The subfamily Acidocerinae

I haven't got to know acidocerines well yet, but there is something intriguing about them, something that calls you to dig deeper in order to get to understand why are they so different (at genus level, at their ecologies and biologies) and yet so (almost frustratingly) similar (at species level- at their external morphology at least).

According to Short & Fikáček (2013), the Acidocerinae are "relatively uniform and difficult to characterize", it is probably due to the fact that members of the subfamily exhibit "an extremely broad range of habitats: many occupy ‘typical’ aquatic habitats such as ponds, marshes, forest pools and stream margins. Some (Agraphydrus, Dieroxenus) are known to occupy hygropetric habitats. Quadriops has never been taken in an aquatic situation and one series was taken from a sap flow in Costa Rica", because as it is expected, different ecologies, may require variations on the structures, which is possibly the reason why you can find so many characters converging along the whole phylogeny.

The subfamily is currently composed by 14 genera and 291 described species:
*In parentheses the number of species per genus

Acidocerus Klug (1)
Agraphydrus Régimbart (37)
Chasmogenus Sharp (43)
Dieroxenus Spangler (1)
Globulosis García (1)
Helobata Bergroth (13)
Helochares Mulsant (180)
Helopeltarium d’Orchymont (1)
Horelophopsis Hansen (2)
Peltochares Régimbart (1)
Quadriops Hansen (6)
Radicitus Short & García (3)
Tobochares Short & García (4)
Troglochares Spangler (1)

In the molecular phylogeny presented by Short & Fikáček (2013), Acidocerinae is the sister clade to the 'Sphaeridiinae lineage' (Rygmodinae+Sphaeridiinae), which is the 'mostly terrestrial group' of hydrophilids. This relationship is strongly supported by the molecular data, but there are currently no morphological synapomorphies for the clade.

(Slightly) Modified from Short & Fikáček (2013).

One of my challenges as an aquatic beetle person would be to find morphological traits that lead to support or question those molecular data.


References

• Hansen, M. (1991). The hydrophiloid beetles. Phylogeny, classification and a revision of the genera (Coleoptera: Hydrophilidae). Biologiske Skrifter, 40, 1–367.

• Short, A. E., & Fikáček, M. (2011). World catalogue of the Hydrophiloidea (Coleoptera): additions and corrections II (2006–2010). Acta Entomologica Musei Nationalis Pragae, 51(1), 83-122.
• Short, A. E. Z., & Fikáček, M. (2013). Molecular phylogeny, evolution and classification of the Hydrophilidae (Coleoptera). Systematic Entomology, 38(4), 723-752.