part of Malpighian tubule Malpighian tubule segment fly_anatomy.ontology Segment of the Malpighian tubule. Each tubule can be divided into distinct segments based on differences in appearance, cell structure and function. The initial segment is involved in calcium homeostasis and excretion, the transitional and main segment are secretory and the lower tubule and ureter are reabsorptive. Type I cells are found in all segments, but Type II cells are only found in the upper tubule (initial, transitional and main segments). FBbt:00005789 epithelial tube