330 ARACHNOIDEA AND PALMOSTRACA. 



has spoken of the frequent brilliancy of males as due to their greater 

 vitality, and refers the relative plainness common in females to their 

 greater need for protection. Darwin refened the greater decorativeness 

 of males to the fact that those which varied in this direction found 

 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 asso- 

 ciated with the more fundamental qualities of maleness and femaleness. 



Classification of Spiders. 



1 . Tetrapneumones, with four lung-books and no tracheEe, e.g. — • 



Mygale, a large lurking spider which has been known to kill 

 small birds, but usually eats insects ; Atypus, Cteniza, and 

 others make neat trap-door nests. 



2. Dipneumones, with two lung-books and trachea; as well, such as — 



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. 

 Atlus salticus. The common house spider is Tegenaria 

 domestica ; the commonest garden spider is Epeira diade- 

 mata. Argyroneta aquatica fills an aquatic silken nest 

 with bubbles 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 Gamasus. 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 



(Sarcoptes scabiei), causing "itch" in man; S. cams, 



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



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



animals. Gall-mites (Phytoptus), on plants. 



