426 C. GORDON HEWITT. 



basal portion of the abdomen. They give off internally a 

 largenuraber of trachea3 whicli ramify among tlie viscera and 

 provide a large portion of the contents of the abdomen with 

 air. 



From the anterior side of the anterior thoracic spiracle a 

 flattened sac arises. On its ventral side this gives off a 

 branch which supplies the muscles of the neck and the ante- 

 rior leg. The sac then narrows into a rather thick- walled 

 cervical tracheal dnct {c.tr.), which passes through the neck 

 alongside the cephalo-thoracic nerve-cord and enters the head. 



Tracheal Sacs of the Head. — The tracheal sacs of the 

 head occupy the greater portion of the head capsule. They 

 entirely fill up all the space which would otherwise behaemo- 

 coel. These tracheal sacs are supplied by the cervical 

 tracheal ducts which, on entering the head capsule, curve 

 dorsally behind the cephalic ganglion. Before curving up- 

 wards each gives off a large ventral duct (fig. 4), which 

 spreads out beneath the cephalic ganglion forming a structure 

 of a tentorial nature upon whicli the ganglion rests. The 

 dorsal cephalic ducts unite behind the cephalic ganglion 

 above the oesophagus. From the point of junction three 

 ducts arise, two lateral ducts and a median dorsal duct. The 

 median dorsal duct {m.d.) opens into a large bilobed dorso- 

 cephalic sac lying on top of the ganglion, and occupy- 

 ing the dorsal region of the head capsule. It gives off 

 branching tracheal twigs supplying the antero-dorsal portion 

 of the optic ganglion (periopticon). Each of the lateral 

 ducts (fig. 4, l.d.) supplies the posterior cephalic sacs. It 

 first communicates with a sac (fig. 13, p.c.s.) lying behind 

 the dorsal portion of the optic ganglion to which it gives off 

 a large number of tracheal twigs. This sac opens into an 

 elongate vertical sac whicli occupies the veiitro-posterior 

 region of the bead capsule. The remaining tracheal sacs of 

 the head are supplied by the tentorial tracheal ducts {tr.d.), 

 which spread out beneath the cerebrum in a fan-shaped 

 manner, and are bilaterally distributed. Each half, in addi- 

 tion to giving off internally tracheal twigs to the optic 



