3 2o8 THE JOINTED ANIMALS 



Group RICINULKI 



Tacked on to the Opiliones is a small group termed Ricinulei, which differs in 

 many important characteristics 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 integ- 

 ument, 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 

 organization, 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 characteristic, however, by which 

 the ticks may be distinguished from the Opiliones is that the abdomen never 

 presents any trace of segmentation; it is confluent with the cephalothorax, the 

 fusion between the two being so complete, that, as in the harvest spiders of the 

 group Palpatores, the anterior sternal plates of the abdomen are thrust far forward 

 between the coxae of the cephalothoracic limbs. As in all Arachnida, the mouth is 

 adapted for sucking, but the jaws are often partially united, and form, with a plate 

 termed the epistome, and the labium a beak. The epistome is often of large size, 

 and is attached to the front border of the carapace; the mandibles are either pincer- 

 like or simply pointed at the tip, forming piercing organs. The palpi, which re- 

 semble a pair of small legs, have their basal segments, or maxillae, united together 



