Fig. 1.

Insect terminal tracheal cells ramify in response to hypoxia. (A) A tracheal branch corresponding to one of the Rhodnius abdominal segments was surgically severed (dotted line in the left panel). A few days later, tracheal branches arising from neighbouring abdominal segments send tracheoles to the segment lacking its own branch, compensating for the lack of oxygen in the affected area. (B) A metabolically active ectopic organ (in red) was transplanted into the abdominal segment deprived from its own tracheal branch. The ectopic organ was immediately invaded by numerous tracheal projections, providing oxygen for metabolism of the organ. (C) Rhodnius larvae were placed in hypoxia and tracheal sprouting increased dramatically. (D) Local hypoxia was chemically created at a particular location of the Rhodnius abdomen (area in blue), and, as a consequence, many tracheal projections from nearby tracheal branches invade the hypoxic area. (E) In a Drosophila 3rd instar larva, the FGF homologue Branchless (green) was ectopically expressed in a single cell on the ectoderm. Tracheal terminal extensions projected by tracheal branches in the proximity are attracted towards the Branchless-expressing cell. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)