  Long periodic orbits constitute a serious drawback in Gutzwiller's theory of
chaotic systems, and then it would be desirable that other classical
invariants, not suffering from the same problem, could be used in the
quantization of such systems. In this respect, we demonstrate how a suitable
dynamical analysis of chaotic quantum spectra unveils the fundamental role
played by classical invariant areas related to the stable and unstable
manifolds of short periodic orbits.
