From Nookipedia, the Animal Crossing wiki
The Violin Beetle (バイオリンムシ, Baiorin mushi) is an insect that appears in Animal Crossing: City Folk and in Animal Crossing: New Leaf. They appear first in June, between 4am and 7pm. They reappear in September and October, from 4 a.m. to 5 p.m., and then disappear in November, when they can be found from 8 a.m. to 5 p.m.. They can be found on tree stumps of regular and cedar trees, and are uncommon.
Donating to the Museum in City Folk
"I've heard this bug got is name because its body resembles a violin... I'm not so sure though. All slick and black and shiny... If you were to ask me, it looks rather more like an awful locust..."
The Violin Beetle can be found on the tree stump, located on the middle level of the insect section.
According to the in-game encyclopedia, "These resemble violins-- hence the name. They are so thin, they fit in wood gaps."
Names in other languages