6S8 MEDICAL ENTOMOLOGY 



into the atrium there is a bent tube, the dorsal wall of which is formed by 

 a rigid chitinous plate, while the ventral wall is folded and consists of 

 elastic chitin. The tube leads into the main tracheal trunks and has 

 attached to its ventral wall just below the folded portion a row of 

 chitinised teeth which are not unlike similar structures found at the 

 entrance of the tracheae of other arthropods. 



On the internal surface of the spiracle a number of large tracheal trunks 

 are seen to emerge from it and to pass inwards ; they may be conven- 

 iently divided into anterior, median and posterior sets. 

 The tracheae * 



The anterior trunk passes forwards along the ventral 



surface close to the salivary gland ; it gives off numerous fine branches for 

 the skin, legs and other adjacent structures. About mid-way in its 

 course a large branch is given off for the supply of the salivary gland, and 

 from this a smaller branch goes to the brain. Near its anterior end 

 a large branch is given off for the supply of the mouth parts, salivary 

 ducts, etc. ; this branch in many cases joins its fellow of the opposite 

 side and forms the so-called ' tracheal plexus ', situated near the brain. 

 At its termination the main anterior trunk gives off a large external branch 

 which divides into two or more branches for the supply of the coxal gland, 

 skin, legs, Gene's organ, etc. The main middle trunk soon splits into 

 two secondary branches ; the anterior of these supplies the dorsal skin, 

 the lateral diverticula, the mid-intestine, the Malpighian tube, oviduct, etc. 

 The posterior branch is almost entirely restricted to the ovary and oviduct. 

 The main posterior trunk also divides into two branches, the anterior 

 of which supplies the ovary and connected structures ; the posterior 

 branch goes to the posterior diverticula, rectum, Malpighian tubes, etc. 



The heart is situated in the middle dorsal line, immediately below 

 the integument and about the level of the stigmal plate. It lies on the 



mid-intestine, and can often be seen pulsating, through 

 The heart and vessels 



the integument, in nymphs. It is oval m shape, and is 



produced into two pouches at its posterior end. On the ventral surface 

 there are two pairs of slit-like openings (ostia), leading into channels 

 which extend up to the dorsal wall. The aorta commences at the 

 anterior end of the heart, and passes forwards in close apposition to the 

 mid-intestine. At its anterior end it expands into a sinus which surrounds 

 the oesophagus and brain, the ventral surface of the sinus becoming 

 fused with the ventral body wall, and the dorsal wall with the sheaths 

 of the mandibles. There are in addition four pairs of lateral vessels 

 which pass into the legs. The course of the circulation is the same 

 as that in the Insecta; the blood from the body cavity is drawn 



