Fig. 8. Vortex production during continuous swimming in the pupa. The solid line traces the path of the tip of the abdominal paddle. Numbers 1–8 indicate half-strokes, and the jets of water associated with them are shown as stippled regions outlined by dashed lines. The open-headed arrows show the direction of the jets, which are orientated at ±26° from dead aft of the swimming path. The jets were traced directly from a video sequence. Each jet appeared in the second half of the stroke as the abdominal tip began to move forward relative to the water. The jets were usually only visible for approximately 50ms, but in this case between the fifth and sixth half-strokes the insect turns upwards; this was accompanied by the release of a persistent jet on the outside of the bend. This jet, labelled number 6 in the drawing, is crossed by a forwardly directed ‘stopping jet’ associated with the change in direction.