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EVOLUTION OF ANIMALS 



Part V 



Fig. 30.32. The viscid lines of an orb web in close-up photograph. The viscid 

 silk collects in droplets when the tension on the basal lines is loosened. Dew 

 gathers on these and they are jeweled in morning sunlight. The web is a trap in 

 which insects are ensnared. (Photograph by Lynwood Chace. Courtesy, National 

 Audubon Society.) 



alone and are inconspicuous because much smaller than the females. In their 

 courtships the males of some species are stealthy; others are acrobatic. Many 

 of them meet a tragic end since the female finally devours her mate. 



Other Arachnids. Mites and ticks are small arachnids (Order Acarina) with 

 the head, thorax, and abdomen closely fused and unsegmented (Fig. 30.34, 

 30.35). They hatch from the eggs as active six-legged larvae that feed and molt 

 into eight-legged nymphs. These feed still more, molt and change into adults, 

 also eight-legged. Ticks and mites are similar except for certain details of struc- 

 ture and size, ticks being much the larger. In both types, a dartiike structure 

 (hypostome) below the mouth acts like an anchor when pushed into the flesh. 

 In ticks the outer surface of the hypostome is armed with recurved teeth; in 

 mites it is smooth. 



Ticks are parasites of mammals, birds, reptiles, and some amphibians. A 

 tick lays hundreds of eggs on the ground, in birds' nests and other homes of 



