Aldrovanda plants were taken from the habitats and transferred into petri dishes filled with water. We inspected the plants for closed traps with dark biomass inside. We sampled only traps from the whorls 3–8 counted from the apex at the German site and whorl 2–12 at the Czech sites, to guarantee that compared traps were more or less of the same size. Filled snap-traps were dissected with small forceps and transferred into small glass vials or Eppendorf tubes filled with 70% ethanol at the German site and with 4% formaline at the Czech sites. All traps were dissected from various plants and various whorls to reduce sampling bias. After sampling, the plants were transferred back to the habitats.
Traps were opened using dissection needles and their content was analyzed and recorded with an Olympus SZX 16 stereo microscope and a colorview III camera (Olympus, Tokio, Japan). Species identification was performed according to Scourfield and Harding (1994) , Streble and Krauter (2006) , and Schaefer (2009) .