The Collaborative International Dictionary of English v.0.48:

backswimmer \back"swim`mer\ n.
   any of numerous predaceous aquatic insects of the family
   Notonectidae (such as Notonecta undulata) that swim on
   their backs and may inflict painful bites; -- also called
   boat bug. --[RHUD]
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

backswimmer
    n 1: predaceous aquatic insect that swims on its back and may
         inflict painful bites [syn: backswimmer, Notonecta
         undulata]