The beetles are minor antagonists in the first movie. They chase Fievel through the sewers to eat him, but he loses them by going over a chasm. Many of the black beetles fell into the sewer lake and drowned, some were eaten by what appeared to be a piranha, or some other predatory aquatic animal.
Strangely they are photo-realistic, while in the same movie, Digit is a cartoony cockroach.
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