Okay, so the other night, I caught a dung beetle and finished my bug collection. Unfortunately, when I donated it, it seems I forgot to donate a bug from the past. I checked in the museum and, sure enough, there's a bare tree in the room the left of the main room.
I can't show a picture, but it's an empty tree second to last on the right of the room, the one with the tarantula and scorpion. What bug or bugs is/are missing?
My best guess would be the Robust Cicada. If you read the sign the order of bugs should be: robust cicada, walker cicada, evening cicada, cicada shell, cyclommatus stag, horned elephant, spider, tarantula & scorpion. You could also check the board outside of the museum and compare it to your encyclopedia bug tab.