XVI 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 sewtuwm, and with free, leg-like palps. The 
eapitulum 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 reflenus and A. persicus 
have been proved to convey a Spirochaete 
disease to fowls, and the latter, under 
the name of the “ Mianeh Bue” has long 
possessed an evil reputation for the 
“poisonous” effect of its bite on human 
beings. In Mexico the “ Turicata” Fic. Pe Magen eescren iiare 
(Ornithodoros turicata) and the “ Gara- Geen Pe ikear 
pata” (O. megnini) are greatly dreaded, 
while human “tick fever” on the Congo has been traced to the 
instrumentality of O. moubata. 
Fam. 3. Ixodidae.—These are the 
more familiar Ticks, possessing a scutwim 
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, /zodes, Haema- 
physalis, Dermacentor, LRhipicentor, 
Rhipicephalus, Boophilus, Margaropus, 
Hyalomma, Amblyomma, and Aponomma. 
Ixodes ricinus is the common English sheep-tick. Species 
Fic. 245.—Female Sheep-tick, 
Txodes ricinus. 
