78 MOSQUITOES OF NORTH AMERICA 



supposed that this liquid was lymph from the cutaneous tissues, which the mos- 

 quito must suck before the flow of blood into the capillaries is brought about. 

 He thought that possibly this liquid was afterwards rejected by disgorging, or 

 that it was held in reserve against future needs. The two smaller diverticula 

 Dufour found to be filled with an amber-colored liquid and with bubbles of air 

 and he states that their volume is variable, according to contents. Thompson 

 and others have observed that all three of the diverticula are usually filled with 

 air. Christophers, in considering the mechanism of feeding, discusses the 

 diverticula as follows : 



" In mosquitoes as usually killed, the proventriculus and anterior portion of 

 the mid-gut are considerably distant from the posterior end of the pumping 

 organ, so that the large delicate walled oesophageal chamber with its extensive 

 diverticula intervene. Immediately after feeding, however, though blood is very 

 evident in the mid-gut, and even in the calyx-like proventriculus, yet in the 

 oesophagus there is no trace. As this latter is so large and has such delicate 

 walls, it is evident that, in the act of feeding, the calyx-like proventriculus must 

 be applied directly to the posterior opening of the pharynx, thus shutting off the 

 capacious oesophageal pouch. The large oesophageal diverticulum probably acts, 

 not only as an air chamber to specifically lighten the body of the mosquito, but 

 also as an air pad to distribute the pressure of the large coagulum formed in 

 the mid-gut after feeding. In a fed mosquito a transparent area is generally to 

 be seen in front of the opaque mass of blood in the abdomen. This transparent 

 area is the abdominal portion of the air-containing oesophageal diverticulum." 



It is undoubtedly the presence of air that led Giles to the view that the 

 diverticula belonged to the tracheal system. He calls them " pneumatic sacs " 

 or " aspiratory vesicles " and claimed that they were part of the tracheal system 

 and not connected with the digestive tract. This view has been thoroughly con- 

 troverted and Thompson states that the diverticula " have a less extensive 

 tracheal supply than any other part of the alimentary canal, except possibly the 

 pharynx and antlia." Giles thought that there were two ventral sacs and that 

 they have an aerostatic function. He makes the interesting statement, which 

 should be investigated further, that " the more gorged the insect or the heavier 

 it be with eggs, the larger will these sacs be found . . , ." 



Eysell appears to have studied the diverticula and their function most care- 

 fully. He calls the large ventral diverticulum " Vorratsmagen " (reservoir- 

 stomach) and the two dorsal diverticula " Flugblasen '' (flight-bladders). He 

 states that the dorsal diverticula are always filled with air and that this is 

 usually also the case with the ventral one. However, after a meal the ventral 

 diverticulum is filled with nectar, fruit juices or blood which remains there for 

 hours, and with low temperature even for days. 



" Sweetened watery solution of litmus is readily taken by mosquitoes. As 

 long as it remains in the reservoir-stomach it retains its beautiful blue color. 

 Transmitted to the mid-gut it turns red at once and also remains red in the 

 hind-gut. Only when the blue color has completely given way to the red has 

 the reservoir-stomach given up the last vestiges of the sugar solution to the 

 mid-gut. By this simple experiment, which can be easily repeated at any time, 

 one can determine with certainty how long nourishment remains in the reservoir- 

 stomach at determined temperatures. 



