Plateumaris discolor (Panzer, 1795)

Geiser, Elisabeth, 2023, Revision of the Palaearctic species of the genus Plateumaris C. G. Thomson, 1859 (Coleoptera, Chrysomelidae, Donaciinae), ZooKeys 1177, pp. 167-233 : 167

Plateumaris discolor (Panzer, 1795)

Type location.

Germany.

Type material.

The holotype is unknown.

Remarks.

Plateumaris discolor was described by Panzer (1795) as Donacia discolor ( Geiser and Geiser 2023), but the morphological variability of P. discolor is within the range of the variability of P. sericea . It was finally synonymised with P. sericea by Askevold (1991) by examination of the endophalli from P. discolor and P. sericea specimens which showed constant characters throughout their distribution area (Fig. 12a ), but some authors continue to regard P. discolor as a species propria ( Silfverberg 2010; Bieńkowski 2014; Rheinheimer and Hassler 2018); therefore, further arguments are discussed below.

Several characters are used to distinguish P. discolor from P. sericea . The first are the antennomeres: in P. discolor A3 and A4 are a little bit longer than A2, whereas in P. sericea A3 is 1.5 × as long as A2 and A4 is twice as long as A2. In fact, the length of the antennomeres is very variable, therefore the difference between "a little bit" and "one and a half" is not clear.

The second is the pronotum disc: in P. discolor it is more punctured and transversely wrinkled whereas in P. sericea it is very finely sculptured. However, the structure of the pronotum disc varies in both “species” in its sculpture and shows an intermediary form in many cases.

The third is the median lobe of the aedeagus, which is also very variable (Fig. 11D, E ). This is shown also in the drawings and pictures in identification keys. Sometimes the aedeagus picture of P. discolor in one key looks most similar to the picture of the aedeagus of P. sericea in another key. When a drawing or photograph was made from different angles of view, the same aedeagus can look different in shape and contour. There exist specimens with " Plateumaris discolor " antennomeres and " Plateumaris sericea " pronotum and vice versa. Also, each shape of the aedeagus can occur with any combination of the antennomere or pronotum characters.

Due to these variations, there are no reliable morphological characters to distinguish P. discolor from P. sericea . Other evidence suggests that they may be separate species: P. discolor is reported to be assigned to acid soil and peat bogs where the larvae develop on Carex , Juncus and related plants, whereas P. sericea prefers various wetland habitats with alkaline soil ( Rheinheimer and Hassler 2018). Their larvae feeds on Sparganium sp. and Iris pseudacorus (Bienkowsky, 2014). However, P. sericea has such a large distribution area and is very abundant even nowadays in contradiction to almost all other Donaciinae species, therefore, it is more likely that the food plant is also widespread and abundant. This is the case with Carex or Juncus but not with Sparganium and Iris . In the key to Donaciinae larvae in Japan Narita (2003) mentions Carex dispalata Boott. and Scirpus fluviatilis (Torr.) A. Gray as food plants for the larvae of P. sericea . This is definitely not a confusion with P. discolor because the latter does not occur in Japan. Therefore, the assignment to the food plants in Bienkowsky (2014) contradicts the study of Narita (2003) and is in general not a suitable argument that P. discolor is a separate species.

Molecular studies by Hendrich et al. (2015) and J. Bergsten (pers. comm. NHRS, 23 Jan 2023) indicate that P. sericea is genetically very variable. In molecular phylogenetic trees, specimens identified as P. discolor are resolved in between P. sericea specimens, sometimes in groups and separated from P. sericea groups, sometimes not. It is likely that some of these specimens identified as P. sericea are "some kind of" P. discolor and vice versa, because morphological characters are not reliable to distinguish them. There is another problem: it is possible that P. sericea consists of several cryptic species but P. discolor may not be one of them.

Material examined.

More than 100 specimens labelled " P. discolor " from different localities in Europe.

Figures

Figure 11. Plateumaris sericea A-C habitus illustrating variability in colours, but there are more D, E aedeagus: two examples of the variability of the shape of the median lobe (photographs by K. Matsumoto). Scale bars: one unit - 1 mm.

Figure 12. Schematic sketch of the endophallus. Median ejaculatory guide blackened A Plateumaris sericea (and specimens identified as P. discolor) with apex of median ejaculatory guide rounded. The endophalli of specimens from Poland, Italy and Japan all look the same as in this sketch B Plateumaris shirahatai with apex of median ejaculatory guide notched (drawings by G. Geiser based on photographs from Askevold 1991 and Hayashi 2020).