Nanoscale structures in fluid media normally require techniques such as freeze fracture electron microscopy and atomic force microscopy for their visualization. As demonstrated in the present study, the surface modification due to nanoscale clusters occurring intrinsically in nematics made of bent-shaped molecules with either rigid or flexible cores leads to microscale structures, which are visible in an optical microscope. The underlying physical mechanism proposed here involves a quasiperiodic change in anchoring conditions on untreated glass plates for the medium made of islands of clusters surrounded by unclustered molecules. The resulting pattern of stripes outlines the director-normal field around line defects in the well-known schlieren texture. The instability, which is seen over most of the nematic range, with increasing visibility under continued cooling of the sample, sets the nematics made of bent-shaped molecules apart from the classical nematics of rod-shaped molecules.