We previously described a remarkable type of cell motility that provided direct, visual evidence for the fluid nature of cell membranes. The movement involved continual, unidirectional rotation of one part of a protozoan, including the plasma membrane and cytoplasmic organelles, in relation to a neighbouring part. The cell membrane in the ‘shear zone’ appeared continuous with that of the rest of the cell. The rotary motor consisted, at least in part, of a non-contractile, microtubular axostyle which extended centrally through the cell. The protozoan was a devescovinid flagellate found in the hindgut of a Florida termite. In this paper, we have confirmed earlier reports of this type of motility in other kinds of devescovinids from Australian termites. By demonstrating continuity of the plasma membrane in the shear zone of the Australian devescovinids as well, we have obtained additional examples that provide direct, visual evidence for fluid membranes. A comparative analysis of rotational motility in various devescovinids revealed 2 different kinds of rotary mechanisms. Hyperdevescovina probably have an internal motor, in which one part of the cell exerts forces against another part, as in the Florida termite devescovinid. Devescovina species, on the other hand, have an external motor, in which flagellar and/or papillar movements exert forces against the surrounding medium. The structure of the axostyle in different devescovinids was compared, and its role in rotational motility discussed with respect to the behavioural data.

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