  FluSI, a fully parallel open source software for pseudo-spectral simulations
of three-dimensional flapping flight in viscous flows, is presented. It is
freely available for non-commercial use under
[https://github.com/pseudospectators/FLUSI]. The computational framework runs
on high performance computers with distributed memory architectures. The
discretization of the three-dimensional incompressible Navier--Stokes equations
is based on a Fourier pseudo-spectral method with adaptive time stepping. The
complex time varying geometry of insects with rigid flapping wings is handled
with the volume penalization method. The modules characterizing the insect
geometry, the flight mechanics and the wing kinematics are described.
Validation tests for different benchmarks illustrate the efficiency and
precision of the approach. Finally, computations of a model insect in the
turbulent regime demonstrate the versatility of the software.
