Simple Summary: Phortica okadai, a vector of Thelazia callipaeda, relies heavily on its chemosensory system for reproduction and population expansion. However, ultrastructural data across its developmental stages remain uncharacterized. This study employed scanning electron microscopy to observe and describe the ultrastructure of P. okadai across four developmental stages: egg, larva, pupa, and adult. It also focused on characterizing the ultrastructural features, morphometric parameters, and distribution patterns of five sensilla types on the adult head. Sexual dimorphism was observed in the length of Type II chaetica sensilla (ChII), Thin basiconic sensilla (TB), and Large basiconic sensilla (LB). The ultrastructural characteristics of P. okadai during developmental stages, such as eggs and larvae, can be utilized for species identification. Additionally, the focused investigation on the head sensilla morphology of P. okadai provides a foundation for its olfactory system research. Phortica okadai, a vector of Thelazia callipaeda, is associated with an increasing incidence of thelaziasis. The complex habitat and chemosensory system of P. okadai are critical for its proliferation and expansion. However, ultrastructural data across developmental stages remain limited. This study used scanning electron microscopy to examine the ultrastructure of P. okadai developmental stages, with a focus on head sensilla. The results showed that the eggs of P. okadai are dark brown and cylindro-oval. The larvae are vermiform, divided into 11 segments. The pupae are marked by a conspicuous respiratory tubercle, and the posterior spiracle contains three distinct spiracular slits. Among five types of sensilla (trichoid, intermediate, chaetica, coeloconic, and basiconic), coeloconic, intermediate, and trichoid sensilla were predominantly found on the antennae, while basiconic and chaetica sensilla were distributed on both the antennae and the maxillary palps of P. okadai. The analysis revealed that the absence of dorsal appendages on the eggs distinguishes P. okadai from D. melanogaster. Males have longer antennae and exhibit sexual dimorphism in the length of sensilla (ChII, TB, and LB). This study provides the first comprehensive ultrastructural characterization of P. okadai developmental stages and head sensilla, laying a foundation for species identification and olfactory system research. [ABSTRACT FROM AUTHOR]

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