Recent experiments demonstrated that most field-collected Triatoma infestans (Klug) (Hemiptera: Reduviidae) adults from northern Argentina either never initiated flight or did so repeatedly in both sexes. This pattern could not be explained by sex, adult age, weight, weight-to-length ratio (W/L), or chance. We examined whether bugs that never initiated flight possessed developed flight muscles, and whether flight muscle mass relative to total body mass (FMR) was related to the probability of flight initiation. Approximately half of the adults that never initiated flight had no flight muscles. The absence of flight muscles was 2.4 times more frequent in males than females. Females had significantly larger flight muscle mass than males. For both sexes, the frequency of bugs with no flight muscles was spatially heterogeneous among individual collection sites. A logistic regression model of flight initiation that included both FMR and W/L provided a better fit than models including either one of these predictors. FMR is a novel predictor of flight initiation in Triatominae, with a stronger effect than W/L. The higher frequency of females initiating flight in our experiments may be explained by females having flight muscles more frequently than males, and having FMR and W/L values more suitable for flying. These findings demonstrate that individuals and natural populations of T. infestans can differ dramatically with regard to flight initiation.