insect tracheal primordium
Primordium of a single tracheal metamere of the embryonic/larval tracheal system from its appearance as a placode during stage 11 to the completion of fusion with adjacent tracheal metameres in stage 15/16. There are 10 pairs of tracheal primordia - one pair in each segment from T2 to A8. Each tracheal primordium originates as a slight depression in the lateral ectoderm during stage 10 known as a tracheal placode. During stage 11, these placodes invaginate to form tracheal pits that elongate and branch. These pits close over during stage 13. Fusion of tracheal primordia begins at stage 14 with fusion of the dorsal trunk primordia and is complete by early stage 16.