Are common and sometimes very puzzling, but they can always be orientated by the aid of the very characteristic striations, on the prism faces, which serve also to distinguish quartz from other minerals of similar appearance.
In many of those ants whose third abdominal segment forms a second " node," the basal dorsal region of the fourth segment is traversed by a large number of very fine transverse striations; over these the sharp hinder edge of the third segment can be scraped to and fro, and the result is a stridulating organ which gives rise to a note of very high pitch.
Trans., 1802) in his usual summary fashion gave a general explanation of these colours, including the law of sines, the striations being supposed to be straight, parallel and equidistant.
The Pentatomidae, bristle-bearing tubercles on the legs are scraped across a set of fine striations on the abdominal sterna.
Twinned crystals are not common, but the presence of polysynthetic twinning is sometimes shown by fine striations running diagonally or obliquely across the cleavage surfaces.