Velocity thresholds for registration of probes. An example of how aphid feeding behaviour was measured using a resolution of 275 pixels per mm2. Subject states can be defined as ‘moving’ or ‘not moving’ by means of two thresholds: the start velocity at which the subject begins to move, and the stop velocity at which the state changes from moving to not moving. Probe starts were recorded if the velocity of the aphid’s centre point dropped below 0.02 mm/s for at least 10 seconds (α). Probe stops were recorded if the velocity of M. persicae aphids exceeded 0. 3 mm/s for at least 2 seconds (β), or 0.35 mm/s for at least 2 seconds in the case of winged N. ribisnigri aphids. To avoid premature probe endings due to short movements during probing (event γ), probe stops were only recorded when velocity increased above 0. 1 mm/s for more than 2 seconds. Figure adjusted from the EthoVision XT Reference Manual (version 8) .