METASTIGMATA 



469 



group. The chelicerae are cutting instruments with their distal 

 ends serrated outwardly, and there is always present a hypostonie 

 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 wlien 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 rejlexus and A. pcrslcus 

 have been proved to convey a vSpirochaete 

 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 

 Ijeings. In Mexico the " Turicata " F:g. 244. 

 (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. mouhata. 



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 patcli on the front 

 part of the body of the distended female. 

 Tliere are ten genera, Ixodes, Haema- 

 2^hyscdis, Dermcicentor, Ehipicentor, 

 Rhijncephalus, Boophilus, Margaropus, 

 Hycdommci, AmUyomma, and Aponomma. 



Ixodes ricimis is the connnon English sheep-tick. Species 



Ornithodoros talaje, 

 under surface, x 5. (After 

 Canestriui.) 



Fig. 245. — Female Sheep-tick, 

 Ixodes ricinus. 



