202 MANUAL OP ZOOLOGY. 



be taken as an example, there are four pairs of legs, adapted 

 for walking. In the Sarcoptes scabiei the cause of the skin- 



disease known as the ' itch ' 

 the two anterior pairs of legs 

 are provided with suckers, and 

 the two posterior are termi- 

 nated by bristles; the mouth, 

 also, is furnished with bristles. 

 In the Ticks (Ixodes) the 

 mouth is provided with a beak, 

 or 'rostrum,' which enables 

 them to pierce the skin, and 

 retain their hold firmly. In 

 the Hydrachnidce, or Water- 

 mites, the head is furnished 



Fig. 58. Acarina. Tetranychus telarius, a with two Or four OCelli, and 

 < viable ' Mite, much enlarged. Qf 



^ 



natatory legs. They are parasitic, during at least a portion of 

 their existence, upon Water-beetles and other aquatic insects. 

 They pass through a metamorphosis, the larva being hexapod, 

 or having only three pairs of legs. 



Another member of the Acarina is the curious little Demodex 

 folliculorum, which is found in the sebaceous follicles of man, 

 especially in the neighbourhood of the nose. It is probable 

 that very few, if any, individuals are exempt from this harm- 

 less parasite. 



ORDER III. ADELARTHROSOMATA. The members of this order, 

 comprising the Harvest Spiders, the Book-scorpions, &c. are 

 distinguished from the preceding by the possession of an abdo- 

 men, which is more or less distinctly segmented, but generally 

 exhibits no line of separation from the cephalo thorax, the two 

 regions being of equal breadth and conjoined together. The 

 mouth is furnished with masticatory appendages, and respira- 

 tion is effected by tracheae, which open on the lower surface of 

 the body by two or four stigmata. 



Family 1. Phalangidce. The well known ' Harvest- spiders' 

 belong to this family. They are characterised by the great 

 length of the legs, and by the filiform maxillary palpi, termi- 

 nated by simple hooks. 



Family 2. Pseudo-scorpionidce (Cheliferidse). The 'Book- 

 scorpion' (Chelifer) is a common little animal in old books. 

 It is distinguished by the fact that the maxillary palpi are of 

 large size, and are converted into nipping-claws, or chelae, thus 

 giving the animal the appearance of a Scorpion in miniature. 



Family 3. Solpugidce. In this family the abdomen is not 

 only very distinctly segmented, but is also clearly separated 



