Walking and take-off in Aphis fabae.

ABSTRACT. A walking aphid ready to emigrate is directed by light, gravity and mechanical stimuli to the upper and outer parts of the plant. There, the new conditions enhance the probability of take-off, as a kinetic response, but the precise moment at which the aphid stops walking and takes flight does not depend on any new external ('releasing') stimulus. The probability of take-off, then, appears to depend partly on substrate stimuli received during walking. Aphids were tested either when suspended by the thorax (and carrying and walking on spheres or paper bands) or when walking on a treadmill with a variable surface structure and aspect relative to gravity. A variety of situations that stimulated faster walking (e.g. a 'cat-walk'), also delayed take-off. Other situations that retarded walking (e.g. being upside-down), similarly delayed taking off, and tended as well to induce probing of the surface. A common feature of these latter situations appeared to be that they elicited traction movements of the legs on the surface: a flexor reflex opposed to the leg extension that regularly precedes a take-off.