Black beetle eyes are sold at the apothecary's in Diagon Alley. The rate was five Knuts a scoop the summer before Harry's first year.
Crushed scarab beetles are used in the Wit-Sharpening Potion (GF27).
Rita Skeeter's animagus form is that of a beetle. She is an unregistered animagus and uses the inconspicuous form of an insect to spy in order to get information to use in her career as a writer and newspaper columnist. (GF37)
While not directly indicative of her beetle form, Rita Skeeter's name gives an indication of her connection to insects; "skeeter" is an American slang term for the mosquito. -BB
Fourth years made the Wit-Sharpening Potion, which uses crushed scarab beetles, shortly after the Second Task of the Triwizard Tournament. It's probably too subtle to be intended as foreshadowing the discovery of Rita Skeeter's ability to turn into a beetle or the presence of the Egyptian sphinx in the Third Task, but perhaps these later plot points were inspiration for this ingredient. -BB