Metachronal rhythms and gill movements of the nymph of caenis horaria (Ephemeroptera) in relation to water flow

Abstract

The gills of some Ephemerid nymphs are always motionless, e. g ., many Bætine forms of English streams. In many others, however, the gills move rapidly in metachronal rhythm, by virtue of which currents are created in the water. These currents are peculiar to the species and probably have an adaptational significance. In many forms already under investigation, e. g ., Cholen dipterum. Siphlurus sp. Ecdyonurus venosus, Ephemerella sp. Leptophlebia marginata and Ephemera vulgata , a common feature is noticeable. This is that in their rhythmical movements both members of each pair of gills beat together, i. e ., and their movements are co-phasedly synchronized. Since, therefore, the effect of the gills on one side of the body is exactly duplicated on the other, whatever may be the precise mechanism for the production of currents, the latter are symmetrical with the longitudinal axis of the body (Eastham, 1932). An intersting exception in the nymph of Caenis horaria . In this animal the currents pass over the body from one side to the other. The gills beat in metachronal rhythm down each side of the body, but though the rhythms are synchronous there is a time phase difference between them. In other words, members of a pair are not co-phasedly synchronized in movement. We have thus in Caenis horaria a bi-laterally symmetrical animal producing movements in the surrounding medium which are not of the nature of an axial flow. It is with this phenomenon that this paper deals.