Star-forming regions that are visible at 3.6 \mu m and H \alpha but not in the u , g , r , i , z bands of the Sloan Digital Sky survey ( SDSS ) , are measured in five nearby spiral galaxies to find extinctions averaging \sim 3.8 mag and stellar masses averaging \sim 5 \times 10 ^ { 4 } M _ { \odot } . These regions are apparently young star complexes embedded in dark filamentary shock fronts connected with spiral arms . The associated cloud masses are \sim 10 ^ { 7 } M _ { \odot } . The conditions required to make such complexes are explored , including gravitational instabilities in spiral shocked gas and compression of incident clouds . We find that instabilities are too slow for a complete collapse of the observed spiral filaments , but they could lead to star formation in the denser parts . Compression of incident clouds can produce a faster collapse but has difficulty explaining the semi-regular spacing of some regions along the arms . If gravitational instabilities are involved , then the condensations have the local Jeans mass . Also in this case , the near-simultaneous appearance of equally spaced complexes suggests that the dust lanes , and perhaps the arms too , are relatively young .