It is a commonplace in comparative morphology that the muscles and their nerves give fundamental data, comparable with that given by the skeletal parts. There have been many attempts to allocate the male genitalia of the Lepidoptera to their proper segments and appendages, but so far with very uncertain results. The chitinous parts have been fully studied, and the results summarized from time to time, especially by Pierce in his introductions to the “Genitalia of the British Noctuidae” and “Geometridae.” But the internal parts have been less completely recorded. The embryology is hopelessly distorted by the fact that the whole group of appendages are formed very late from an imaginal bud; the nerves give uncertain evidence, because all of them are branches of a single trunk, passing back from the fused terminal ganglia; the muscles have been only partially and I suspect inaccurately reported, and the two forms cited by Snodgrass in his “Principles of Insect Morphology” are further degenerate types, Bombyx having largely lost the clasper and much of the vinculum, and having no recognizable distinction of juxta and valve; Carpocapsa being less reduced in having preserved the juxta as a separate sclerite, but also completely lacking uncus and clasper.