Ultrastructure of mouthpart and terminalia in Musca crassirostris (Diptera: Muscidae) with molecular confirmation from Thailand.

Musca crassirostris Stein, 1903 (Diptera: Muscidae) is a medically and veterinary important hematophagous fly whose feeding activity causes significant irritation and blood loss in livestock and is occasionally associated with human myiasis. Although this species is widely distributed and epidemiologically relevant, its feeding-related functional morphology and genital diagnostic characters have remained insufficiently documented at the ultrastructural level. Here, we present the first comprehensive scanning electron microscopy (SEM)-based characterization of the labellar feeding apparatus and male and female terminalia of M. crassirostris, integrating stereomicroscopy and mitochondrial cytochrome c oxidase subunit I (COI) DNA barcoding for species confirmation. SEM images revealed well-developed prestomal teeth and serrated pseudotracheal structures, features directly associated with hematophagous feeding. Distinctive genital characters are also present and facilitate accurate species identification. The COI barcoding confirmed species identity with complete concordance among BOLD, GenBank, and species delimitation analyses, providing new reference data for Southeast Asian populations. By linking ultrastructural morphology with feeding function and diagnostic utility, this study establishes a robust morphological-molecular framework that advances current understanding of M. crassirostris and supports its identification and surveillance in medical and veterinary entomology.