1. The current-resisting powers of Rhithrogena semicolorata Curtis have been investigated and measured, and compared with those of Ecdyonurus sp.

  2. The part played by the gills as organs of adhesion is considered, and they are found to contribute little to the current-resisting powers of the nymph. The claws are regarded as being of more importance to the animal in clinging than the gills.

  3. The dorso-ventral flattening of the animal is considered to be a creviceseeking adaptation, and not a current-resisting one.

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