206 



ENTOMOLOGY 



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 successful as the normal method. 



Why does the insect do this ? The little nectar secreted at the base 

 of the pistil appears to be of no consequence, at present, and the stigmatic 

 fluid is not nectarian; 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. Ordinarily it is 

 the flower which has become adapted to the insect, which is enticed by 



FIG. 258. Pronuba yuccasella, fe- 

 male, gathering pollen from anthers of 

 Yucca. Enlarged. 



FIG. 259. Pronuba moth ovipositing in flower of 

 Yucca. Slightly reduced. 



means of pollen or nectar, but here is a flower which though entomo- 

 philous in general structure has apparently adapted itself in no way 

 to the single insect upon which it is dependent for the continuance of its 

 existence. More than this, the insect not only labors without compensa- 

 tion in the way of food, but has even become highly modified with refer- 

 ence to the needs of the plant, its special modifications being unparal- 

 leled among insects with the exception of bees, and being more puzzling 

 than the more extensive adaptations of the bees when we take into con- 

 sideration the impersonal nature of the operations of Pronuba. Further 

 investigation may render these extraordinary interrelations more in- 

 telligible than they are at present. 



