part of open tracheal system development tracheal pit formation in open tracheal system GO:0035202 Formation of the tracheal pits, the first tube-like structures to form in the open tracheal system. Once cells are determined to their tracheal cell fate, the tracheal pits arise by invagination of each ectodermal cluster of tracheal placode cells, between 5 and 7 hours after egg laying. An example of this is found in Drosophila melanogaster. biological_process tracheal sac formation tracheal placode invagination anatomical structure formation involved in morphogenesis