INSECTS IN RELATION TO PLANTS 



20 5 



FIG. 257. 



the pistil and actually thrusts pollen into the stigmatic tube and 

 pushes it in firmly. The ovules develop 

 into seeds, some of which are consumed 

 by the larvae, though plenty are left to 

 perpetuate the plant itself. Three species 

 of Pronuba are known, each restricted 

 to particular species of Yucca. Riley 

 says that Yucca never produces seed 

 where Pronuba does not occur or where 

 she is excluded artificially, and that 

 artificial pollination is rarely so success- 

 ful as the normal method. 



Why does the insect do this ? The lit- 

 tle nectar secreted at the base of the pistil 

 appears to be of no consequence, at pres- 

 ent, and the stigmatic fluid is not necta- 

 rian ; indeed, the tongue of Pronuba, used 

 in clinging to the stamen, seems to have 

 lost partially or entirely its sucking power, 

 and the alimentary canal is regarded as functionless. 



FIG. 258. 



Pronuba yuccasella, fe- 

 male, gathering pollen 

 from anthers of Yucca. 

 Enlarged. 



Ordina- 



Pronuba moth ovipositing in flower of Yucca. Slightly reduced. 



rily it is the flower which has become adapted to the insect, 

 which is enticed by means of pollen or nectar, but here is a 



