Studies of testicular development of Heliothis virescens (F.) revealed that the paired larval testes fuse into a single testis in the prepupal stage. An epithelial tissue encapsulates and connects each larval testis to the body wall. This tissue is lost before formation of the fused testis. The testicular sheath consists of 4 additional membranes, the “tunica interna,” the “membrana communis,” the “tunica externa,” and the “capsula lobuli.” Dorsoventral twisting of the testis occurs in the prepupal stage after fusion of the testis, causing the follicles to rotate up to 845°. There is a tremendous increase in testicular volume during larval development. Maximum size of the testes is reached when prepupal larvae attain a body length of 26 mm. Volume of the testes progressively decreases in pupal and adult stages.