228 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



After a number of asexual generations have been produced, special 

 larger, sausage-shaped crescents appear within the red corpuscles. 

 These are the gametocytes, or sexual forms. If a female mosquito 

 sucks blood from a person having mature male and female stages of 



the parasite in the blood, such 

 i^a, parasites are taken mto the diges- 



tive tract of the mosquito, where 

 union of the male and female 

 gametocytes takes place. After 

 conjugation the resulting zygote 

 forms an ookinete or cyst that 

 enters the lining of the stomach 

 of the mosquito, in the outer 

 walls of which a complicated de- 

 velopment then ensues for about 

 twelve days, ending with the 

 formation of a large number of 

 spindle-shaped structures called 

 sporozoites. The cyst then bursts 

 and the sporozoites migrate to the 

 salivary gland of the mosquito. 

 After that time, if the female mos- 

 quito bites an uninfected human 

 host she infects him with the sporo- 

 zoites, which enter red blood cells. 

 Animals are not the only group 

 having complicated parasitic 

 cycles. The various smuts, mildews, and rusts are plant parasites 

 that annually take their toll throughout the country. Wheat rust is 

 probably one of the most destructive of the parasitic fungi. This 

 rust has been the most dreaded of plant diseases because it destroys 

 the harvest upon which the civilized world is most dependent. Wheat 

 rust has long been associated with barberry bushes. As early as 

 1760, laws were enacted in New England providing for the destruction 

 of barberry bushes near wheat fields, although nothing was actually 

 known of the relationship between the barberry and rust until com- 

 paratively recent years. It is now known that wheat rust may pass 

 part of its life as a parasite on the barberry, whence it migrates to the 

 wheat plant and there undergoes a complicated life history. Since 

 the nourishment and living matter of the wheat are used as food by 



Diagram of eggs, larva, pupa, and 

 adult of Culex (left) and the malarial 

 carrying Anopheles (right). How could 

 you tell the eggs, larvae, and adults of 

 these two genera apart ? 



