(ESOPHAGEAL DIVERTICULA 77 



" The appendages of the alimentary canal are : — 



" The Salivary Glands. — The salivary glands consist of six tubular acini lying 

 three upon either side. A duct can be seen traversing almost the entire length of 

 each acinus. Shortly after leaving the acinus, the three unite to form a single 

 duct. Beneath, and in contact with the lower surface of the suboesophageal 

 ganglion, the ducts of each side unite to form a common salivary duct which 

 passes forwards and enters the chitinous first portion of the alimentary canal 

 close to the base of the proboscis. 



" The Malpighian Tubules. — These are five in number and open into the first 

 portion of the hind-gut immediately beyond the pylorus. Their blind ends are 

 held in position in the neighborhood of the rectum by tracheal branches. They 

 pass forwards in loops above their origin, so that, in transverse section, as many 

 as ten may be seen cut across. 



" The Vascular System. — As in most insects where the respiratory system 

 ramifies throughout the whole body, the vascular system is not well developed. 

 A dorsal vessel or heart and an anterior prolongation of this (aorta) are the only 

 closed blood-vessels. Apart from the dorsal vessel the blood circulates in large 

 blood spaces, which lie between the lobes of the fat-body and among the muscles 

 and viscera. 



" The dorsal vessel passes close beneath the tergal plates throughout the 

 abdomen. It is very thin walled, and is not provided with valves. The upper 

 portion is attached to the dorsum at intervals by suspensory fibres (muscular), 

 so that a festooned appearance is given in longitudinal section. There is, how- 

 ever, no true division into compartments. Laterally large cells (pericardial 

 cells) are arranged throughout its entire extent, and fibres of a muscular nature 

 (alary muscle) pass from the body wall and end in branches in close connexion 

 with the dorsal vessel. 



" At the first abdominal segment the dorsal vessel dips down beneath the 

 mesophragma, lying as it does so in direct contact with the cuticle. In the 

 thorax it again arches upwards, and lies between the lower portions of the 

 antero-posterior wing muscle close above the anterior portion of the mid-gut. 



" In the anterior third of the thorax it divides into two smaller portions which 

 pass outwards, and coming in contact with the salivary ducts enter the neck. 



" Blood spaces without definite walls occur throughout the body. The thorax 

 especially contains large spaces among the muscles, and the complex fat-body 

 which lies between and supports the organ is everywhere bathed with blood 

 fluid." 



The diverticula of the oesophagus are of sufficient interest to warrant further 

 remarks. There are three of these diverticula, arising near the posterior end of 

 the oesophagus. Two of them are dorso-lateral and small, the third one is ven- 

 tral and very large and extends into the abdomen. Dufour examined these 

 organs carefully and speculated regarding their possible function, assuming that 

 they were in some way connected with the digestive process. He called the 

 smaller pair ''' bourses veniriculaires" the large one " pause " or paunch. In 

 fact this paunch is the so-called " sucking stomach," " food-reservoir " or crop of 

 many other insects. In the mosquito it is, when expanded, globose, ovoid. Du- 

 four states that when empty it is folded into plaits. Graber has determined that 

 in flies this organ actually serves as a food-reservoir by feeding them with colored 

 liquid. Dufour found that when the stomach of the mosquito is distended with 

 blood the paunch is filled with a colorless or faintly amber-tinged liquid. He 



