Over 97% of the 203 male medflies monitored in a lifetime study of their behaviour exhibited what we term supine behaviour (temporary upside-down orientation) starting an average of 16.1 days prior to their death (mean lifespan of 61.7 days). Supine onset increased the mortality risk by 39.5-fold and a unit increase in supine level increased mortality by 26.3%. The discovery that behavioural traits in insects can be used as biomarkers of their health and to predict their time to death has important implications regarding research on morbidity dynamics, behavioural neuroethology and gerontology, and the interpretation of longevity extension in model organisms.