Feeding of two species of Scydmaeninae "hole scrapers", Cephennium majus and C. ruthenum (Coleoptera: Staphylinidae), on oribatid mites

Abstract

Prey preferences and feeding-related behaviour of two Central European speci es of Scydmaeninae, Cephennium majus and Cephennium ruthenum, were studied under laboratory conditions. These beetles capture mites by using unique premental suckers and then penetrate the prey's cuticle by slowly scraping a hole with their short mandibles. A mechanism for interlocking the sides of beetle's labrum with closed mandibles was discovered, confirming that the predator's mouthparts can be tightly sealed around the penetration site, which enables them to inject digestive juices and ingest liquefied tissues of the prey through a tiny hole. The feeding behaviour of Cephennium can be divided into three phases: (i) attack and stabilization of the attachment site (4-5 min); (ii) penetration of the mite's cuticle (ca. 40 min); and (iii) feeding (several hours). Results of prey choice experiments using over 1200 identified mites belonging to 23 families of Oribatida (55 species) and 6 families of Mesostigmata (9 species) demonstrated that C. majus feeds mostly on members of the Oribatida: Phthiracaridae (72.07% of offered phthiracarids, 26.7% of eaten oribatids), Ceratozetidae (100% and 24.7%, respectively), Achipteriidae (95.59% and 21.7%) and Liacaridae (61.97% and 14.7%); C. ruthenum fed mostly on Phthiracaridae (48.68% and 41.6%), Achipteriidae (37.29% and 24.7%) and Ceratozetidae (82.35% and 15.7%). The entire feeding behaviour from attack to completion of feeding in C. majus took 2.00-10.37 h when eating oribatids ranging in body length from 0.34 to 0.70 mm. Interestingly, it took much longer for C. majus to kill ptyctimous Phthiracarus, body length 0.39-0.45 mm, than any non-ptyctimous oribatids, including the much larger (0.64-0.70 mm) Chamobates subglobulus. The two species of Cephennium differed greatly in their preference for eating Liacaridae due to their large size, which was acceptable for the larger C. majus, but at the edge of acceptability for the smaller C. ruthenum. Comparative analysis of mites eaten by all the studied species of Scydmaeninae resulted in a preliminary identification of morphological types of Oribatida acceptable and non-acceptable for ant-like stone beetles. The most readily accepted oribatids are typically subglobose, with a rigid cuticle, smooth or only shallowly and finely microreticulate surface of the idiosoma, covered with sparse setae or nearly asetose and with short or moderately long legs. In contrast, rejected mites have one or a combination of the following characters: body flattened, not subglobose; the cuticle relatively soft; idiosoma deeply sculptured or coarsely reticulate; legs long and spiny; the body covered with a protective crust of soil particles. Crotonioidea, Carabodidae, Damaeidae and Hermanniellidae seem to be particularly well-protected against scydmaenine predators.