Aim The objective of this research was to clarify the morphological structure of the internal reproductive system in Hyphantria cunea, and characterize its dynamic changes during reproductive processes, providing a theoretical basis for understanding the species' reproductive mechanisms and population monitoring.

Methods The internal reproductive systems of male and female H. cunea adults were dissected and observed under optical microscope at four reproductive stages: pre-mating, post-mating, pre-oviposition, and post-oviposition stages. Structural characteristics were systematically described, and morphological differences were compared across these reproductive states.

Results The internal reproductive system of the female H. cunea adult consisted of a pair of ovaries, a pair of lateral oviducts, a median oviduct, a bursa copulatrix, a spermatheca, and female accessory glands. The male system consisted of a pair of fused testes, a pair of male accessory glands, a pair of vasa deferentia, a pair of seminal vesicles, a pair of duplex ejaculatory ducts, and a simplex ejaculatory duct. The simplex ejaculatory duct is divided into seven segments based on constrictions observed on it. In unmated females, both the bursa copulatrix and spermatheca were empty. After mating, a spermatophore was present in the bursa, and the spermatheca contained seminal fluid. Ovarioles of unmated females were densely packed with oocytes, whereas after oviposition inter-oocytic spaces increased and the proportion of the vitellarium relative to total ovariole length decreased. In unmated males, segments S8 and S7 of the simplex ejaculatory duct were completely filled with secretions, which were depleted after mating.

Conclusion The study comprehensively characterized the structure of the internal reproductive systems in both female and male adults of H. cunea and elucidated their stagge-specific morphological dynamics. These results establish a theoretical foundation for further research into the reproductive biology of H. cunea and provide valuable references for predicting oviposition peaks in forest ecosystems.