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Striking regularities are found in the northwestern arm of the M31 galaxy.Star complexes located in this arm are all spaced 1.2 kpc apart and havesimilar sizes of about 0.6 kpc. Within the same arm region Beck et al. (1989)detected a regular magnetic field, and we found that its wavelength is thespacing between the complexes. In this arm, groups of HII regions lie insidestar complexes, which, in turn, are located inside the gas and dust lane. Incontrast, the southwestern arm of M31 splits into a gas and dust lane upstreamand a dense stellar arm downstream, with HII regions located mostly along theboundary between these components of the arm. The stellar density in thesouthwestern arm is much higher than in the northwestern arm, and the former isnot fragmented into star complexes. The age gradient across this arm have beenfound in earlier observations. According the classical SDW theory, thesedrastic differences may be due due to their different pitch angles: about 0degree for northwestern part of the arm and about 30 degree for thesouthwestern segment. Data on M31, M51, M74, and some other galaxies suggestthat star complexes are mostly located in the arm segments that are notaccompanied by a dust lane upstream, i.e. do not host spiral shock wave. Theregularities in the distribution of complexes along a density wave spiral armare most probably due to the development of the Parker-Jeans instability, whichbuilds up star complexes if the initial SFR in the arm is low and, as asequence, magnetic field is regular along the arm.

Spiral arm structures revealed in the M31 galaxy