XVIII 



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 

 Ornitkodoros, though recent discoveries 

 of new forms have tended towards their 

 fusion. Argas reftex-us and A. pcrsicus 

 have been proved to convey a Spirochaete 

 disease to fowls, and the latter, under 

 tin- name of the " Mianeh Bug" lias long 

 possessed an evil reputation for the 

 "poisonous" effect of its bite on human 

 beings. In Mexico the " Turicata " FIU. -jn.- Ornithodorostalaje, 



. . . , ., under surface, x 5. (After 



i h-ii.tthodoros tuncata) and the Uara- cam^trini.) 



p;ita" (0. mcgnitii) are greatly dreaded, 



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



instrumentality of 0. ////'/<"/". 



Fam. 3. Ixodidae.- -These are the 

 move familiar Ticks, possessing a scutum 

 or shield, which covers (lie whole luck 

 of the male, which is capahle. 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, Txodes, Haema- 



Derm ,/<; ,t/u,'. 



FlG. 245. Female Slirrp-tirk, /, / ; x _ 

 ].i-il,-ti fit- 1 ii 11.1. 



Ehipicephalus, Boophilus, Margaropus, 



H>i<tl(nnina, Aviblyoin nt, and A/xnuun nut. 



Ixodes ricinus is the common iMi-lish sheep-tick. Species 



