NO. 8 ANATOMICAL LIFE OF THE MOSQUITO SNODGRASS 75 



posterior intestine, or rectum (Rect), which is much enlarged an- 

 teriorly and tapers back to the anus. The inner wall of the pyloric 

 funnel is armed in some species with numerous small spines directed 

 posteriorly. These pyloric spines have been described and well il- 

 lustrated by Trembley (1951) in species of Anopheles, Aedes, and 

 Culex. In the anterior end of the rectum are six small, soft, conical 

 rectal papillae (D, rp) projecting inward from the rectal wall. Five 

 Malpighian tubules (A, Mai) arise from the pyloric region of the 

 intestine as in the larva. 



The oesophageal diverticula are said to be empty on emergence of 

 the mosquito from the pupa. Within an hour after ecdysis, however, 

 according to Marshall and Staley (1932), the air that was pumped 

 into the stomach begins to pass forward into the diverticula, and in 

 12 to 22 hours the stomach is empty. 



The function of the oesophageal diverticula in relation to food 

 intake has been studied by a number of investigators, but, though 

 using the same experimental methods of feeding, the latter have not all 

 come to the same conclusions. The subject has recently been well re- 

 viewed by Trembley (1952) and by Megahed (1958), and good 

 bibliographies are given by both these writers. In general it is found 

 that ingested blood goes directly to the stomach, while fruit juices and 

 sugar solutions go first into the diverticula, to be later delivered to 

 the stomach. According to Trembley, blood in small amounts may 

 occasionally go to the diverticula, and sugar solutions sometimes go 

 direct to the stomach. The work of Megahed on Culicoides gives 

 essentially the same results, the stomach being ordinarily the receptacle 

 for blood, the diverticula for concentrated sugar solutions, but water 

 and dilute sugar solutions go direct to the stomach. Most observations 

 seem to apply to the female insect. Day (1954), however, in experi- 

 ments on male mosquitoes, found that the sexes react similarly : "blood 

 went to the mid-gut and sugar to the diverticulum in the male in spite 

 of the fact that males do not ingest blood under natural conditions." 



The "switching mechanism" that determines whether the ingested 

 food goes into the stomach or the diverticula, Day (1954) has pro- 

 posed, is governed by the different kinds of sense organs in the wall of 

 the cibarial pump (buccal cavity). If receptors of one type are 

 stimulated by sugar it may be supposed that they cause a relaxation of 

 sphincter muscles of the diverticula ; if others are sensitive to blood 

 components, they may effect a relaxation of the cardiac sphincter of 

 the stomach. In the neck of the ventral diverticulum, Day notes the 

 presence of a group of spines, which would appear to assist in keeping 



