372 PHYLUM ARTHROPODA. 



favour in the eyes of their mates, were consequently more successful 

 in reproduction, and thus tended to entail brilliancy on their male 

 successors. The careful researches of Prof, and Mrs. Peckham greatly 

 strengthen the position of those who believe in the efficacy of sexual 

 selection. In the Evolution of Sex it has been suggested that 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 associated with 

 the more fundamental qualities of maleness and femaleness. 



Classification of Spiders 



I Tetrapneumones or Mygalomorpha, with four lung-books and 

 no tracheae ; the fangs of the chelicerae move vertically, 

 parallel to each other, e.g. 



Mygale, a large lurking spider which has been known to 

 Jdll small birds, but usually eats insects ; Atypus, Cteniza^ 

 and others make neat trap-door nests. 



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

 tracheae as well ; the fangs of the chelicerae move somewhat 

 horizontally toward each other. 



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

 Tarantula, the latter with poisonous qualities which have 

 been much exaggerated ; jumping spiders or Attidse, e.g. 

 Attus salticus. The common house spider is Tegenaria 

 domestica ; the commonest garden spider is Epeira 

 diademata. Agyroneta aquatica 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, but there is sometimes 

 a clear boundary line ; both are unsegmented except in Opilioacams, 

 which has a segmented abdomen. 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 Gamastts. 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 tracheae. Cheese -mite (Tyroglyphus}. Itch-mite 

 (Fig. 196) (Sarcoptes scabiei}^ causing "itch" in man; S. 



