PROC. BIOL. SOC. WASH. 92(4), 1979, pp. 743-752 DESCRIPTION OF THE LARVA AND PUPA OF CYLORYGMUS LINEATOPUNCTATUS (COLEOPTERA: HYDROPHILIDAE: RYGMODINI) Paul J. Spangler Abstract. — The larva, pupa, habitat, and distribution of the Chilean water beetle, Cylorygmus lineatopunctatus d'Orchymont, are described and illus-trated. The larva and pupa are interpolated into previously published keys to the immature stages of hydrophilid beetles. In late January and through February, 1978, I was able to participate in entomological fieldwork in Chile through the combined auspices of the J. I. Molina Institute of Studies and Publications of Chile and the Smithsonian Institution. During that cooperative project I concentrated my efforts on collecting aquatic Coleoptera, and I was pleasantly surprised to find adults and the larval and pupal stages of the rare rygmodine hydrophilid beetle, Cylorygmus lineatopunctatus d'Orchymont. As mentioned previously (Spangler, 1974), C. lineatopunctatus is the only representative of the tribe Rygmodini known from South America. All other taxa of the Rygmodini, ca 18 genera and ca 53 species, have been described from New Zealand, the subantarctic islands of New Zealand, or from Australia. Larvae of two rygmodine genera, Thomosis and Namostygnus, were described recently by Ordish (1974), therefore, the following description adds a third larval description to our knowledge of the Rygmodini. The description of the pupa of Cylorygmus given below is the first for any of the rygmodine genera. In addition to describing the larva and pupa of C. lineatopunctatus, I am taking this opportunity to summarize and comment on new information on the bionomics and distribution of the species. The monotypic genus Cylorygmus was described by d'Orchymont (1933) from a single specimen which he found among hydrophihds in the Hamburg Museum. The type-species of the genus, C. lineatopunctatus d'Orchymont, was a female collected in 1896. Since d'Orchymont's description of the genus and species in 1933, the species was referred to in the literature only five times as follows. Blackwelder (1944) cited C. lineatopunctatus in his checklist of Latin American Coleoptera. Moroni (1973) included it in his list of water beetles known from Chile. Spangler (1974) designated a neotype for the type which was destroyed when the Hamburg Museum was damaged during World War II, illustrated the neotype and distinguishing characters.