1. A bout of settling activity on a leaf or other surface has both excitatory and inhibitory effects upon subsequent flight activity (antagonistic induction and depression of flight by settling); flight has similar after-effects upon settling; and the changing balance between these reciprocal after-effects determines the general course of behaviour.
2. The strength and duration of settling responses made on landing by free-flying aphids were regulated by jolting them off the surface into flight again prematurely. This treatment diminished but did not otherwise appear to distort the after-effects of a landing on flight and was used to establish the following points.
3. The balance between the excitatory and inhibitory after-effects of any one landing on flight depends (i) on the excitatory states of both flight and settling before the landing occurs, and (ii) on the strength of the settling stimuli received from the surface after landing.
4. The after-effects of settling on flight are predominantly excitatory when flight is already strong relative to settling but predominantly inhibitory when flight is relatively weak; the after-effects of strong settling are consistently greater, both ways, than those of weak settling.
5. Landings on different surfaces known to provide weak and strong settling stimuli still produce their typically different after-effects on flight when the settling responses actually performed on the two surfaces are kept equal.
6. The after-effects of settling on flight evidently do not require any peripheral feed-back (although this can add to the after-effects), and result from central crossinhibition of flight when settling is centrally excited by external stimuli.