MITES AND TICKS. 
241 
single claw. This group has a more extensive range than the last, being repre¬ 
sented by a number of forms in Central and South Europe, and extending even to 
the Arctic Circle. The best known family is the Phalangiidce, which is exceed¬ 
ingly rich in genera and species, and appears to be almost cosmopolitan in 
distribution. The body is often soft-skinned, small, and sometimes almost of the 
size and shape of a pea, while the legs, on the contrary, are exceedingly long and 
slender, and even thread-like. Still more curious are the members of the family 
Trogulidce, in which the integument is hard and thick, while the legs are short 
and stout, and the front part of the head is produced forwards on each side into 
a distinct plate, meeting its fellow of the opposite side to form a hood, hollowed 
out below, and concealing the jaws and moutli-parts. 
Group Ricinulei. 
Tacked on to the Opiliones is a small group termed Ricinulei, which differs 
in many important characters from the harvest-spiders. The mandibles, for 
instance, consist of only two segments, and the palpi of only four (five with the 
maxilla). Moreover, the anterior part of the carapace is furnished with a movable 
hood, or cucullus, completely concealing the mouth; and the abdomen consists of 
only five segments. The legs are short, stout, and have two minute claws. 
Considering the differences in structure presented by the various groups of 
Opiliones, it is not surprising that corresponding differences occur with respect to 
habits. The species with short, stout legs, and relatively heavy bodies, like 
Trogulus and Stylocellus, are very sluggish, Trogulus lifting its legs one at a time, 
and with apparent effort, and at the slightest danger ceasing all movement. This 
immobility, coupled with the protective covering of earth that adheres to its 
integument, conduces to the creature’s safety by enabling it to escape observation. 
The Palpatores and Laniatores, with their long slender legs and light bodies, 
are much more active, and run off with speed when alarmed. Apart from the 
agility which it confers, the extreme length of limb possessed by these Phalangiidce 
stands them in good stead by enabling them to stand on tiptoe and out of reach 
when threatened with destruction from armies of ants, which in tropical countries 
kill and devour every creature small enough to be overcome by numbers. 
The Mites and Ticks, —Order Acari. 
The mites and ticks constitute a group which, for diversity of structure, 
number of species and individuals, and minuteness of size, has no equal in the class. 
Many are wholly parasitic in habit, and have become so profoundly modified in 
organisation, and their affinities with the rest of the Arachnida so masked by 
degeneration, that some authors have proposed to remove the Acari into a class by 
themselves. Nevertheless, most of the species which lead a free life and have 
departed least from the type of structure characteristic of the Arachnida, show so 
many points of resemblance to the Opiliones, that it is by no means easy to draw 
a hard-and-fast line between them. One leading character, however, by which the 
ticks may be distinguished from the Opiliones is that the abdomen never presents 
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