240 MANUAL OF ZOOLOGY. ■ 



rudimentary jaws or stilets. The abdomen is undeveloped, 

 and there are four pairs of rudimentary legs. They esdiibit 

 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. 78, V), of which the Cheese-mite may 

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

 for walking, and the mouth is provided with distinct mandibles. 

 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 en- 

 ables them to pierce the skin, and retain their hold firmly. 'In 

 the Hydrachnidce (fig. 78, f), or Water-mites, the head is fur- 



Fig. 78.— Arachnida. a Pycnogonum Hitorale; b Tetra7tychus ielariTis, one of the 

 "Sociable" mites; c Hydrachna globuluSf one yf the "Water-mites." 



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

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

 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 {Ganasidte) live upon plants; the 

 Wood-mites {Oribatida) and Harvest-ticks {Leptidm) are to be 

 found amongst moss and herbage, or creeping upon trees or 

 stones; whilst the true Ticks {Ixodida) aXts.ch themselves para- 



