THE ACAEIXA OR MITES. 



57 



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 subtriangular piece, called the 

 capitulum, or head. This capitulum bears 

 the palpi, the mandibles, the mandibular 

 sheaths, and the hypostome. The last 

 three organs together form the proboscis, 

 or haustellum. The hypostome (fig. 107) 

 is a median piece (really of two pieces) 

 bearing beneath many recurved teeth, or 

 denticles. The more basal of these denti- 

 cles are situated in rows, and the number 

 of these rows is used in the differentiation 

 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 also are known to be incon- 

 stant. 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 

 the tick by force the body may be torn 

 from the attached capitulum. 



The palpi are inserted at the sides of 

 the mouth parts and are of four joints, but 

 commonly one sees only two, for the basal 

 joint is short and broad, and the apical is 



very small and often situated in a depression near the tip of the third joint. The 

 palpi are usually somewhat concave on the side toward the mouth parts, so that they 

 may sheath these parts. The comparative length of the second and third joints of 

 the palpi give useful characters in separating the genera of ticks. 



On the dorsum of the capitulum 

 of adult female ticks are two de- 

 pressed, pitted areas known as the 

 porose areas. No one has yet de- 

 termined their function. All fe- 

 male ticks of the family Ixodidae 

 which do not have these organs 

 fully exposed are immature. Va- 

 rious speciee, and even genera, 

 have been based on immature 

 forms, owing to a failure to recog- 

 nize this point. The genera Fhauloixodes, Herpetobia, Sarconyssus, and Gonixodes 

 are of this class. The scutum or shield is usually irregularly hexagonal in shape. 

 On each lateral margin is a pale eyelike spot or ocellus; in some genera there are 

 no eyes. The posterior margin of the body in most forms is marked by a number 

 (8 to 10) of short impressed furrows, which outline a series of lobes or festoons; these 

 are more distinct in the male than in the female, and when the latter is distended 

 with blood they are barely \isible. On the under side, or venter, of the body there 

 are two median apertures. The anterior one is not far from the beak and is the genital 

 pore; the posterior one is toward the tip of the body and is the anus. In many forms 

 there is a curved groove behind the anus, and from it a median furrow extends toward 



Fig. 105.— OrnitJiodoros turicata. (Original.) 



Fig. 100.— Ventral furrows: a, Ixodes; b, Dermacentor. 

 (Original.) 



