EVOLUTION OF INTERSPECffiS INTEGRATION AND ECOSYSTEM 



703 



todes increase from the elasmobranchs to 

 the mammals, indicating specialization 

 parallel to that of the hosts. 



Cyclomorphic populations with two hosts 

 are also found among herbivorous insects. 

 For example, the aphid (Ilormaphis hama- 

 melidis) causing the cone gall of the witch 

 hazel {Hamamelis virginiana) has various 

 species of birches for alternate hosts 

 (Betula nigra, B. papijracea, B. spinosa). 

 The winter eggs of this aphid are laid 



causing the spiny gall of the witch hazel 

 also has various species of birches for alter- 

 nate hosts (Betula alba, B. fontinalis, B. 

 nigra, B. papxjracea, B. pendula, B. pti- 

 mila). Both the witch hazels (Hamame- 

 lidaceae) and the birches (Betulaceae) are 

 ancient types of plants. 



The two species of aphids, although clas- 

 sified in different genera, have a close 

 morphological relationship and similar life 

 cycle. These species exemplify cyclic iso- 



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^ ^y A 



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s.'-:-' 





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Fig. 250. Life cycle of a tapeworm (Diphyllobothrium oblongatum) . The eggs (1-3) 

 develop a coracidium (4), which hatches in the water. Diaptomus oregonesis (5) eats the 

 coracidium and develops a procercoid (5A). Herring or minnows (6) eat the Diaptomus, 

 and plerocercoids (6A, 6B, 6C) develop and encyst on the stomach wall or mesenteries. 

 Infected fish are fed to young birds by their parents, and the mature tapeworms (7A, 7B, 

 7C) develop and are shed (7) into the water with the feces. (From Thomas.) 



on the twigs of the witch hazel. The stem 

 mother hatches from one of these eggs 

 in the spring and attacks the lower sur- 

 face of the leaves. Her continuous secre- 

 tory stimulation causes the cone-shaped 

 gall to develop on the upper surface. 

 The generation produced in these galls 

 migrates to birches upon which a num- 

 ber of generations are produced that 

 differ markedly from each other. The forms 

 and the number of generations seem to be 

 fixed genetically and are not modified by 

 the environment (see pp. 123, 347). In the 

 fall, a generation that migrates to the witch 

 hazel produces a wingless sexual genera- 

 tion, the females of which lay winter eggs. 

 The aphid (Hamamelistes spinosiis) 



lation (p. 616), the sexual generation of 

 Hormaphis occurring in August through 

 October, and that of Hamamelistes in June. 

 Mordvilko (1928, 1935) thinks that the an- 

 cestors of these aphids first evolved in sub- 

 tropical regions where birches were lacking, 

 and that the witch hazel spread north and 

 the birches south. When the two plants 

 came to live in the same region, the life 

 cycle of the aphids as we see it today could 

 have arisen. 



Parasitic fungi like the rusts (Uredina- 

 les) often have complicated life cycles 

 that include alternate hosts (pp. 614, 643). 

 The white pine blister rust has a uredo 

 stage on currants and gooseberries. 



The adaptations of cyclomorphic species 



