METASTIGMATA 



469 



group. The chelicerae are cutting 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 



Omithodoros, though recent discoveries 



of new forms have tended towards their 



fusion. Argas refiexus 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 



(Ornitliodoros turicata) and the " Gara- 



pata " (0. inegnini) 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, Ixodes, Haema- 

 2)hr/scdis, JDermacentor , Bhipicentor, 

 Bliipiicephalus, Boophilus, Margaropiis, 



Eyalomma, AmUyomma, and Apionomma. 



Ixodes ricinus is the common English sheep-tick. Species 



Turicata " Fio. 244. — Omithodoros talaje, 

 under surface, x 5. (After 

 Canestrini.) 



Fig. 245. — Female Sheep-tick, 

 Ixodes ricinus. 



