The Collaborative International Dictionary of English v.0.48:

Dynastidan \Dy*nas"ti*dan\, n. [Gr. ?, fem. of ?. See Dynast.
   The name alludes to the immense size of some species.]
   (Zool.)
   One of a group of gigantic, horned beetles, including
   Dynastus Neptunus, and the Hercules beetle (Dynastus
   Hercules) of tropical America, which grow to be six inches
   in length.
   [1913 Webster]