A REVISION OF THE IXODOIDEA, OK TICKS, OF 

 THE UNITED STATES. 



STRUCTURE. 



The Ixodida^, or ticks, are all mites of considerable size; even the 

 young or " seed-ticks " are visible to the naked eye, while a full- 

 grown engorged female may be half an inch long. Their abundance 

 on many of the domestic animals and occasional occurrence on man 

 have rendered them well-known objects of disgust in every clime. 



The body is covered by a tough leathery skin, which in the female 

 becomes greatly distended as she engorges herself with the blood of 

 the host. Before distention the tick is of a somewhat triangular 

 outline, flat, with prominent, slender legs and a beak-like rostrum in 

 front. When the female becomes swollen these characters may be 

 hardly noticeable and the whole creature may look like some large 

 seed or bean. In most of the forms there is on the front part of the 

 dorsum a corneous shield known as the scutum. In the male this 

 scutum covers the greater part of the body, but in the female only a 

 small part in front. Articulated to the anterior margin of this 

 scutum, and usually within a slight emargination, is a small sub- 

 triangular piece, called the capitulwm., or head. This capitulum 

 bears the palpi, the mandibles, the mandibular sheaths, and the 

 hypostome. The last three organs together form the prohoscis., or 

 haustellum. The hypostoTiie is a median piece beneath (really of two 

 pieces) bearing many recurved teeth or denticles. The more basal 

 of these denticles are in rows, and the number of these rows has 

 been used in the diiferentiation of species, but is subject to some 

 variation. At the tips of the mandibles are two or three processes, 

 known as the apophyses; these have also been used in specific classi- 

 fication, but are noAv also known to be inconstant. The hypostome 

 and mandibles are inserted into the host when the tick feeds, and so 

 firmly do the recurved teeth of the hypostome hold that if one tries 

 to remove a tick l)y force the body may be torn from the attached 

 capitulum. 



The palpi are inserted at the sides of the mouth j^arts and are of 

 four segments, but connnonly one sees only two, for the basal is short 



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