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 mouth-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 

 Troyulus 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 



VOL. vi. 1 6 



