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such feces; in the insect they continue development, and finally 

 the insect is eaten by a mammal and development continues to its 

 natural conclusion. This is the life cycle of many parasitic worms, 

 as for example the rat tapeworm, Hymenolepis diminuta, often 

 secondary in the meal moth and other grain insects, and some- 

 times primary in man who eats insect infected grain which has 

 been polluted by rodents. 



These are merely some of the types of transmission, of which 

 there are also many modifications. I am dwelling on these differ- 

 ent biological relationships because I want first to impress upon 

 you the vital necessity of a knowledge of every phase of the life 

 cycle of each unit in a disease complex that is the vertebrate 

 hosts, the disease organisms, and the invertebrate vectors and 

 hosts. 



Necessity of Knowing Insect Life History. 



An excellent example of the necessity of knowing the life history 

 of the invertebrate vectors is found in the ticks, many of which are 

 disease carriers. Nuttall has described at least six types of life 

 cycle among the ticks. 



Representing the first type is Argas persicus, the carrier of 

 several spirochaete diseases of fowls, which attaches to a host in 

 the larval stage, engorges and drops to molt, then reattaches, but 

 drops twice to molt in the nymphal stage and as an adult drops to 

 oviposit after the engorgement, then reattaches and engorges 

 again and again. It is obvious that this tick can transmit dis- 

 ease organisms to several different hosts during the course of its 

 development. 



A second type is represented by Ornithodorus moubata, the carrier 

 of several spirochaete relapsing fevers of man. It does not attack 

 a host during its larval stage, but has five different nymphal hosts, 

 one for each stage, and many adult hosts. The individual tick 

 therefore has unlimited possibilities of disease transmission. One 

 single infected tick carried in the personal effects of a traveler 

 might start outbreaks of the fever in many localities. 



The third type may be represented by Dermacentor andersoni 

 or venustus the carrier of Rocky Mountain spotted fever, which 

 has one larval host, one nymphal host and one adult host. 



The fourth type may be represented by Rhipicephalus everts i. the 

 carrier of Nuttallia equi of horses, which attacks only two hosts, 

 one during the larval and nymphal periods and one in the adult 

 stage. 



The fifth type is represented by Boophilus annulatus the carrier 

 of Babesia bovis, cause of Texas fever, which has but one host dur- 

 ing its entire development. It is obvious that only an hereditary 

 organism can be transmitted by a tick with such a life history 



