  We suggest several reciprocal swimming mechanisms that lead to a locomotion
only in viscoelastic fluids. The first situation is to have a difference
between the two amplitudes of the oscillatory arm motion for a three-sphere
microswimmer. The second situation is when one of the frequencies of the arm
motion is twice as large as the other one for a three-sphere microswimmer. The
third situation is when the sphere sizes are different for a two-sphere
microswimmer. In all these three cases, the average velocity is proportional to
the imaginary part of the complex shear viscosity of a surrounding viscoelastic
medium. It is essential for a micromachine to break its structural symmetry in
order to swim in a viscoelastic fluid by performing reciprocal body motions.
