This post gives the details on the leather version of the Trouser Worm. This is essentially the same fly as the foam version except it uses the leather from a rabbit strip as the tail instead of foam.
The leather has some advantages over the foam. While the action on the foam version is stunning I suspect it may occasionally be over-the-top for Carp in certain moods. The leather still has great action but it is slightly more subtle. Using leather simplifies the process and there is something to be said for natural materials.
There are also some disadvantages. If you are looking for a full-on headstand all the time then the foam can't be beat. The leather loses a little buoyancy and drops the full headstand after it is fished for a while. Fortunately It still falls, rides and lays hook-up and the leather usually sits slightly off the bottom. This is probably good enough for most situations.
(Hard to see the fly....he took it deep!)