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.
tracheal sac formation