Mortality of adult males of melon flies, Bactrocera cucurbitae, fed sucrose and protein was studied in the present study. Flies fed only sucrose plus protein (optimal food) or only sugar from the day of adult eclosion shows long survivorship, or no significant mortality, in the study period. However, flies fed only protein from the day of eclosion or no food shows rapid change of survivorship from 100% to 1.7% and 8.4% respectively within four days. At the same time, flies diet changed from optimal diet to protein or optimal diet to no food on 7^th day results survivorship of only 0% and 6.6%, and on 11^th day survivorship of 0% in only 96 hours. However, no significant mortality of adults was observed when flies were shifted from the optimal diet to sugar-only on 7^th and 11^th day onwards. Therefore, the experiment demonstrates that flies have an absolute requirement of carbohydrate in the adult diet. It also indicates that flies have limited capability to convert lipids to metabolic energy.