backswimmer (Notonecta triguttata)
A species of aquatic insect that swims on its back. The name is usually written in katakana.
I found a backswimmer in the pond.
Usually written in katakana; the kanji form 松藻虫 is rare.
Kanji form is rarely used; the katakana spelling is standard.
The name マツモムシ literally means 'pine seaweed bug', likely from its habitat among aquatic plants. The exact derivation is uncertain.