Joint unit CIPF-Incliva 1 article published in JoVE Biology Optical Cross-Sectional Muscle Area Determination of Drosophila Melanogaster Adult Indirect Flight Muscles Estela Selma-Soriano1,2,3, Rubén Artero1,2,3, Beatriz Llamusi1,2,3 1Department of Genetics and Interdisciplinary Research Structure for Biotechnology and Biomedicine (BIOTECMED), University of Valencia, 2Incliva Health Research Institute, 3Joint unit CIPF-Incliva We report a method to quantify muscle area, which is an indirect method to determine muscle mass in Drosophila adults. We demonstrate the application of our methodology by analyzing the indirect flight muscles in a Drosophila model of Myotonic Dystrophy disease.