92 



GENERAL ZOOLOGY 



unable to pollinate themselves. It is now clear that the 

 color, scent, nectar, and in many cases the form of flowers 

 have been developed in connection with their insect visitors. 

 The insects most concerned in the pollination of flowers are 

 flies, butterflies, wasps, and bees. 



Some insects visit flowers for the sake of the nectar. Pol- 

 lination results from the insect brushing itself against the 

 pollen-bearing organs and subsequently rubbing this pollen 

 on to the pistil in a neighboring flower in the search for 

 nectar. But the case of Pronu'ba (Fig. 53) is somewhat 



different. Pronuba is a 

 white moth a little over 

 a centimeter (about half 

 an inch) long, which 

 lives in the flower of 

 the yucca, or Spanish 

 bayonet, a familiar plant 

 of the dry southwestern 

 plains. During the day 

 the female remains 

 quiet, but at dusk (in 

 the breeding season) she 

 begins laying her eggs within the pistil of the flowers, among 

 the ovules, which, when the flower is fertilized, are to grow 

 into seeds. Upon these seeds the larva of the Pronuba will 

 feed. If this were all, there would be no peculiarity deserving 

 of mention; but the female goes a step farther and makes 

 sure of a supply of seeds for the larva by collecting pollen 

 from the stamens and thrusting it into .the pistil. The ad- 

 vantage to the larva is obvious, since its supply of food is 

 rendered certain ; the advantage to the plant probably lies in 

 the fact that not all the seeds thus provided for are eaten by 

 the larva before reaching maturity. This association may be 

 cited as an illustration of symbiosis. 



FIG. 53. Pronuba Moth. Natural size. 

 (After Riley) 



