238 A BOOK OF INSECTS 



pellet, using her front legs — " very much as a cat does 

 when cleaning her mouth." After scraping the pollen 

 from one stamen she proceeds to another, and so on until 

 she has formed a ball which is often thrice as large as her 

 own head. This she holds firmly between her tentacles, 

 and usually, though not invariably, flies to the bloom of a 

 neighbouring plant. Here she lays one or more eggs in 

 the ovary of a flower ; and after so doing actually climbs 

 up the pistil, thrusts her pellet of pollen into the stigmatic 

 tube, and pushes it firmly home. As a result of this 

 forceful pollination, the ovules develop into seeds, some 

 of which are consumed by the larva, though plenty are 

 left to perpetuate the plant. 



Three species of Pronuba are known, each being 

 attached to a distinct species of yucca; and Professor 

 Riley's investigations lead him to believe that these plants 

 never set seed in districts where their attendant moths 

 are not found, or when the insects are excluded artifici- 

 ally. The habits of Pronuba are all the more wonderful 

 because she herself derives no benefit from what she does. 

 Her proboscis is imperfectly formed, her alimentary canal 

 is practically functionless, and she has never been seen to 

 feed. Doubtless her ancestors did so, and were first 

 attracted to the yucca in search of nectar or pollen ; but 

 to-day the actions of the moth are disinterested in the 

 fullest sense of the word. Moreover, we must not assume 

 that the moth consciously performs the act of pollination 

 for the sake of her offspring. The whole procedure is 

 instinctive. 



Quite as remarkable, and even more complicated, is 

 the interdependence of fig-trees and certain small gall- 

 wasps. In order to understand this relationship, the 

 reader must be told that a fig is really a hollow inflores- 

 cence—a chamber bearing numerous simple flowers on its 



