Its morphology was studied with optical, X-ray micro-CT and synchrotron micro-CT documentation methods. Here we report an unusual beetle larva in Eocene Baltic amber. 37 million years ago or less, Sadowski et al., 2017 Sadowski, Schmidt & Denk, 2020) two additional species have been reported ( Wichard, Gröhn & Seredszus, 2009 Bukejs, Alekseev & Jäch, 2015). 99 million years ago Cai, Maier & Huang, 2018). A younger fossil of Elmidae was reported from Myanmar amber (ca. 105 million years ago Peris et al., 2015) yet the identity of this fossil as a representative of Elmidae has been doubted ( Bukejs, Alekseev & Jäch, 2015). The oldest possible record is a specimen in Cretaceous amber from Spain (ca. While representatives of Elmidae seem to play an important role in modern freshwater ecosystems, the fossil record of the group is quite scarce ( Peris et al., 2015). Larvae show a considerable variation from elongate cylindrical bodies to flattened, more or less onisciform ones. Few larvae seem also to feed on wood ( LeSage & Harper, 1976), representing true wood-borers ( Valente-Neto & Fonseca-Gessner, 2011), but still being aquatic.

Adults and larvae seem to mostly feed on algae, which are scraped off from surfaces, in some cases also encrusting animals such as bryozoans are consumed. In a more modern function, representatives of Elmidae can be used for monitoring water quality ( Hilsenhoff, 1982 Grasser, 1994 Moog & Jäch, 1995 Braukmann, 1997 Garcia-Criado, Fernandez-Alaez & Fernandez-Alaez, 1999 Garcia-Criado & Fernandez-Alaez, 2001).

In some areas of the world it seems they have even been consumed by humans (as a kind of seasoning) and therefore even had a commercial value ( Philippi, 1864 Kodada & Jäch, 2005a p. In certain running waters, representatives of Elmidae may be the dominating animal life form ( Kodada & Jäch, 2005a p. All adults are associated with running waters, and most of them indeed live within the water. 471), others seem quite long-lived with animals surviving up to ten years ( Kodada & Jäch, 2005a). Some adults seem to be rather short-lived, surviving only for three weeks, as in the ingroup Larainae ( Kodada & Jäch, 2005a p. The group has a worldwide distribution with more than 1,300 formally described species. One of these merolimnic beetle groups is that of Elmidae, the riffle beetles. In many cases, larvae and adults are aquatic, but nevertheless these differ strongly in their overall morphology, and coupled to that also in details of their ecology. In the extant freshwater fauna, beetles are making up 14% of all formally described species of Insecta ( Sæther, 1993). This group entered freshwater habitats many times independently, leading to several diversification events ( Wootton, 1988). These representatives with at least one aquatic life stage are also known as “merolimnic” ( Merritt & Cummins, 1996).Īmong these aquatic forms are also many different types of beetles. Especially the myriads of aquatic larvae (for challenges of this term in this aspect see Haug, 2020) of dragonflies, damselflies (both Odonata), mayflies (Ephemeroptera), stoneflies (Plecoptera), flies and midges (both Diptera), and many more represent an enormous share of the modern freshwater ecosystem. However, a much larger biomass is represented by another group of crustaceans (that often is still not recognised as such): Insecta. Many may immediately think of crustacean ingroups as major components of freshwater faunas, such as representatives of Copepoda or Cladocera, or also crayfish (Astacida) or crabs (Brachyura). Freshwater is a major type of habitat in modern-day ecosystems and was so in the past, at least from the Devonian onwards (about 400 million years ago) ( Gueriau et al., 2016).

Riffle beetle larvae