CHAPTER XXIV 

 Order Acarina (continued) : The Ticks 



Family IXODID^ : Ticks. 



The species of this family are in all stages of their exist- 

 ence blood-sucking parasites of vertebrate animals ; and, as 

 with so many other blood-suckers, the evil that they do is 

 not confined to mere robbery of blood, but lives after them 

 in the form of zymotic contamination of their victims. 



In the IxodidcB the body is more or less oval, with the 

 segmentation and even the distinction between cephalothorax 

 and abdomen obliterated, though in one genus {Ornithodorus) 

 a notch on either side behind the last pair of legs just 

 suggests a division into these two regions. At or near the 

 front end is a movable " beak," or capitulum, which consists 

 of a strongly chitinised basal piece — the basis capituli — sur- 

 mounted by the pedipalps, chelicerae, and other mouth-parts. 



The basis capituli is a flattened ring, which encircles the 

 bases of the chelicerae ; the pedipalps are broadly implanted 

 in its sides ; dorsally its cuticle is produced over the chelicerae 

 to form a laterally inflexed cover, or " sheath " for each of 

 them ; ventrally it is prolonged in the middle line to form a 

 spatulate rasp — the hypostome. 



Each chelicera (Fig. 120) consists of a long shaft, to the 

 distal end of which are hinged, side by side, two stoutly 

 uncinate cusps, which can be moved laterally so as both to 

 cut and to make fast in the skin of a victim. The shaft and 

 a considerable portion of the " hooks " are invested in a 

 sheath formed by the prolongation of the dorsal cuticle of 

 the basis capituli, the sheath being clo.sely beset with rows of 

 microscopic barbules. 



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