(I) τy, θy after long vowels, diphthongs, and consonants become ς; after short vowels τy, θy become σς (not = ττ cross78), which is simplified to ς.
a. In the above cases τy passed into τς. Thus παντ-yα, παντσα, πανσσα, πάνσα (Cretan, Thessalian), πᾶσα ( cross37 D. 3).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].