Mushinuyun

Socially-awkward Uehara has wanted to be an entomologist since he was a kid, and he spent all his time chasing that dream. When he failed, he was forced to come back to his home island, where his mother was completely unsympathetic to his problems. He arrived to his home island along with many other tourists, who were there to observe an unusual astronomical event. Afterward, Uehara encountered a strange bug whose bite transformed his body (including his penis). What is happening to Uehara, and what will become of his life?

(Source: MangaHelpers)