1. In experiments with apterous adult aphids walking on non-plant surfaces, rostrum protraction and probing of the surface were elicited by a variety of situations having the common effect of interfering mechanically with the normal pattern of locomotory movements.
2. A uniform wax ridge which fitted the span of the aphid's legs but afforded a poor grip to the tarsi and broke up the stepping rhythm, combined with an over head light, generated a cyclical alternation between walking and probing.
3. When the uniform substrate was interrupted by some form of obstacle to locomotion further probes were elicited, and the more abruptly and completely locomotion was arrested, the more promptly these appeared.
4. Visual stimulation (from black models of aphid size) and olfactory stimulation (from the aphids' siphuncle wax) also caused walking aphids to stop and probe.
5. Readiness to probe varied greatly with age and pretreatment. Aphids denied access to a host plant for 1-2 days probed more frequently at random in response to the substrate but less frequently at objects encountered.
6. Walking and probing interact in the manner of antagonistic reflex systems, and the diverse types of stimulus which elicited probing may all have done so indirectly by first inhibiting locomotion.