedissequum photos (this feature should be comparable to X. dives) clearly points towards X. stackelbergi in my opinion. Also on Steven's album the leg ring you (and Piet) where mentioning is clearly visible. I guess with ring on femur3 it is meant that the distal end of the femur is lighter in X. stackelbergi? While searching for the original photo I also found a shot where I tried to get a ventral picture, but only caused it to taking off and got this blurry result. However, the joint of femur and tibia is lighter, so the hind leg is not continuously dark. It is not a clear ring but I also wonder about the reliability of that feature. Furthermore, in the lateral view one can see that the spot on tergite 3 converges at the lateral border to some extend, so all in all, considering the new photos, to me it feels quite good for X. stackelbergi.  Really looking forward to your opinion. (Is Fred Fly still active in the forum)? Edited by Carnifex on 03-06-2020 22:35 Cheers, Lorin Font Color All requests are from the urban area of Vienna, if not otherwise stated. My Diptera observations (and other living forms) can be found here, and corrections or comments over there would also be appreciated. |
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| Sundew |
Posted on 04-06-2020 02:02
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Member Location: Berlin and Baden-Württemberg, Germany Posts: 3890 Joined: 28.07.07 |
Dear Lorin, I might possibly persuade Fred Fly to have a look at our discussion, but he is dealing with other dipteran families now and very busy altogether. However, I hope we can bring some light in our puzzle ourselves! I checked the keys once more and had another close look at my picture collection of X. dives and X. stackelbergi. Here is what I found out (see the picture compilation): 1. The best distinctive characters (as mentioned in the Dutch paper) seem to be the ventral pattern of the male abdomina and the colour of the hairs on the edge of the lower calypter and the plumula - both not applicable to our pictures. 2. In my X. dives pictures, the dark wing part below the stigma is typically extending over vein R 2+3. I noticed a lightened spot in cell r1 close to the vein fork below that is not mentioned in the descriptions. 3. I am quite sure that my male in the middle row of pictures is X. stackelbergi because of the colour of the hind leg. The dark wing zone below the stigma is restricted to cell r1 as described. The lightened spot is hardly recognizable. 4. The male of the lowermost picture row I had also labeled "X. cf. stackelbergi" because of the shape of the yellow tergite marks and the wing darkening just in cell r1. Its hind legs, however, are very dark. - On closer inspection I see clearly the lightened spot in cell r1, which is reminiscent of X. dives. So maybe the presence of this light spot is more important than the extension of the grey zone into cell r2+3? Only a comparison of a large number of collected specimens could clarify that hypothesis. 5. But - good news! Another character caught my eye, and I wonder why the descriptions don't explicitely mention it. It is the shape of the male abdomen viewed from above, i.e., the length:width ratio. All my X. dives males have a long, slender abdomen with nearly parallel sides, whereas the X. stackelbergi male has a rather broad abdomen with rounded sides. The drawings 6 and 7 in the Dutch paper clearly show it, too. Also Steven Falk's males have this shape - as well as yours. Conclusion: You actually found X. stackelbergi, and I have pictures of only one male of this species; the other has to be re-labeled as X. dives. Problem solved! What do you say? If we meet females in the future, we need a good portrait photo. The frons pattern should be distinctive. Best wishes, Claudia (Sundew) Sundew attached the following image:  [295.56Kb] Edited by Sundew on 04-06-2020 02:11 |
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| Carnifex |
Posted on 22-06-2020 19:59
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Member Location: Vienna, Austria Posts: 1867 Joined: 23.06.15 |
Hi Claudia, I like your comparison and ideas. The main obstacle I see right now is to get enough specimens of X. stackelbergi to check different features. I am actually astonished about the reduced extension of the wing spot in your lower fly. That worries me a bit about the reliability of that feature. The clear triangle I am actually not sure about. First, with the overall lighter wing spot in your X. stackelbergi, the contrast to the triangle is of course much weaker, but I still can make out the shape in your photo. Also, in Steven Falks (dead) specimens, although the wings are not straight, one can see lighter areas in the cell as well. Body shape also is a critical feature, as the segments are quite flexible. An overall comparison between many individual would be ideal, and maybe you are right in that the borders of the third segment are more straight in X. dives, but I feel not confident enough to apply this feature right now. For example, see here: https://www.inatu...s/41689370 These photos all should depict the same individual but the abdominal shape is quite different. I hope one time I will be able to catch a male stackelbergi, to have a closer look by myself. Thus far, it is still an unsatifsyingly described pair of species, but that makes it interesting at same time :-) Cheers! Lorin Edited by Carnifex on 22-06-2020 19:59 Cheers, Lorin Font Color All requests are from the urban area of Vienna, if not otherwise stated. My Diptera observations (and other living forms) can be found here, and corrections or comments over there would also be appreciated. |
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| Sundew |
Posted on 24-06-2020 03:02
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Member Location: Berlin and Baden-Württemberg, Germany Posts: 3890 Joined: 28.07.07 |
Well, abdomen shape is often used in diptera keys, the tergites are not thus flexible, only the lateral membrane is. To me, in X. stackelbergi the maximal width of the abdomen is only reached on the border between tergites 2 and 3 and narrows in an arc in both directions, whereas in X. dives the maximal width extends between tergites 2 and 4. Length:width ratio in X. stackelbergi is about 1.9 and in X. dives about 2.1, but of course you are right that repeated measurements of a series of males is needed to verify this difference. I am quite optimistic that it could turn out to be statistically significant. As to your linked specimen, it is obviously the same male on the same leaf, and only the last picture shows an exact view from above, so I'd say this is X. stackelbergi. Abdomen shape fits, the hind legs are not black but brown, and you can see a dark ring on the hind tibiae. I do not share your opinion that the species pair is unsatisfyingly described. We have much more differential characters than, e.g., in Syrphus species, where you have to check the distribution of microtrichiae in certain parts of wing cells... One character mentioned by the Dutch authors we did not pay attention to is: Hairs on the margin of the lower calypter and the plumula yellow in X. stackelbergi and dark brown in X. dives. I find the calypters very inconspicuous here, but I cropped two pictures that should show the region - see below. Once you know the important body parts, you 'll focus on them in the future (hopefully, the autofocus of the camera is willing fo follow  ).Good luck, Claudia Sundew attached the following image:  [283.79Kb] |
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but don't see the image in the post.