Violin beetle

The violin beetle (バイオリンムシ, Baiorin mushi) is an insect that appears in and in. They appear first in June, between 4am and 7pm. They reappear September and October, from 4 a.m. to 5 p.m., and then disappear in November, when they can be found 8 a.m. to 5 p.m.. They can be found on tree stumps of regular and cedar trees. They are uncommon.

Donating to the Museum in City Folk
Upon donating to the Museum, Blathers will say this:

"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.

Further information
According to the in-game encyclopedia, "These resemble violins-- hence the name. They are so thin, they fit in wood gaps."