INSECTS IN RELATION TO PLANTS 



203 



an 



three drooping sepals forms the floor of an arched passageway leading 

 to the nectar. Over the entrance and pointing outward in a movable 

 lip (Fig. 254, /), the outer surface of which is stigmatic. An entering 

 bee hits and bends down the free edge of this lip, which scrapes pollen 

 from the back of the insect and then springs back into place. Within 

 the passage, the hairy back of the bee rubs against an overhanging anther 

 (an) and becomes powdered with grains of pollen as the insect pushes 

 down towards the nectar. As the bee backs out of the passage it en- 

 counters the guardian lip again, but as this side of the lip can not re- 

 ceive pollen, immediate close pollination 

 is prevented. Of course, it is possible for 

 bees to enter another part of the same 

 flower or another flower of the same plant, 

 but as a matter of fact, they habitually 

 fly away to another plant; moreover, as 

 Darwin found, foreign pollen is prepotent 

 over pollen from the same flower. It may 

 be added that bees and other pollenizing 

 insects ordinarily visit in succession several 

 flowers of the same kind. 



Orchids. The orchids, with their 

 fantastic forms, are really elaborate traps 

 to insure cross pollination. In some 

 orchids (Habenaria and others) the nec- 

 tar, lying at the bottom of a long tube, 

 is accessible only to the long-tongued 

 Sphingidae. While probing for the nectar, 

 a sphinx moth brings each eye against a 

 sticky disk to which a pollen mass is 

 attached, and flies away carrying the mass 

 on its eye. Then these pollinia bend down 

 on their stalks in such a way that when 



the moth thrusts its head into the next flower they are in the proper 

 position to encounter and adhere to the stigma. The orchid Angracum 

 sesquipedale, of Madagascar, has a nectary tube more than eleven inches 

 long, from which Darwin inferred the existence of a sphinx moth with a 

 tongue equally long. 



Milkweed. The various milkweeds are fascinating subjects to the 

 student of the interrelations of flowers and insects. The flowers, like 

 those of orchids, are remarkably formed with reference to cross pollina- 



FIG. 254. Section to illustrate 

 cross pollination of Iris, an, 

 anther; /, stigmatic lip; , 

 nectary; s, sepal. 



