To determine the optimal indicators for larval instars and developmental stages and morphologi-cal characteristics of Sepsis thoracica, laboratory rearing methods were adopted, and four morphological pa-rameters, including larval body length, body width, mouth hook length and mouth hook width, were measured. Larval instars were deduced base on Dyar’s rule and frequency histograms. The obtained results were veri-fied using Crosby growth rule and linear regression analysis. Additionally, the morphological changes and recognition characteristics of the insect were systematically observed and recorded. The results showed that the larvae of S. thoracica can be divided into three instars, with the mouth hook length and width serving as the optimal indicators for larval instar identification. The eggs of S. thoracica are oval in shape and are de-posited inside the surface layer of cow dung. The larvae are maggot-shaped, with distinct morphological di-fferences among the 1st to 3rd instars. They possess a pair of heavily ossified mouth hooks and a cephalo-pharyngeal skeleton. The pupae are oval-shaped, and their color gradually darkens over time. The adults are black with metallic luster. Males have specialized forefoot femora and tail organs, while females are slightly larger in size without specialized structures. The study established definitive criteria for classifying larval in-stars of S. thoracica and provided comprehensive morphological descriptions of its developmental stages, of-fering a theoretical foundation for research on growth and development, population control and potential re-source utilization of the insect.