Gintama: Season 2, Episode 16

2 - 16: Rhinoceros Beetles Teach Boys that Life is Precious

Kagura's beetle was killed during a beetle fighting match. She tries to drag Gintoki and Shinpachi to go beetle hunting with her. That only succeeds when they know there is a high demand for the beetles - meaning they can make money of it. The trio encounters the Shinsengumi who also happens to be capturing beetles for reason unknown.