Why Do Ephemerella Nymphs Scorpion Posture: A "Ghost of Predation Past"?

The behavior of intact Ephemerella infrequens (Ephemeroptera, Ephemerellidae) and Ephemerella with cerci amputated was observed in response to encounters with live predaceous stoneflies (Megarcys signata, Plecoptera: Perlodidae), tethered-live and tethered-model predators, freshly excised stonefly antennae, and thin wires. The incidences of defensive posturing (scorpion posture), movement (crawl, swim, drift) and freezing (no response, stop) behavior were video-taped in response to these predatory stimuli. Scorpion postures occurred with significantly higher frequency when Ephemerella was touched by live and model Megarcys than by Megarcys antennae and wires, with no differences between the two former or the two latter treatments. Frequency of posturing was not altered by prey orientation toward or away from predators or by direction of predator approach with respect to current (upstream, downstream, or beside prey). Thus, hydrodynamic cues associated with live and model Megarcys rather than simple tactile, chemical or visual cues are probably most important in stimulating Ephemerella to scorpion posture. Removal of Ephemerella cerci resulted in a decreased incidence of scorpion posturing, and modification of frequencies of movement and freezing behaviors with predator-prey orientation. These results suggest that cerciless mayflies may have impaired or altered sensory systems. However, cercal removal did not increase Ephemerella's risk of predation, since numbers of intact and cerciless prey consumed during predation trials did not differ. Gut content analysis of Megarcys showed that they were size selective on smaller Ephemerella, which are known to scorpion posture with significantly lower frequency than large Ephemerella. We speculate that the posture is an evolutionary relict, a "ghost of predation past," that is not as effective in smaller individuals, which are also softer-bodied and have smaller spines. Present-day preference for small Ephemerella or other mayfly species over large Ephemerella may be attributed to the historical effectiveness of the scorpion posture, long prey handling times, difficulty in prey capture, or absence of an appropriate behavioral attack stimulus. None of these factors was affected by removal of cerci.