Dynamic imaging of swarming by light microscopy. Stills from movies of swarming motile masses of P. vortex. A. Rotating colonies spinning out from a central mass on MH (1.5% w/v) agar. Material between rotating colonies (all are in motion) is highly reflective basal material with a small number of cells. B. Start of detachment of rotating colony from the edge of a swarming culture on MH (1.5% w/v) agar. Black arrows indicate direction of rotation, with the bulk of cells withdrawing (red arrow) to leave colony isolated. C. As (B), 5 min later with further withdrawal of main mass of cells; a few minutes after this the spinning colony moved away in the direction of the blue arrow. Ba = reflective basal material. D. Moving mass of cells on MH (0.3 % w/v) agar; first image with the center of the tip shown as a green dot and the track of subsequent movement shown as the position of the tip one red dot every 20 s. The tip of a moving mass avoids a second moving strand of cells to which it is interconnected. E. From the same image stack as D but taken 10 min later with same (last) position shown in both (D) and (E) as a blue dot.