Seasonal variation in wing morphometry of the stable fly Stomoxys calcitrans (Diptera: Muscidae) in Thailand: Climatic effects on phenotypic traits.

The stable fly Stomoxys calcitrans (Linnaeus, 1758) (Diptera: Muscidae) is a globally significant mechanical vector of medical and veterinary importance. Understanding morphological variation offers insight into environmental influences on development, dispersal, and population structure in insects such as the stable fly. This study aimed to investigate how seasonal and climatic variations affect wing size and shape in S. calcitrans, as these traits are closely associated with biological functions including flight performance, host-seeking behavior, and vectorial capacity. Wing size and shape variation were examined across three geographically distinct sites in Thailand using geometric morphometrics, followed by multiple linear regression analysis to evaluate the influence of climatic variables on size, and a two-block partial least squares analysis to assess their influence on shape. Overall, S. calcitrans exhibited significantly larger wing size during the cool season compared to the hot and rainy seasons, particularly in females (p < 0.05); no significant seasonal differences were observed in males based on pooled data (p > 0.05). Regression estimates revealed significant sex-specific responses to climatic variables (p < 0.05): male wing size decreased with increasing temperature and wind speed, whereas female wing size declined with temperature and rainfall but increased with relative humidity. Mahalanobis distance analysis confirmed significant seasonal effects on wing shape in both sexes, with the most pronounced differences occurring between the cool and rainy seasons. Wing shape variability also showed sex-specific climatic associations: in males, no climatic variables were significantly associated with wing shape (p > 0.05), whereas in females, wing shape was associated with wind speed (p < 0.05). These findings demonstrate the wing morphometric responses of S. calcitrans to seasonal and climatic influences, offering important insights into the environmental drivers of phenotypic variation and their potential implications for dispersal, population structure, and vector management strategies.