  Arguments are given that support the interpretation of the lattice QCD
glueball and gluelump spectra in terms of bound states of massless constituent
gluons with helicity-1. In this scheme, the mass hierarchy of the currently
known gluelumps and glueballs is mainly due to the number of constituent gluons
and can be understood within a simple flux tube model. It is also argued that
the lattice QCD $0^{+-}$ glueball should be seen as a four-gluon bound state.
The flux tube model allows for a parameter-free computation of its mass, which
is in good agreement with lattice QCD.
