METASTIGMATA 



469 



group. The chelicerae are cuttiug instruments with their distal 

 ends serrated outwardly, and there is always present a hypostome 

 beset with recurved teeth which serve to maintain a firm hold 

 on the tissues into which it is thrust. On either side of the 

 chelicerae are the four-jointed palps, leg-like in the Argasidae, 

 but more rigid and rod-like in the Ixodidae, where their inner 

 margin is often hollowed so as to enclose the chelicerae and 

 hypostome when the palps are apposed. There is a conspicuous 

 pair of spiracles near the coxae of the fourth pair of legs. 



Fam. 2. Argasidae. The Argasidae are leathery Ticks 

 without a shield or scutum, and with free, leg-like palps. The 

 capitulum is never more than partially visible when the adult 

 animal is viewed dorsally. Their hosts 

 are always warm-blooded animals. Two 

 genera are usually recognised, Argas and 

 Ornithodoros, though recent discoveries 

 of new forms have tended towards their 

 fusion. Argas reftexus and A. persicus 

 have been proved to convey a Spirochaete 

 disease to fowls, and the latter, under 

 the name* of the " Mianeh Bug " has long 

 possessed an evil reputation for the 

 "poisonous" effect of its bite on human 

 beings. In Mexico the " Turicata " 

 (Ornithodoros turicata) and the " Gara- 

 pata " (0. megnini) are greatly dreaded, 



while human "tick fever" on the Congo has been traced to the 

 instrumentality of 0. moubata. 



Fam. 3. Ixodidae. These are the 

 more familiar Ticks, possessing a scutum 

 or shield, which covers the whole back 

 of the male, which is capable, there- 

 fore, of little distension, whereas it 

 forms only a small patch on the front 

 part of the body of the distended female. 

 There are ten genera, fxudes, Hamm- 

 p j l1/sa /;.^ Dermocentor, Rhipicentor, 



_. - 



Rhipicephalus, Boophilus, Margaropus, 

 Hyalotnma, Amblyomma, and Aponomma. 



Ixodes ridnus is the common English sheep-tick. Species 



FIG. 244. Ornithodoros talaje, 

 under surface, x 5. (After 

 Canestrini.) 



FIG. 245. Female Sheep-tick, 



Ixodes ricinii*. 



