INSECTS AND HUMAN DISEASE 145 



and it is known as the Rocky Mountain spotted fever 

 tick, Dermacentor andersoni (venustus). Both males and 

 females can act as vectors, and an infected adult may pass 

 on the germs of the disease, through its egg, to the next 

 generation, so that they also become capable of transmitting 

 spotted fever. The larvae and nymphs too may become 

 infected and retain the disease germs .through all their 

 moults, with the result that, when adult, they can transmit 

 the infection to another host. 



The spotted fever tick is the only known natural vector 

 of the disease, from which it derives its name, and the area 

 over which spotted fever is rife coincides with the range of 

 the tick, just in the same way that the presence of malaria 

 and yellow fever is coincident with the distribution of their 

 mosquito vectors. The tick occurs at various elevations, 

 ranging from five hundred feet to nearly nine thousand feet 

 above sea level. Favourable or unfavourable local conditions 

 for the creature's development alone regulate its prevalence 

 or scarcity in a given locality. The abundance of these ticks 

 depends largely on the amount of vegetation, which acts as 

 a protection for the developmental stages when the ticks 

 are not upon a host ; small and large mammals are also 

 necessary adjuncts of this tick, the former to act as hosts 

 for the immature stages, the latter for the adults. 



Dermacentor andersoni (venustus) has such a peculiar 

 life-history that it is well worth considering in some detail. 

 Oviposition in the female extends over some thirty or so 

 days, and during this time small ovoid brownish eggs, to 

 the number of about four thousand, are laid. After sixteen 

 to fifty days, depending on atmospheric conditions, small, 

 brown, active, six-legged larvae emerge. Before the larvae 

 or " seed ticks," as they are often called, are able to grow 

 they must partake of a feast of blood, and, usually, they 

 affix themselves to some small mammal, such as a ground 

 squirrel, and remain attached and feeding for from three 

 to eight days, by which time they have become engorged. 



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