water bug
US /ˈwɑː.t̬ɚ ˌbʌɡ/
UK /ˈwɑː.t̬ɚ ˌbʌɡ/
1.
any of various insects that live in or on water, especially a large aquatic bug (family Belostomatidae) that can inflict a painful bite
:
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I saw a large water bug swimming in the pond.
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Some people confuse cockroaches with water bugs.