A ring-like structure observed at the base of the ciliary cap of insect spermatids. This structure may anchor the axoneme to the ciliary cap membrane and/or act as a diffusion barrier, proposed to be analogous to the annulus of mammalian sperm flagellum. [ http://www.ncbi.nlm.nih.gov/pubmed/25447994 http://www.ncbi.nlm.nih.gov/pubmed/4903810 ]