1. Individual aphids were allowed to fly for set periods, to land on a host leaf, a non-host leaf or a card and make settling responses to it, and then to take off again, without handling, in a laboratory flight-chamber with continuous recording of the rate of climb as a measure of flight activity.
2. The after-effect of a landing was typically a boosting or rebound of flight, which was sometimes accompanied within seconds by depression of it.
3. The boosting effect of landings summated, and the depressing effect waned, through a series of 1 min. flights.
4. Both effects were greater after landings on a leaf than after landings on a card, where the settling responses were weak and flight was therefore inhibited only briefly.
5. Normally the depressing after-effect was greatest with landings on the host leaf, where the settling responses were strongest and flight was inhibited the longest.
6. When flight had been brought up to a highly excited state by previous landings, a landing on the host had little or no depressing after-effect upon fight and now boosted it more than landings on a non-host leaf or card.
7. The strength of the flying aphids' phototactic response in the horizontal (yawing) plane varied with the strength of their photokinetic response measured as the rate of climb.
8. The results effectively precluded the possibility of fight rebound being due to the rest gained while settled; they support the conclusion that both the rebound and the depression of flight following a landing are after-effects of the temporary inhibition of fight by the settling responses.
9. These two after-effects of settling on fight duplicate the reciprocal after-effects of flight on settling described previously as antagonistic induction and antagonistic depression. Both appear to depend on central nervous interaction rather than peripheral feedback.