  The local electronic states around a single As (Te, Se) vacancy are
investigated in order to shed light on the role of ligands in a series of
iron-based superconductors. Such a vacancy can produce a local hopping
correction ranging from $-0.22$ eV to 0.12 eV and always induce two in-gap
resonance peaks in the local density of states (LDOS) at the fixed symmetrical
bias voltages, which are rather robust and irrelevant to the phase of
superconducting order parameter. The LDOS images near the defect predominantly
possess $0^o$ and $45^o$ stripes. These energy-dependent charge modulations
created by quasiparticle interference are originated in the nesting effect
between the inner (outer) hole Fermi surface around $\Gamma$ point and the
inner (outer) electron Fermi surface around $M$ point.
