ORDER ACARINA : THE TICKS 273 



of legs. Eyes, when present, are situated on the edge of the 

 scutum. The pedipalps, which are rigid and form a distinct 

 sheath for the mouth-parts, have the 4th segment sessile and 

 vestigial, so that they appear to consist of only 3 segments. 

 In the legs the coxae are generally armed with tubercles or 

 spurs, and the tarsi carry a pulvillus as well as a pair of claws. 



There are certain constant grooves on the body of a hard 

 tick. Of particular importance, from the taxonomic stand- 

 point, is the anal groove — a fine groove, usually in the form 

 of a semicircle with the concavity forwards, that half encircles 

 the anus. This groove is absent in a few forms. 



The female Ixodine differs from the male not only in 

 being much larger and more distensible, so that in some 

 species when she is gorged she is as large and rotund as a 

 large filbert ; but also in having a small scutum, which covers 

 only the front end of the body — instead of covering the 

 whole dorsum, as it does in the male ; and further in the 

 presence on the dorsum of the basis capituli of two round or 

 oval glandular depressions known as arecE porosa. 



The larva and nymph of the Ixodinse, whether prospective 

 female or male, resemble the female in having a small scutum. 

 The Ixodinse are fixed parasites, the adult attaching itself 

 permanently to its host. When the female is fertilised and 

 gorged, and has matured her eggs, she drops from her host, 

 delivers her eggs, and forthwith dies. 



The "hosts," or victims, of Ixodinae include land- 

 vertebrates of all kinds ; but man is not often attacked by 

 adult Ixodines, though the larvae will readily burrow into 

 his skin if they get a chance. One species of Dermatocentor 

 is said to carry the Piroplasma of Rocky Mountain Spotted 

 Fever in man ; but with this exception Ixodinae are of no 

 particular importance in human pathology, though when 

 they do bite man the consequences may be unpleasant. 



It is among domestic animals that the Ixodinse work havoc ; not 

 merely by abstraction of blood, though even in this vi'ay, if they attack a 

 young or weakly animal in force, they may do mortal damage ; but by 

 spreading among them the infections of certain destructive diseases due 

 to specific piroplasms and spirochastes. As in the normal course of things 

 a given tick in any one stage of its existence feeds only on a single host, 

 if it should become infected from that host it will not have any oppor- 

 tunity of passing on the infection directly, but the infection remains 



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