Non-invasive in vivo spectroscopy using a monitor calibrator: A case of planarian feeding and digestion statuses

•A monitor calibrator has been applied for non-invasive in vivo spectral measurements.•A simple measurement technique without the need of specialized cuvettes was proposed.•A monitor calibrator enabled quantitative tracking a planarian digestion dynamics.•It was possible to discriminate planaria by their state depending on the type of food.