From Benford's Gilbert and Sullivan Lexicon:
During the initial run of H.M.S. Pinafore, Carte clashed with his backers, and the Comedy Opera Company was dissolved –– with hard feelings on both sides. Carte formed a partnership with Gilbert and Sullivan and the triumvirate went on to produce The Pirates of Penzance. Ever since Pinafore, the company had been distressed by American entrepreneurs who had been producing unauthorized Gilbert and Sullivan operas without paying royalties. Carte decided the best way to solve that problem would be to open their new show in New York rather than London. So the trio came to New York and opened The Pirates of Penzance at the Fifth Avenue Theater on December 31, 1879. The opera opened in London’s Opera Comique the following spring and had a run of 363 performances –– considerably less well received than its predecessor but still a great success.
The story line satirizes the Victorian proclivity toward an excessive devotion to duty, even when such devotion leads to ridiculous actions. This illustrates another Gilbertian brand of humor: the ultimate result of carrying a good thing too far. Sullivan’s music is generally described as a parody of Italian opera, nowhere better illustrated than in the famous number where the pirates belt out “With cat-like tread, upon our prey we steal,” as they stomp across the stage.