aquatic leaf beetles, Donacia sp. - Donacia

Donacia vs. Plateumaris (updated comment)

Dear all,

I like to contribute to the discussion how to discriminate these genera.

The decisive character is a rather microscopic one, not to bee seen on any of the photos in the guide:
Donacia has the elytral suture normal apically, while in
Plateumaris the dorsal edge of the suture is bent outwards, exposing the inner face of the sutural fold (which is turned upwards then, somewhat replacing the ordinary sutural fold). The elytra fit tightly together as in Donacia, though!

You are right in suspecting that some of the "Donacia" images will show Plateumaris species.

To give a hint, some rules, that are drawn from European species, but will probably apply to yours as well:

- dorsally pubescent: Donacia only
- enlarged hind legs: Donacia only
- elytral tips subtruncate: Donacia only

In general,
species of Plateumaris are stouter and the elytra parallel-sided, rather convex tranversally, narrowing to apex in the terminal 3rd. Also the legs are rather stout, and hind femora but a trifle longer than those of middle legs. The dorsal surface is often very shiny, lacking microsculpture.

Donacia spp. have the elytra more wedge-shaped (narrowing to apex begins at about the middle of elytra), often the elytral disk is somewhat flattened with oblique shallow depressions.

I will add opinion on some more photos - this one shows Donacia sp.

regards, Boris

… Boris Büche, 20 December, 2006 - 6:52am

Donacia sp.

Don't think it's possible to take this image to the sp. level...

Det. & comment per EGRiley:

Donacia sp., can't tell which.

Family Chrysomelidae, Subfamily Donaciinae

Mike Quinn, Austin

Texas Beetle Info
http://www.TexasEnto.net/beetles.htm

… Mike Quinn, 22 December, 2005 - 12:17pm

distinguishing characters?

Is there some other way to distinguish Donacia species from Plateumaris? The only difference mentioned in this key to Alabama Donaciinae is whether the sutural margin of the elytra is straight to the apex [Donacia] or sinuate near the apex [Plateumaris]. Judging from the drawings, it's a small difference that would be difficult to see in most photos.
I wonder whether at least some of the photos on the Donacia page are actually Plateumaris species; it would be nice to be able to sort these out and put descriptive text on the Guide page(s) for future reference.

… Robin McLeod, 23 December, 2005 - 12:45am

Key to Tribes, Genera, and Subgenera of Donaciinae...

Ya might need to squint to see this but here's a link to a detailed

Key to the Donaciinae (Arnett et al, 2002) pg. 622

(via Goooogle Scholar)

Mike Quinn, Austin

Texas Beetle Info
http://www.TexasEnto.net/beetles.htm

… Mike Quinn, 23 December, 2005 - 8:18am

distinguish Donacia from Plateumaris

Per EGRiley's Guide to the Leaf Beetles of the GSMNP:

Subfamily Donaciinae

Inner edge of apical portion of elytral suture exposed

Plateumaris fulvipes
*Plateumaris metallica
*Plateumaris rufa

Inner edge of apical portion of elytral suture concealed

Donacia biimpressa
*Donacia caerulea
Donacia confluenta
Donacia liebecki
Donacia subtilis

I'll ask Ed if he has any other suggestions, though collecting does come to mind...

Mike Quinn, Austin

Texas Beetle Info
http://www.TexasEnto.net/beetles.htm

… Mike Quinn, 23 December, 2005 - 7:32am

I'm guessing...

that in order for the elytral suture to be exposed, the tips of the elytra would have to be separated at least slightly. In Stephen's photo, the elytral tips are jammed tight together, concealing whatever might be beneath (=Donacia ?), whereas in Joyce's photo, the elytral tips are obviously separated, exposing a tiny bit of something at the point where they separate (=Plateumaris ?) but I'm not sure what I'm looking at, or even what an "exposed... " might look like in a live specimen.
The overall shape of the elytra in Joyce's photo seem more angular, compared to the more rounded ones in Stephen's shot, which makes me wonder whether there's other markings or features that would allow identification of particular individuals to species. Could some of the species names provide a clue? Examples: fulvipes = fulvous legs or feet, rufa = reddish, biimpressa = two "dimples" somewhere, caerulea = bluish, etc.
It seems that, until recently, the assumption at BugGuide was that every Donaciinae was a Donacia species, presumably because Plateumaris had never been considered.

… Robin McLeod, 23 December, 2005 - 8:26pm

Larger photo

I've enlarged the image, does that help? Click on it to get full size.

… tom murray, 23 December, 2005 - 7:45am