1. Movements of the legs during the swimming of adult Dytiscus marginalis and Hydropphilus piceus have been analysed from films. In both beetles the middle and hind pairs of legs play an active part and the front pair is held stationary beneath the thorax.
2. The two legs of a segment are retracted simultaneously in Dytiscus, the two limb pairs acting alternately. In Hydrophilus retraction of a hindleg is simultaneous with that of the contralateral middle leg. Movement is rectilinear in both insects, but the head of Hydrophilus rotates a little to the side opposite that on which the hindleg is retracting.
3. Following amputation of a single hindleg, Dytiscus swims along a relatively straight path as a result of modifications in the movements of the two middle legs and the remaining hindleg. Contrary to the description of Bethe & Woitas, the hindleg continues to alternate with the contralateral middle leg, the only change in rhythm being a delay in the retraction of the other middle leg. Some of the changes in action of these legs are similar to those used in turning of the normal insect and those which produce circling in unilaterally blinded insects.
4. Forced rotation of Dytiscus elicits compensatory movements which tend to produce circling in the opposite direction. The eyes, antennae and legs are the sites of receptors whose asymmetric stimulation produces this response. It is suggested that a similar asymmetry in the inflow from these sense organs is responsible for the modified movements following limb amputations.
5. In the swimming of nymphs of Anax the legs retract slightly before shortening of the abdomen takes place. Although all the segments take part, a large proportion of this shortening occurs in segments 6-8 which contain the branchial chamber. Water is ejected through a small aperture during this contraction but when water is drawn in the anal valves are open wide.
6. The pressure in the branchial chamber rises to about 30 cm. water within 0.03 sec. and the reaction from the jet enables the animal to attain speeds of 30-50 cm./sec. within the first centimetre of propulsion. The duration of this pressure and of the impulse on the animal correspond with the time during which the abdomen contracts longitudinally.