j6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 39 



blood corpuscles out of the diverticulum when the circular muscles in 

 the neck of the diverticulum are contracted. 



The salivary glands. — The salivary glands of the mosquito consist 

 each of three lobes (fig. 29 B), of which the middle lobe is shorter 

 than the other two. The glands lie at the sides of the anterior end of 

 the ventriculus (A, SlGld; the left gland is displaced in the figure). 

 The two ducts extend into the back of the head, where they unite in a 

 single outlet tube (fig. 24 A, SIDct), which ends at the base of the 

 hypopharynx in a small syringelike swelling that acts as a salivary 

 ejection pump (SIP). On the elastic dorsal wall of the pump is in- 

 serted a dilator muscle (18) from the floor of the cibarial pump. The 

 salivary pump discharges through the salivary canal (sc) of the hypo- 

 pharynx in the female; in the male the duct traverses the labium 

 (fig. 22 F). The salivary secretion in species of Anopheles, according 

 to Metcalf (1945), contains both an anticoagulin and an agglutinin, 

 but in other pest species neither appears to be present. 



The salivary glands are of particular interest in connection with the 

 transmission of disease by mosquitoes. They offer the only avenue of 

 escape for disease organisms from the body cavity of the mosquito into 

 the blood of an alternate host. The sporozoites of malaria, for ex- 

 ample, that penetrate into the salivary glands are carried in the saliva 

 of the biting mosquito directly into the vertebrate host, which is 

 necessary for the completion of the complex life history of the 

 malaria parasite, Plasmodium. This suggests the question of how it 

 became obligatory for some parasites to divide their developmental 

 history between two different animals, but the known facts give no 

 answer. Mosquitoes do not bite each other, and there is no way by 

 which the malaria parasite can be normally transferred from one 

 vertebrate to another. 



The reproductive system. — The organs of reproduction in the 

 Diptera include the parts common to all insects, but their structure in 

 two respects is exceptional. Each testis appears to correspond with a 

 single testicular tube in other insects; the egg tubes of each ovary are 

 extremely small, and all are enclosed in a cellular sheath. 



The male organs of the mosquito include a pair of testes (fig. 30 E, 

 Tes) , a pair of testicular ducts, or vasa deferentia ( Vd) , which enlarge 

 posteriorly to form a pair of semindl vesicles (SV) that in some 

 species are united (D). The vesicles end in a very short common 

 ductus ejaculatorius (Dej), which receives a pair of large accessory 

 glands (AcGld) and then opens directly into the base of the aedeagus 

 (Aed). In the normal condition the reproductive organs lie beneath 

 the alimentary canal, but, with the inversion of the terminal segments 

 of the abdomen, the relation is reversed (fig. 30 A) — the ejaculatory 



