ORIENTATION BY HELICAL MOTION .3. MICROORGANISMS CAN ORIENT TO STIMULI BY CHANGING THE DIRECTION OF THEIR ROTATIONAL VELOCITY

Organisms that move along helical trajectories change their net direction of motion largely by changing the direction, with respect to the body of the organism, of their rotational velocity (Crenshaw and Edelstein-Keshet, 1993, Bull. math. Biol. 55, 213-230). This paper demonstrates that an organism orients to a stimulus field, such as a chemical concentration gradient or a ray of light, if the components of its rotational velocity, with respect to the body of the organism, are simple functions of the stimulus intensity encountered by the organism. For example, an organism can orient to a chemical concentration gradient if the rate at which it rotates around its anterior-posterior axis is proportional to the chemical concentration it encounters. Such an orientation can be either positive or negative. Furthermore, it is true taxis-orientation of the axis of helical motion is direct. It is neither a kinesis nor a phobic response-there is no random component to this mechanism of orientation.