34 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



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 abdo- 

 men. This transparent area is the abdominal portion of the air-con- 

 taining oesophageal diverticulum. 1 



Christophers' view, however, that food isuot taken into the 

 large diverticulum, is incorrect. Eysell has carefully studied 

 the functions of the three diverticula and his terminology is 

 based upon the results. He states that the two dorsal diver- 

 ticula are always filled with air. The third diverticulum, or 

 reservoir stomach, is generally also filled with air. However, 

 after the insect has fed, the large diverticulum is found filled 

 with nectar, fruit juice, or blood, which may be retained there 

 for hours, and with low temperature even for days. 



A solution of litmus with sugar and water is readily taken by mos- 

 quitoes. As long as it remains in the reservoir-stomach it retains its 

 beautiful blue color. Taken into the mid-gut it reddens at once and 

 also remains red in the hind-gut. Only when the blue color has com- 

 pletely given way to the red has the reservoir-stomach given up to 

 the mid-gut the last vestiges of the solution. When the nourishing 

 liquids have been given up by the reservoir-stomach, which is fur- 

 nished with longitudinal and circular muscles and furthermore assisted 

 by ventral pressure, to the mid-gut, air again takes its place. 



Christophers examined his mosquitoes in this last condition 

 when the blood had been transferred from the reservoir stom- 

 ach to the mid-gut and again replaced by air in the former. 



The process of ecdysis of the imago appears to be practi- 

 cally the same in all the Nemocera. Miall '"' mentions the se- 

 cretion, through the spiracles of the enclosed imago, of a 

 layer of air beneath the pupal skin in Chironomiis and Simu- 

 lium. In the case of Simulium the air accumulated inside 

 the pupal skin performs still another function, namely, to carry 

 the emerging imago uninjured from the submerged pupa to 

 the surface of the water. When the pupal skin is ruptured 

 the air and the fly escape together and the fly is carried rap- 

 idly to the surface within a ball of air. The escape of Simu- 

 lium from the pupa, according to Osten Sacken, 3 seems to have 

 been first described by Verdat in 1821. 



'L. c., pp. 6-7. 



3 The Natural History of Aquatic Insects, 1895, p. 133, 187-188. 

 :t On the Transformations of Simulium. Amer. Entom. and Bota- 

 nist, vol. 2, pp. 229-231 (1870). 



