468 ARACHNIDA ACARINA CHAP. 



last three legs are inserted at the margin of the body, has about 

 thirty species, found among moss and dead leaves. Nothrus is 

 a short-legged genus with flat or concave dorsal plate, often 

 produced into very remarkable spiny processes. There are 

 twenty -two species found under bark and among moss and 

 lichen. Hoploderma {Hoplophora) is remarkable for its power of 

 shutting down its rostrum and withdrawing its legs in a manner 

 which leaves it as unassailable as a tortoise or an armadillo. 



Though the Oribatidae are all eyeless, they are distinctly 

 sensitive to light, not wandering aimlessly till they reach a 

 shadow, but apparently making straight for a dark spot when 

 subjected to strong illumination. Some species have a curious 

 habit of collecting dirt and debris on their backs, so as entirely 

 to obscure the often very remarkable disposition of the spines and 

 processes with which they are furnished. 



The next two families include the animals commonly known 

 as Ticks, the largest and most familiar of the Mite tribe. Of 

 recent years they have attracted much attention as the conveyers, 

 to man and domestic animals, of certain diseases due to blood- 

 parasites (see p. 457, n.\ and our knowledge of their structure and 

 d habits has greatly increased in conse- 



quence. Hitherto they have generally 

 been considered to constitute a single 

 family, the Ixodidae, but a section of 

 them so differ from the rest as to require 

 their removal to another family, theArga- 

 sidae, so that it is necessary to employ a 

 super - family name IXODOIDEA to 



embrace the whole group. 

 Fio. 243. Capitulum of Boo- 



phiius austraiis, ventral Ticks are parasitic on mammals, birds, 



te'JuiT^af'pi?! and / e P tiles > some shewing a marked 

 i, the mandible or cheii- partiality for a particular host, others 

 bein S much more catholic in their tastes. 

 Both sexes in the Argasidae, but the 

 females only of the Ixodidae, are capable of great distension, 

 but when unfed they are all somewhat flat animals with laterally 

 extended legs and rather crab-like movement. 



All Ticks possess a small, movable "false-head" or capitulu m 

 bearing mouth-parts which are exceedingly characteristic of the 



