Mix butter, parsley, lemon juice and tarragon.
Split Langoustine tails through top shell being careful not to cut through softer undershell.
Spread Langoustine tails so meat is on top.
In 2-quart oblong baking dish, arrange Langoustine tails; brush each with butter mixture.
TO HEAT BY TIME Cover with wax paper.
Set Power Select at MEDIUM for 11 minutes.
TO HEAT BY INSTA-MATIC Cover completely with 2 pieces of plastic wrap; heat on COOK 7.
Let stand, uncovered, 3 minutes before serving.
Serve with remaining butter sauce.