Flight dispersal of Triatoma infestans (Klug), the main vector of Chagas disease, is probably the most important mechanism of house reinfestation at a village scale after residual insecticide spraying. Recent experiments demonstrated that some field-collected T. infestans adults may never initiate flight whereas others may do it repeatedly, and that adult age, weight, weight-to-length ratio or chance could not explain much of this heterogeneity. We examined whether bugs that never initiated flight possessed developed flight muscles, and whether flight muscle mass was related to the probability of flight initiation in experimental huts under natural climatic conditions. We found that 22% of 252 male and 9% of 154 female field-collected T. infestans were not able to initiate flight because they had no flight muscles. Among bugs with developed flight muscles, flight initiation probabilities were significantly and positively associated to flight muscles mass relative to total body mass, and negatively associated to weight-to-length ratio according to a repeated-measures logistic regression model. These variables explained the higher flight initiation probabilities observed in females than in males with flight muscles. The flight muscle dimorphism introduces another level of heterogeneity, within and between natural populations, as shown by the pronounced differences found in the frequencies of flight muscle morphs between collection sites in northern Argentina. Identifying the causes of this dimorphism would allow to understand and to predict flight dispersal potential in different bug populations, thus contributing to improved control strategies of T. infestans.