1. Flight has a cumulative ‘priming’ effect (excitatory after-effect) on the settling responses of an aphid landing on a surface, even when the flight has been interrupted by one or more previous landings. But when the bouts of flying between landings were kept brief (1 min.) there was also an opposite, depressing or inhibitory after-effect on settling which reduced and in certain conditions regularly outweighed the priming effect.
2. Which of the two after-effects predominated depended on the kind of surface landed on each time, on the strength of the settling responses at previous landings and on the total time the aphid had been flying.
3. The depressing effect as measured on a standard host leaf was negligible when the flight had been interrupted by landings on a card, where the settling responses were weakest and soonest inhibited by flight again, greater after landings on a non-host leaf and greatest after landings on the host leaf, itself where settling responses were strongest.
4. When two successive landings were both made on the same surface, a depression of settling at the second landing was more likely after a strong previous settling response than after a weak one; it was also more likely when the two landings were made on a non-host than when they were made on a host.
5. Flight and settling are nervous antagonists and the terms ‘antagonistic induction’ and ‘antagonistic depression’ are applied, respectively, to the excitatory and the inhibitory after-effects of flight on settling. These are components in the co-ordination of flight and settling as successive activities and do not necessarily describe the sequences observed.