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 débris 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 
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, the Arga- 
sidae, so that it is necessary to employ a 
super - family name — IxoDOIDEA — to 
Fig. 243.—Capitulum of Boo- embrace the whole Sehae : 
philus australis, ventral Ticks are parasitic on mammals, birds, 
ee SEO a as and reptiles, some shewing a marked 
m, the mandible or cheli- partiality for a particular host, others 
EOS digits ™ the being 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 capitulum 
bearing mouth-parts which are exceedingly characteristic of the 
