The Killer Bug was developed by Frank Sawyer (the genius behind the Pheasant Tail Nymph). Frank was the riverkeeper on the Avon, and part of his job was to rid the river of grayling, seen then as an undesirable fish. The Killer Bug (originally called Grayling Killer Bug) was tied to represent a scud, and it was very effective.
Here in the US it is very effective for trout and sunfish - even in waters that don't have scuds.