Co-ordination of two kinds of flight activity in an aphid.

1. The interaction between 'migratory flight', defined as flight oriented vertically and horizontally towards a large overhead light source, and 'targeted flight' oriented towards a yellow leaf-like object seen to one side against a dark background, has been analysed in Aphis fabae in a laboratory flight chamber. Landings were prevented by regulating a down-current of air so as to balance the flier's rate of climb and by momentary withdrawal of the target when the flier came close. 2. The longer an aphid had flown before the target appeared the more persistently it homed-in on the target and the more its rate of climb was depressed by the presence of the target. 3. This 'priming' of targeted flight during migratory flight could be counteracted by suitably repeated bouts of presentation of the target. Such a treatment tended rather to prime migratory flight responses to the target stimulus, that is avoidance of the target and increase in the rate of climb. 4. The visual stimulus from the target had both excitatory and inhibitory effects on the rate of climb, mixed in varying proportions. Antecedent events in the behavioural chain governed the magnitude and relative timing of these effects. 5. The excitatory effect usually followed an inhibitory one but it could occur first or even alone, and was thus a separate effect not comparable with post-inhibitory rebound.