The nature of the two-dimensional (2D) to three-dimensional (3D) morphological transition in the highly strained epitaxy of InAs on GaAs(001) is discussed, based on irt situ scanning tunneling microscope and atomic force microscope studies, combined with photoluminescence (PL) and PL excitation spectroscopy results. A re-entrant 2D-3D morphology change is observed, in which quasi-3D (Q3D) clusters appear, disappear, and reappear well in advance of the formation of 3D islands. We suggest that the Q3D clusters may act as a kinetic pathway to 3D island formation, spreading out the 2D-3D transition over a delivery range of similar to 0.3 monolayers. Large (> similar to 50 nm wide) 2D clusters sitting on top of the wetting layer (WL) undergo morphological changes with increasing strain and ultimately lose their material to 3D islands. Small (<20 nm) 2D clusters decorating the WL appear to contribute to the commonly observed redshift of the InAs WL PL peak prior to 3D island formation. A diffusion-limited formation of 3D islands for our growth conditions is indicated by the behavior of the 3D island density as a function of growth rate.