insect tracheal pit
Deep and narrow pit formed by the invagination of a tracheal placode in a late extended germ band embryo. Invagination of the placode begins during stage 11. Branches begin to bud from the pit during stage 11, with a dorsal and ventral stem being apparent by mid-stage 12. The pit closes over during stage 13. There are 10 pairs of tracheal pits - one pair in each tracheal metamere (each segment from T2 to A8). Pits are located in the central one third of each parasegment, with the most anterior in parasegment 4 and the most posterior in parasegment 14.