Eupeodes corollae? => Scaeva dignota

Gestart door IngridA, juni 26, 2017, 19:19:45 PM

Vorige topic - Volgende topic

IngridA

I would like to know which species this is
It concerns this observation
https://observation.org/waarneming/view/140827427
unsicher Eupeodes corollae observed at 2017-06-14 BRD - Cham (Landkreis)

Puppe geklopft von Fichte

Fotos:













Vriendelijke groet, Ingrid
(met verontschuldigingen dat ik niet in het Nederlands schrijven kan, zie mijn introductie in m'n eerste bericht.)

Jeroen van Soolingen

This is Scaeva, because of the swollen forehead. The spots reach the lateral margin of the tergites, so it is Scaeva dignota.
Groeten,
Jeroen van Soolingen

IngridA

Thank you very much for your help Jeroen.
I did´nt found this species in my books. I read it immigrated to Germany a few years ago.
Vriendelijke groet, Ingrid
(met verontschuldigingen dat ik niet in het Nederlands schrijven kan, zie mijn introductie in m'n eerste bericht.)

Arp

#3
Ingrid pointed me to this observation as she had received an admin-comment by Bert Oving telling her that this should be Scaeva albomaculata, not S. dignota - so now she has two opposing opinions by two admins. Strictly speaking Bert is admin for observado and Jeroen is not, so she should listen to Bert, but the comment didn't come with much explanation and looking at images on the internet (yes, I know ... but I can't find the proper literature for download) it woud seem that the explanation given by Jeroen at the time did have some merrit at least.

Additionally it could be a new species record for Germany (or at least a scarce one) if it is indeed S. albomaculata, so we should get the name right I suppose

So, if the two are difficult to ID - so difficult that admins disagree - it might be worthwhile to have a discussion on the proper ID here so that it may be used for future reference?

The keys that I own do not feature all species of Scaeva (I don' t "do" Diptera much). Checking this nice document (excellent info on the larvae!!) :

Laska et al. (2006) Taxonomy of the genera Scaeva, Simosyrphus and Ischiodon (Diptera: Syrphidae): Descriptions of immature stages etc etc... - Eur. J. Entomol., vol.103, pp637-655. (PDF)

The reference section points me to this key, but it seems to be unavailable online (to me):

DUŠEK, J.; LÁSKA, P. (1985) A review of the genus Scaeva Fabricius  (Diptera,  Syrphidae)  with  the  description  of  a  new  species from Chile.
Acta Entomol. Bohemoslovaca, vol.82, pp.206–228.
(not online?).

Scaeva are good flyers and have a high capacity for dispersion, there is even a record of the southern species S. albomaculata from Sussex, England (1938), so the species not previously being recorded for Germany (?if that is the case?) is not really a problem, but this specimen was reared from a pupa found in Bayern and that makes it a different ballgame, indicating larval development not just vagrancy.

Steven Falk has a good photo of albomaculata on Flickr, the images on the website by Cyrille.Dussaix should be okay (albomaculata / dignota) and usually the IDs on insecte.org are fairly reliable too (albomaculata / dignota). Additionally insecte.org offers some nice side-by-sides for various species (but no albos in there).

Going by what Jeroen had offered as an analysis (marks do not reach the lateral margins of the tergites) and looking at the similarities in the shape of the spots between the few images I've quickly dug up as "reliable"(?) reference, I personally see more resemblance with dignota, but I do not know what to look for really...

So: Are the marks anything to go by for separating albomaculata and dignota? Or are these too variable, or maybe even sexually dimorphic?
Are there any other characters, like hairs (Ingrids images offer a lot of detail I think)

It would be nice if someone could explain the proper analysis for future reference :)
Groetengedoe, Arp (wrn - JungleDragon)

Auratus

You got it right Arp.

The shape and/or orientation of the spots on tergite 3 and 4 are one of the distinguishing differences between S. albomaculata and S. dignota. The spots of S. albomaculata (and S. pyrastri) are said to be at an angle to the front border of the tergite wheras those of S. dignota are parallel to the front border.

There are however some other Scaeva which share this character. One of those, is S. selenitica which resembles S. dignota a lot. However, we can easlily distinguish the males of both species by the angle between the eyes on the front. The angle for S. selenitica is about 130 degrees whereas that of S. dignota is about 90 degrees. Also, just as Jeroen mentioned,  the spots reaching the lateral margin of the tergites is distinctive for S.dignota.

So, in my opinion, this is a male S. dignota

Groet,

Jan

Arp

Thank you Jan, for the clear diagnosis based on ID-ing characters we can all recognize (I think).  :duim:

Meantime, Bert Oving also answered in a comment with the observation. He puts forward another character that had him confused, or so it would seem: The white band on the side of the thorax being sharply demarcated from the dark central dorsum. Indeed, looking at series of images of (presumed) dignota and albomac. it would seem that on dignota the white margin usually seems more "fluffy", probably due to more hairs being present(?) On Ingrid's images above this is less so - maybe due to the animal still being fairly teneral?

Anyway, I thought it worthwhile to add this "character" to the discussion for future reference.

So, the obs is back to dignota and all experts seem to be in accordance again  :blink:

Thanks to all (jeroen, Bert, Jan) for clearing things up  :duim:
Groetengedoe, Arp (wrn - JungleDragon)