226 MANUAL OF ZOOLOGY. 



rudimentary jaws or stilets. The abdomen is undeveloped, 

 and there are four pairs of rudimentary legs. They exhibit 

 no traces of either circulatory or respiratory organs, and the 

 sexes are united in the same individual. 



Family 3. Acarida. This family includes the Mites, Ticks, 

 and Water-mites, some of which are parasitic, whilst others 

 are free, and some are even aquatic in their habits. The mouth 

 is formed for suction. There is no definite line of demarcation 

 between the unsegmented abdomen and the cephalothorax. 



In the true Acari (fig. 75, b\ of which the Cheese-mite may 

 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 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 Hydrachnida (fig. 75, c], or Water-mites, the 



Fig. 75. Arachnida. a Pycnogonum littorale ; b Tetranychus telarius, one of the 

 " Sociable" mites ; c Hydrachna globulus, one of the " Water-mites." 



head is furnished 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. 



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 



