that spread Disease. 



41 



between the main staircase and the statue of Sir Eichard Owen, 

 there is shown, on the North side, a series of ticks arranged in 

 three groups. The first group consists of ticks that convey disease 

 to human beings, and inckides Ornitlwdoros mouhata Murray, 

 Ornitlwdoros savigniji Aud. and Derinaccnior vcimstus Banks. 



Ornitlwdoros mouhata (fig. 14) is- very widely distributed in 

 Africa, and the regions wliere it is Iviiown to occur are marked in 

 red in the map that is placed by the side of the specimens. This 

 tick, which sometimes attacks domestic animals as well as human 

 beings, transmits the blood-parasite Spirochacta duttoni Novy and 

 Knapp, the cause of the human relapsing fever of tropical Africa. 



Fig. 13. — Organisms of African Relapsing Fever, 



Sinrochaeta duttoni, and red blood-corpuscles ; X 1,500. 



The larval stage of the tick is not well developed in this species, 

 and is inert, the tick emerging from the eggshell as a nymph. 

 After freeing itself from the larval skin, the nymph is ready to 

 feed. It casts its skin several times before becoming adult, a 

 moult taking place after each meal of blood. The female tick does 

 not lay eggs until it has fed on blood. The spirochaetes are taken 

 up by the tick whilst sucking blood from an infected person. They 

 make their way into the ovaries of the tick, x^enetrating into the 

 undeveloped eggs and multiplying within them. The first nymphal 

 stage that develops from these eggs is capable of conveying the 

 disease. The spirochaetes can be transmitted to the third genera- 

 tion of ticks, even if the second generation was fed on blood free 

 from spirochaetes. 



