  In this work, we investigate the influence of the anisotropic heating on the
spontaneous instability and evolution of thin Harris-type collisionless current
sheets, embedded in antiparallel magnetic fields. In particular, we explore the
influence of the macroparticle shape-function using a 2D version of the PIC
code ACRONYM. We also investigate the role of the numerical collisionality due
to the finite number of macroparticles in PIC codes. It is shown that it is
appropriate to choose higher order shape functions of the macroparticles
compared to a larger number of macroparticles per cell. This allows to estimate
better the anisotropic electron heating due to the collisions of macroparticles
in a PIC code. Temperature anisotropies can stabilize the tearing mode
instability and trigger additional current sheet instabilities. We found a good
agreement between the analytically derived threshold for the stabilization of
the anisotropic tearing mode and other instabilities, either spontaneously
developing or initially triggered ones. Numerical effects causing anisotropic
heating at electron time scales, become especially important for higher mass
ratios (above $m_i/m_e = 180$). If numerical effects are carefully taken into
account, one can recover the theoretical estimated linear growth rates of the
tearing instability of thin isotropic collisionless current sheets, also for
higher mass ratios.
