A high-precision force plate method for long-term mass monitoring of fruit flies under controlled temperature and humidity

Long-term monitoring of body mass in small insects provides critical insights into physiological regulation, metabolism, and behaviour, yet remains technically challenging owing to their extremely low mass and sensitivity to environmental fluctuations. Conventional microbalances and load cells offer high sensitivity but may have limitations in long-term stability or compatibility with controlled environmental conditions. Here, we present an optical, high-precision, force plate-based method that enables continuous, long-term monitoring of body mass in individual fruit flies under controlled temperature and humidity conditions. The system achieves sub-milligram resolution while maintaining mechanical stability and compatibility with environmental chambers, allowing uninterrupted measurements over extended periods. We validate system performance through continuous long-term recordings of individual fruit flies, revealing gradual changes in body mass. This approach provides a versatile platform for precise, long-term mass measurement and integrated physiological-behavioural analysis in insects and other small animals.