Can't help with the entomology Dave, butI can tell you a bit about the etymology. It derives from one of the Germanic languages: schijten, pronounced "shiten" (and you thought it was slang, didn't you?) The past participle is schitten, whence our modern "shit"; this ("shit") is also the correct past particple, although you could make a reasonable case for saying "shitted". "Shat" is incorrect.

Steve