Morphology of the trachea
The
tracheal system of Drosophila is a highly branched tubular system
formed by an ectodermal monolayer epithelium. The trachea are lined with
a characteristic cuticule, which formes minute spiral ridges, the taeniola.
Two large longitudinal trachea, the lateral truncs, span at the dorsal side
of the larva from the anterior to the posterior spiracles. Two crossbranches
connect the lateral trunks anteriorly in the 3rd thoracic segment and posteriorly
in the 8th adnominal segment. The spiracels open through the cuticule and
allow entry of gases. The anterior spiracles remain closed until to the
third larval instar. This may be an adaption to the life conditions: the
anterior half of the Drosophila larva is constantly immersed in the
food medium. 
There
are stereotypic branches from the lateral trunks in the 3rd thoracic and
in all abdominal segments. These branches in turn produce finer intrasegmental
branches. The terminal branches are very thin cytoplasmic extensions which
connect in many tissues almost every single cell. Although the branching
pattern is similar in all three larval instars, it becomes more complex
as thetime progresses.
Media list
Morphology of the trachea, schematic
Morphology of the trachea, in vivo