The Collaborative International Dictionary of English v.0.48:

Mantis \Man"tis\, n. [NL., fr. Gr. ? a prophet.] (Zool.)
   Any one of numerous species of voracious orthopterous insects
   of the genus Mantis, and allied genera. They are predacious
   long-bodied large-eyed insects of warm regions, are
   remarkable for their slender grotesque forms, and for holding
   their stout anterior legs in a manner suggesting hands folded
   in prayer. The common American species is Mantis Carolina.
   [1913 Webster]

   Mantis shrimp. (Zool.) The Squilla, a tropical marine
      burrowing crustacean with large grasping appendages. Also
      called mantis crab and mantis prawn.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

mantis crab \mantis crab\ n.
   Same as Squilla.

   Syn: Mantis shrimp.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mantis crab
    n 1: tropical marine burrowing crustaceans with large grasping
         appendages [syn: mantis shrimp, mantis crab]