Spider curls are a great exercise for the often neglected short or "inner" head of the biceps. The big advantage of spider curls over preacher curls is the constant tension, especially at the top portion of the movement.

Dumbbell Spider Curls

There are two slight variations and coaching points to further stimulate and load the muscle to illicit a greater training effect:

1. Make the dumbbell heads touch during the set, and try to push the dumbbells together – while at the same time squeezing and contracting the biceps. Pushing the dumbbells together really helps to enforce this. At the top of the lift, pause for a second before beginning the eccentric or lowering phase of the rep.
2. A great way to extend the set once you fatigue is to change the force angle by moving the elbows further back and shortening the range of movement (see video). This will allow you to get a few extra reps.