32 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



the new instar. The lateral spiracles are then closed again, since they 

 are not functional in the larva for respiration. In the same manner, 

 at the ecdysis of the pupa the tracheal trunks in Culex are said by 

 Hurst (1890) to break up into segmental pieces, which are pulled out 

 through the temporarily opened spiracles. The soft inner tissue of 

 the respiratory siphon is withdrawn into the body where it is finally 

 absorbed. The siphon itself is shed with the larval cuticle, and its two 

 tracheal trunks break off at the base. 



The tracheal system of the young larva on hatching is filled with a 

 liquid. According to Frankenberg's (1937) observation on Culex, 

 air enters the tracheae only when the end of the respiratory siphon 

 comes above the water surface. One of the dorsal longitudinal trunks 

 fills first, and then the other. The air is drawn into the tracheae as 

 the embryonic liquid diffuses through the tracheal walls. 



The dorsal blood vessel. — The dorsal blood vessel of the mosquito, 

 particularly in Anopheles quadrimaadatus, has been elaborately de- 

 scribed by Jones (1954). Structurally it differs in no essential respect 

 from the vessel of other insects, except for a dilatation, or sinus, of 

 the aorta in the thorax. The larval organ is a simple muscular tube 

 extending along the midline of the back from the eighth abdominal 

 segment into the head. The part in the abdomen, known specifically as 

 the heart, is perforated along the sides by eight pairs of segmental 

 openings, or ostia. The part in the thorax, called the aorta, is im- 

 perforate. In the head the aorta goes beneath the brain, where it is 

 open ventrally allowing the blood to be freely discharged into the head 

 cavity, whence it flows backward through the body to reenter the heart 

 through the ostia. The larval heart, Jones says, always beats forward 

 at an average of 85.2 pulsations a minute, but it has no nerve connec- 

 tions. Along the sides of the heart are attached the usual fan-shaped 

 segmental groups of muscle fibers, the so-called alary muscles, that 

 support the heart on the body wall. 



The alimentary canal. — In the mosquito larva the alimentary canal 

 (fig. 12) is a relatively simple tube. It consists of the usual three 

 parts of the arthropod digestive tract, an ectodermal stomodaeum, an 

 endodermal mesenteron, and an ectodermal proctodaeum. The sto- 

 modaeum begins in the head with the pharynx (Phy), which is fol- 

 lowed by a narrow oesophagus (Oe) that goes through the neck into 

 the thorax, where it enters the first part of the mesenteron, known as 

 the cardia (Car). (This term, borrowed from vertebrate anatomy, 

 has no literal significance in the insect.) Within the cardia the oesopha- 

 geal walls are reflected to form the usual entrance funnel of the 



