570 MEDICAL ENTOMOLOGY 



has been proved to be carried by Amblyomma hebraeum, Koch, C. L. The 

 virus (parasite ?) of the human disease known as ' Rocky Mountain Fever ' 

 is known to be transmitted by Dermacentor andersoni (venustus), Stiles. 



In addition to the above diseases, ticks also transmit several of the 

 Spirochaetae. Spirochaeta duttoni, the parasite of ' African Relapsing 

 Fever,' is transmitted by Ornithodorus moubata, Murray ; S. theileri, 

 another spirochaete found in the blood of cattle in South Africa, is carried 

 by Margaropus annulatus, variety decoloratits. Spirochaeta marchouxi, 

 which causes spirochaetosis in fowls, is transmitted in nature by Argas 

 persicus, Fisch-Wald. 



Certain species of ticks are believed to be the invertebrate hosts of 

 mammalian and reptilian haemogregarines. Christophers has described 

 the sexual cycle of Leucocytogregarina canis in the alimentary tract of 

 Rhipicephalus sanguineus. In this case the sporozoits are not found in 

 the salivary glands of the tick, but remain in a cyst in the alimentary 

 tract ; probably they regain the vertebrate host when the dog eats 

 the tick, the sporozoits being liberated by the action of the digestive 

 fluids on the cyst wall ; this has, however, not been proved for the 

 haemogregarine of the dog. Certain haemogregarines of frogs and 

 lizards are believed to undergo their sexual cycles in ticks parasitic on 

 their hosts. 



A few ticks are naturally infected with flagellates of the genus Crithidia. 

 In Madras Rhipicephalus sanguineus is infected with Crithidia christo- 

 phersi, Novy, and Haemaphysalis bispinosa with Crithidia haemaphysa- 

 lidis, Patton. These two flagellates are of peculiar interest as the)' are 

 transmitted hereditarily. Bishop has also recorded a flagellate from 

 Ixodes ricinus in England, and O'Farrel a Crithidia from Hyalomma 

 aegyptium in the Soudan. 



EXTERNAL ANATOMY * 

 (Plates LXXII and LXXIII) 



The body of a tick is oval or elliptical in contour, flattened dorso- 



ventrally, the dorsal surface being gently convex ; there is no distinct 



cephalothorax demarcated off from the rest of the 



abdomen, but the two are fused together to form the 



HIStB LXXIII 



body. Fixed into a special opening, the camerostoma, 

 is the capit ulum (rostrum), often erroneously spoken of as the head ; 



The account of the external anatomy given here applies only to the Ixodini, such as 

 any common species of Hyalomma or Rhipicephalus. The important external characters 

 of the Argatini are noted on page 580. 



