26O MANUAL OF ZOOLOGY. 



Besides the Cheese-mite (A. domesticus], another well-known 

 species is the Acarus destructor, which feeds upon various zoo- 

 logical specimens, and is very annoying to the naturalist. 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 terminated 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. (fig. 99, c\ or Water-mites, the head is fur- 

 nished with two or four ocelli, and there are four pairs of hairy 

 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. The Garden-mites (Trom- 

 bididce) and Spider-mites (Ganasida) live upon plants; the 

 Wood-mites ( Oribatidce) and Harvest-ticks (Leptida) are to be 

 found amongst moss and herbage, or creeping upon trees or 

 stones ; whilst the true Ticks (Ixodidce) attach themselves para- 

 sitically by means of their suctorial mouth to the bodies of 

 various Mammals, such as sheep, oxen, dogs, &c. Several 

 Mites (Thalassarachna, Pontarachna, &c.) have been found to 

 inhabit salt water, and several species of Trombididcz live habit- 

 ually between tide-marks. 



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 harmless 

 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 

 abdomen, which is more or less distinctly segmented, but 

 generally exhibits no line of separation from the cephalothorax, 

 the two regions being of equal breadth and conjoined together. 

 The mouth is furnished with masticatory appendages, and 

 respiration is effected by tracheae, which open on the lower 

 surface of the body by two or four stigmata. 



Family i. Phalangidcz. 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, terminated 

 by simple hooks. 



Family 2. Pseudoscorpionida (Cheliferidcz). The "Book- 

 scorpion" (CJielifer) is a common little animal in old books. 

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



