Figure 3.

Transverse (horizontal) section through gastral segments 1–4, photographed using SEM. Fig. a, a cross section through the crop (C) and a transparent membrane of the air sac (TM). Tracheal ducts (TD) are visible. Bar = 1 mm. Fig. b, a section through an air sac (AS) and one trachea extending almost across the entire picture (TR) is visible. Bar = 1 mm. Fig. c, the morphology of the endothelium (EN); Bar = 1 μm. Fig. d, the intima of an air sac. Bar = 1 μm. Fig. e, the outside of the air sacs with numerous thick tracheae. Bar = 100 μm. Fig. f, numerous tracheae are seen at the end of the air sacs. Bar = 10 μm.