782 



THE DIPTERA 



to them, of which the two posterior dorsal dilators run from the occiput to 

 the dorsal plate; the two anterior dilators (two pairs in the female) taking 

 their origin from the vertex are inserted also into the dorsal plate, and the 

 five-paired latero-ventral dilators arising from the lateral posterior angle of 

 the head pass upwards and a little forwards to the latero-ventral plates. 

 When these muscles contract the triradiate pharynx becomes nearly circular, 

 and when they relax the walls come together. 



In this way the pharynx pumps the blood from the victim up the blood 

 tube in the proboscis into the alimentary canal. 



4. Oesophagus. — The short oesophagus runs from the pharynx to the oeso- 

 phageal valve. Anteriorly it is narrow, but posteriorly it expands so much 

 that this portion is sometimes called the crop. It is lined with thin chitin, 

 and has many bands of muscles attached to it. Its posterior end lies on a 

 level with the origin of the first pair of legs, and at this situation it gives off 

 three pouches, two dorsal and one ventral. The large ventral pouch opens 

 into the oesophagus by a single opening in the middle line, and extends back- 

 wards under the alimentary canal to the level of the fifth, sixth, or seventh 

 abdominal segment, when fully distended. In shape it is fusiform. The 

 two small latero-dorsally placed pouches open into the sides of the oesophagus. 

 All these sacs are lined with thin chitin, and have some slight musculature. 



Fig. 386. — ^The Alimentary Canal of Anopheles maculipennis in situ. 



(After Nuttall and Shipley, Journal of Hygiene.) 



I, Proboscis; 2, buccal cavity; 3, pharynx; 4, oesophagus; 5, oesophageal 

 pouches; 6, salivary glands; 7, proventriculus; 8 and 9, mid-gut; 10, Mal- 

 pighian tubes; 11, ileum; 12, colon; 13, rectum; 14, rectal papillae; 15, anus. 



5. The Oesophageal Valve or Proventriculus . — The oesophageal valve is an 

 annular thickening of the intestinal wall, due partly to circular muscles, which 

 are capable of closing the lumen of the gut when they contract, and partly 

 to an invagination of the more anterior portion of the gut into the more 

 posterior, thus forming a valve. Projecting from it are six small protuber- 

 ances, more marked in the larva than in the adult insect. Christophers 

 considers that this portion of the alimentary canal is the proventriculus of 

 other insects. It is not lined with chitin. 



6. Mid-Gut or Chylific Ventricle. — The mid-gut is a straight tube running 

 from the oesophageal valve at the level of the first pair of legs to the posterior 

 limit of the sixth abdominal segment ; but this varies as to whether it is filled 

 with food or not. It consists of two parts : an anterior narrow, and a posterior 

 more distended, often called the ' stomach,' which begins on a level with the 

 second abdominal segment, and is the receptacle for the food. The wall of 

 the stomach is composed of the following layers : — ■ 



(1) A delicate internal cuticle. 



(2) A single layer of large cylindrical or cubical epithelial cells with large 

 nuclei; their condition varies with the state of distension of the stomach. 



(3) An elastic basement membrane. 



(4) Muscular fibres, circular and longitudinal, forming a loose network. 

 In this last layer there are numerous air-tubes or trachegs, which probably 



help to keep the gut in positiou in the coelom,- 



