The Collaborative International Dictionary of English v.0.48:

Scorpion \Scor"pi*on\, n. [F., fr. L. scorpio, scorpius, Gr. ?,
   perhaps akin to E. sharp.]
   1. (Zool.) Any one of numerous species of pulmonate arachnids
      of the order Scorpiones, having a suctorial mouth, large
      claw-bearing palpi, and a caudal sting.
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   Note: Scorpions have a flattened body, and a long, slender
         post-abdomen formed of six movable segments, the last
         of which terminates in a curved venomous sting. The
         venom causes great pain, but is unattended either with
         redness or swelling, except in the axillary or inguinal
         glands, when an extremity is affected. It is seldom if
         ever destructive of life. Scorpions are found widely
         dispersed in the warm climates of both the Old and New
         Worlds.
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   2. (Zool.) The pine or gray lizard (Sceloporus undulatus).
      [Local, U. S.]
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   3. (Zool.) The scorpene.
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   4. (Script.) A painful scourge.
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            My father hath chastised you with whips, but I will
            chastise you with scorpions.          --1 Kings xii.
                                                  11.
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   5. (Astron.) A sign and constellation. See Scorpio.
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   6. (Antiq.) An ancient military engine for hurling stones and
      other missiles.
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   Book scorpion. (Zool.) See under Book.

   False scorpion. (Zool.) See under False, and Book
      scorpion.

   Scorpion bug, or Water scorpion (Zool.) See Nepa.

   Scorpion fly (Zool.), a neuropterous insect of the genus
      Panorpa. See Panorpid.

   Scorpion grass (Bot.), a plant of the genus Myosotis.
      Myosotis palustris is the forget-me-not.

   Scorpion senna (Bot.), a yellow-flowered leguminous shrub
      (Coronilla Emerus) having a slender joined pod, like a
      scorpion's tail. The leaves are said to yield a dye like
      indigo, and to be used sometimes to adulterate senna.

   Scorpion shell (Zool.), any shell of the genus Pteroceras.
      See Pteroceras.

   Scorpion spiders. (Zool.), any one of the Pedipalpi.

   Scorpion's tail (Bot.), any plant of the leguminous genus
      Scorpiurus, herbs with a circinately coiled pod; -- also
      called caterpillar.

   Scorpion's thorn (Bot.), a thorny leguminous plant
      (Genista Scorpius) of Southern Europe.

   The Scorpion's Heart (Astron.), the star Antares in the
      constellation Scorpio.
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The Collaborative International Dictionary of English v.0.48:

Nepa \Ne"pa\, n. [L. nepa scorpion.] (Zool.)
   A genus of aquatic hemipterous insects. The species feed upon
   other insects and are noted for their voracity; -- called
   also scorpion bug and water scorpion.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water scorpion \Wa"ter scor"pi*on\ (Zool.)
   See Nepa.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water scorpion
    n 1: long-legged aquatic insect having the front legs fitted for
         seizing and holding prey and the abdomen extended by a long
         breathing tube