Chap. VI] 



ANIMALS 



1 33 



sterile, depends on Pronuba yuccasella for its fertilization (Fig. 69). As 

 the insect is just as dependent on the Yucca for its multiplication, it is 

 difficult to say which of the two organisms determines the geographical 

 distribution of the other. 



The process of pollination in the Yucca is extremely peculiar. The moth lays 

 its eggs in the ovary, in which the larvae have to develop at the expense of 

 the } T oung seeds. In order to render the development of the seeds possible, the 

 moth effects pollination by introducing pollen into the stigma. Since many seeds 



Fig. 69. Yucca filamentosa flower and (after Kerner) the moth (Pronuba yuccasella). 



Natural size. 



are produced and only a few larvae, both organisms profit equally. Other species 

 of Yucca are pollinated by other species of Pronuba ; for example, Yucca Whipplei 

 in California by Pronuba maculata, Yucca brevifolia in the Mohave desert by 

 Pronuba synthetica. 



The dependence of certain plants on their pollinators is also strikingly 

 exhibited in red clover. This plant is pollinated exclusively by humble- 

 bees. In New Zealand there are no humble-bees, and red clover therefore 

 remains sterile. In recent times, therefore, humble-bees have been im- 

 ported to New Zealand in order that pollination and consequent seed- 

 formation may take place 1 . 



Further observations, especially in any countries that have remained 



1 Belt, II. 



