Abstract: The objective of this experimental investigation is the volumetric visualization of the near wake topology of the vortex structures generated by a flapping-wing micro air vehicle. To achieve the required visualization domain (which in the present experiments amounts to a size of 60,000 cm3), use is made of robotic particle image velocimetry, which implements coaxial illumination and imaging in combination with the use of helium-filled soap bubbles as tracer particles. Particle trajectories are determined via Lagrangian particle tracking and information of different phases throughout the flapping cycle is obtained by means of a phase-averaging procedure applied to the particle tracks. Experiments have been performed at different settings (flow speed, flapping frequency, and body angle) that are representative of actual flight conditions, and the effect of reduced frequency on the wake topology is investigated. Furthermore, experiments have been carried out in both tethered and free-flight conditions, allowing an unprecedented comparison between the aerodynamics of the two conditions. Graphic abstract: [Figure not available: see fulltext.].