1. Previous evidence that fresh-winged adult aphids become increasingly ready to settle down on a leaf the longer they have flown beforehand, and that this effect is appreciable after less than 1 min. flight, was confirmed with aphids allowed to fly and alight freely without interference.
2. The settling responses initiated after flight were quickly inhibited again by an unsuitable leaf, with reversion to flight, but repeated flights had a cumulative effect.
3. The excitability of the settling responses increased as an after-effect of flights in which the excitability of flight itself (measured by the rate of climb) was not falling and was even increasing.
4. It is concluded that settling is not released by flight but is positively ‘primed’ while it is inhibited by flight, these two activities having been shown to be centrally antagonistic. Settling may be further promoted by some peripheral physiological feedback during long flights when the rate of climb falls, but this is a secondary, merely reinforcing effect.
5. The primary effect is given the name ‘antagonistic induction’. It resembles successive induction or rebound in lower reflex action As a principle of nervous integration at the behavioural level it calls in question previous assumptions concerning internal drives in aphids and other animals.
6. Host plant selection occurred as a result of the sequence of settling responses being broken off at an earlier stage on one kind or age of leaf than another. Most commonly the difference of behaviour appeared only after a first probe, but following very short flights discrimination was sometimes observed without any probing.