Key events: segmentation, tracheal pits arise, posterior midgut invagination reaches the posterior pole

Stage 11 lasts for about 2 h (5:20-7:20 h) and is terminated by the first signs of germ band retraction. During this time no major morphogenetic changes take place.

At the onset of stage 11 the segmental furrows that subdivide the germ band into metameric units become apparent. Within thorax and abdomen, segmental boundaries appear as relatively deep folds restricted to ventralmost epidermal levels. Within the prospective head, the gnathal segments – mandible, maxilla and labium – become visible immediately ventral to the cephalic furrow, which becomes shallower and finally disappears. The clypeolabrum and the hypopharyngeal lobe represent rudiments of the head segments.

10 tracheal pits are visible at stage 11. The anteriormost pits open into the boundary between the 1st and 2nd toracic segments and will give rise to the anterior spiracles while the posteriormost pits open into the 8th abdominal segments and will form the posterior spiracles. The remaining pits will grow and their extensions eventually fuse to give rise to the tracheal tree.

The salivary glands arise from two ventral ectodermal primordia located at within the maxillary and labial segment. The pre-gland cells undergo cell shape changes and form the salivary gland placode, where invagination is initiated. Subsequently, more cells are internalized and two tubes are formed that run into a common opening at the ventromedial surface of the epidermis.

The anterior midgut primordium continues to grow posteriorly while the posterior midgut primordium has reached about 15% egg length, where it will bend ventralwards to continue anteriorly. At the boundary between posterior midgut primordium and hindgut two buds form. These are the primordia of the Malpighian tubules.

Cell death is a conspicuous phenomenon in Drosophila embryogenesis that becomes evident in the second half of stage 11, extending throughout most of stage 12. The majority of cell death figures at this stage are located between the epidermis and the nervous system, forming large groups intermingled with macrophages. Macrophages are large, round cells that appear singly in the vicinity of dead cells and will eventually phagocytose all dead cells.