Abstract: 【Aim】To clarify the structure and developmental process of the internal reproductive system of Monochamus alternatus adults and provide a theoretical basis for its reproductive mechanism research and population dynamics monitoring.【Methods】The internal reproductive system of M. alternatus adults at different ages (day-old) was anatomically observed under optical microscope to clarify its structural characteristics and developmental classification criteria, and the developmental process of the internal reproductive system of female and male adults was determined by divison of developmental levels. 【Results】The female internal reproductive system of M. alternatus adults is composed of one pair of ovaries, one pair of lateral oviducts, common oviduct, copulatory pouch, spermathecal duct, spermatheca, spermathecal gland , ovipositor and genital cavity. According to the morphology and color of ovarioles and spermathecal glands, the sexual maturation process of female adults can be divided into the following stages: Transparent stage (stage Ⅰ), primary vitellogenesis stage (stage Ⅱ), secondary vitellogenesis stage (stage Ⅲ) and egg maturation stage (stage Ⅳ). Ovarioles grow continuously during the ovarian development, and the color changes from transparent milky white to yellow. The morphology and color of the spermathecal gland changes from flat and translucent to full and yellow. The male internal reproductive system of M. alternatus adults is composed of one pair of testes, two pairs of accessory glands, one pair of vas deferens, one pair of vesicula seminalis, one pair of ejaculatory ducts, circulator and aedeagus. According to the morphology and color of the testes, vesicula seminalis and accessory glands, the sexual maturation process of male adults can be divided into four stages including transparent stage (stage Ⅰ), initial stage of spermiogenesis (stage Ⅱ), peak period of spermiogenesis (stage Ⅲ) and maturing stage (stage Ⅳ). The testicular long diameter changes little. The color of the testis, vesicula seminalis and circulator changes from milky white and translucent to white. The morphology and color of the accessory gland changes from small and translucent to swollen and transparent. The development of the internal reproductive system of both female and male adults is mainly concentrated at the stages Ⅱ-Ⅲ. In terms of the developmental process of the internal reproductive system, male adults develop more rapidly and their internal reproductive system is already mature at the 12-day-old, while the internal reproductive system of female adults is mature at the 15-day-old. 【Conclusion】 This study has clarified the basic structure and developmental processes of the internal reproductive system of female and male adults of M. alternatus, not only providing a theoretical basis for further research on the reproductive mechanism of M. alternatus, but also providing a reference for predicting the peak ovioposition period of M. alternatus in production practice.

Key words: Monochamus alternatus, internal reproductive system, ovary, testis, developmental grading