The ability of epithelial cells to divide along their long cell axis, known as "Hertwig's rule", has been proposed to play an important and wide-ranging role in homogenising epithelial cell packing during tissue development and homeostasis. Since the position of the anaphase spindle defines the division plane, how divisions are oriented requires an understanding of the mechanisms that position the mitotic spindle. While many of the molecules required to orient the mitotic spindle have been identified in genetic screens, the mechanisms by which spindles read and align with the long cell axis remain poorly understood. Here, in exploring the dynamics of spindle orientation in mechanically distinct regions of the fly notum, we find that the ability of cells to properly orient their divisions depends both on cortical cues and on local tissue tension. Thus, spindles align with the long cell axis in tissues in which isotropic tension is elevated, but fail to do so in elongated cells within the crowded midline, where tension is low. Importantly, these region-specific differences in spindle behaviour can be reversed by decreasing or increasing the activity of non-muscle Myosin II. In addition, spindles in a tissue experiencing isotropic stress fail to align with the long cell axis if cells are mechanically isolated from their neighbours. These data lead us to propose that isotropic tension is required within an epithelium to provide cells with a mechanically stable substrate upon which localised cortical Dynein can pull on astral microtubules to orient the spindle.

Isotropic myosin-generated tissue tension is required for the dynamic orientation of the mitotic spindle