Fig 1. Schematic representation of the actometric set up.

Each blue rectangle represents an actometric unit (10×5×2cm) arranged over a metal plate. Each unit presented three light barriers, indicated by red asterisks, distributed homogenously in the longitudinal direction in order to detect insect movement. The whole apparatus contained 40 units in total. Transparent acrylic arenas that fit tightly into the individual units were closed with a transparent lid to contain the insects during the assays. Insect movements interrupted the light beams generating signals recorded by a personal computer by means of an ad hoc software.