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. Inthe Zvolution 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 
1 Tetrapneumones or Mygalomorpha, with four lung-books and 
no trachez ; the fangs of the cheliceree move vertically, 
parallel to each other, e.g.— 
Mygale, a large lurking spider which has been known to 
kill small birds, but usually eats insects; Atypus, Cleniza, 
and others make neat trap-door nests. 
z. Dipneumones or Arachnomorpha, with two lung-books and 
trachece as well; the fangs of the chclicereze move somewhat 
horizontally toward each other. 
The web-spinners, eg. Hfezra; wolf-spiders, e.g. Lycosa, 
Tarantula, the latter with poisonous qualities which have 
been much exaggerated ; jumping spiders or Attide, e.g. 
Altus salticus. The common house spider is Tegenarza 
domestica; the commonest garden spider is petra 
diademata. Agyroneta aguatica 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 débris. 
The abdomen is fused with the cephalothorax, but there is sometimes 
a clear boundary line; both are unsegmented except in Of¢/oacarus, 
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 
tracheze 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 tracheze. Cheese- mite (Zyroglyphus). Itch - mite 
(Fig. 196) (Savcoptes scabtec), causing “itch” in man; S. 
