298 AN INTRODUCTION TO ZOOLOGY 



One more illustration may be cited to show how remarkable 

 the relations between certain plants and animals may become. 

 The flowers of the genus Yucca depend upon a single species of 

 insect, the Pronuba moth, for their cross-pollination. This little 

 white moth visits the flowers in the evening. It scrapes some 

 pollen from a stamen, holds it underneath its head, and carries it 

 to another flower. It clings to the pistil of this, and, thrustii 

 its ovipositor through the wall of the ovary, lays an egg. It thei 

 mounts the pistil, and forces the pollen it has brought down int( 

 the stigmatic tube. Another egg is laid in another part of 

 ovary, and more pollen is inserted into the stigmatic tube. The 

 processes may be repeated half a dozen times in a single flower. 

 The advantage to the flower is, of course, the certainty of beii 

 cross-pollinated and of producing seeds. These seeds provide 

 supply of food for the larvae that hatch from the eggs laid by th( 

 moth in the ovary. The seeds are so numerous that the few eaU 

 by the larvae may well be spared (267). 



c. Parasitism 



A parasitic animal is one that lives upon another organisi 

 This is also a definition of a predaceous animal. We ma] 

 distinguish between the two, however, by assuming that parasite 

 are always carried on or in the bodies of their victims, when 

 predaceous animals are free-living. The fleas are commoi 

 examples of external parasites. The malaria parasite (p. 

 Fig. 42) and the round worm (p. 160, Fig. 81) are internal para- 

 sites. The life histories of these species, as given on the 

 referred to, will suffice to illustrate parasitic habits, but certaii 

 characteristics resulting from this kind of life will be pointed out 

 briefly. 



Parasites as a rule are comparatively simple in structure, 

 is due to degeneration and not to a low position in the anil 

 kingdom. In many cases the reproductive organs are exceed- 

 ingly well-developed, whereas organs of locomotion, digestion, 

 etc., become almost functionless. 



