When I began researching the taxon that was to be the subject of this post, I was surprised to discover that it had weaseled its way onto this site once before. Back in the day, I used an example of the beetle genus Sunorfa to illustrate a post about a closely related genus for which I had been unable to find an image (before a reader pointed me in the direction of one). Sunorfa is a member of that wonderful group of miniature gargoyles, the Pselaphinae (I had the pleasure/pain of sorting a handful of pselaphines at work just the other week; their minute size [usually only a millimetre or two long] makes them a real challenge to work with but their bizarre morphologies make it impossible to resent them). Most species of Sunorfa are found in tropical rainforest litter in southern Asia and Australasia from Sri Lanka to Fiji with the highest diversity of species in New Guinea. In addition, a handful of species are found in the Seychelles. I haven't come across any direct indication of what Sunorfa are doing in all these places but presumably, like other pselaphines, they are predators of even smaller arthropods.
Distinctive features of Sunorfa compared to other pselaphines include a strong transverse sulcus (groove) across the rear part of the pronotum, and a cylindrical abdomen in which the upper tergites and lower sternites are fused into single continuous rings. They also have characteristic foveae (deep depressions) on the top of the head, the base of the elytra including on each side at the 'shoulders', and in the middle of the metasternum (the rear underside section of the thorax) (Chandler 2001). Similar foveae are found in one form or another, in one place or another, on most pselaphines. They have received a lot of attention in taxonomic studies (their appearance and distribution is one of the most reliable features in distinguishing pselaphine taxa) but their function is less well known. Chandler (2001) expressed the opinion that foveae in different parts of the body serve different purposes. Those on the thorax have solitary, sensilla-like setae at their centres and probably represent sensory structures of some kind. Conversely, foveae on the head and abdomen lack such setae and commonly connect to one another internally to form solid tubes. These tubes may function as struts, providing the body with rigidity and strength as the animal is reduced down in size.
Chandler, D. S. 2001. Biology, morphology, and systematics of the ant-like litter beetle genera of Australia (Coleoptera: Staphylinidae: Pselaphinae). Memoirs on Entomology, International 15: 1–560.