356 PHYL UM ARTHROPODA. 



sexual selection may help to establish the brilliancy of males, and that 

 natural selection may help to keep the females plain, but that the 

 decorative and other differences between the sexes are primarily asso- 

 ciated with the more fundamental qualities of maleness and femaleness. 



Classification of Spiders. 



1. Tetrapneumones or Mygalomorpha, with four lung-books and 



no tracheae ; the fangs of the chelicerae move vertically, 

 parallel to each other, e.g. 



JMygale, a large lurking spider which has been known to 

 kill small birds, but usually eats insects ; Afypits, Cfem'za, 

 and others make neat]trap-door nests. 



2. Dipneumones or Arachnomorpha, with two lung-books and 



trachea? as well ; the fangs of the chelicera? move somewhat 

 horizontally toward each other. 



The web - spinners, e.g. Epcira ; wolf - spiders, e.g. Lycosa, 

 Tarantula, the latter with poisonous qualities which have 

 been much exaggerated ; jumping spiders or Attida?, e.g. 

 Attus salticits. The common house spider is Tegenaria 

 domcstica ; the commonest garden spider is Epeira 

 diadeinata. Agyroneta aqnatica fills an aquatic silken 

 nest with bubbles of air caught at the surface. 



Order 7. ACARINA. Mites and Ticks. 



Mites are minute Arachnoids inclined to parasitism. They occur in 

 the earth, or in water, salt and fresh, or on animals and plants. They 

 feed on the organisms they infest or upon organic debris. 



The abdomen is fused with the cephalothorax ; both are unsegmented. 

 According to the mode of life, the mouth-parts are adapted for biting 

 or for piercing and sucking. Respiration may be simply through the 

 skin ; in the majority there are tracheae with two stigmata. A heart 

 seems usually absent, but it is present in Gainasns. Many of the 

 young have only three pairs of legs when hatched, but soon gain 

 another pair. When some mites are starved or desiccated, and to some 

 extent die, certain cells in the body unite within a cyst, and are able in 

 favourable conditions to regrow the animal. 



Examples. 



(a) Without trachea?. Cheese-mite (Tyroglyphus], Itch-mite 



(Fig. 175) (Sarcoptes sca^iei], causing "itch"inman; S. cam's, 

 causing "mange" in dogs. Follicle-mite ( Demodex folli- 

 culoruvi), common in the hair follicles of man and domestic 

 animals (Fig. 174). Gall-mites (Phytoptiis\ on plants. 



(b) With trachea?. Harvest-mites (Trombidium), whose minute 



hexapcd larva? are troublesome parasites in summer on 

 insects, many mammals, and man. The so-called " red- 

 spider 5: (Tetrarhynchus fe/earius} spins webs, and lives 

 socially under leaves. Water-mites, e.g. Hydrachna on 



