84 RECIPROCAL NUTRITIVE DISJUNCTIVE SYMBIOSIS 



by butterflies and are pollinated by bees almost entirely. Other but- 

 terfly flowers include several species of Phlox (Fig. 33) and numerous 

 orchids. Many composites are pollinated by butterflies but they 

 are not strictly butterfly flowers since they may be pollinated by 

 various kinds of insects. 



The hawk moths, being night-flying insects, visit mostly white, 

 fragrant, night-blooming flowers. The Jimson weed ( Datura tatula) 

 and the night-blooming tobacco {Nicotiana noctiflora) are good 

 examples of haw^k-moth flowers. Not all haw^k-moth flowers are 

 white, however. The evening primrose {Oenothera biennis), for 

 example, which is yellow, is pollinated largely by these insects. 



Perhaps the most interesting of all cases of pollination by moths 

 is that of the yuccas which are pollinated by small moths belonging 

 to the genus Promiha. The flowers of the yuccas are pendulous and 

 the style hangs down farther than the stamens but it is impossible 

 for the pollen to fall from the anthers to the stigma because the 

 stigma is cup-shaped and the stigmatic portion is on the inner surface 

 only. The female moths begin work soon after sundown. Each one 

 collects some pollen from the anthers and holds it in her specially 

 constructed mouth parts. She then usually flies to another flower, 

 pierces the ovary with her ovipositor, and, after laying one or more 

 eggs, creeps down the style and stuffs a ball of pollen into the stigma. 

 It is difficult to imagine what would cause a moth to stuff pollen into 

 a stigma for one hesitates to believe that she knows what the result 

 will be. Yet this symbiotic relation is obligate for both the yucca 

 and the moth, since in the absence of the moth the yucca produces 

 no seed while without the yucca the moth cannot complete its life 

 cycle, and if the moths should fail to pollinate the yuccas the result 

 would ultimately be the extinction of both plant and insect. The 

 yucca produces a very large number of ovules. Part of these are 

 eaten by the moth larvse and the remainder mature into seeds. 



52. Fly Flowers.— Syrphid flies, and some other nectar-feeding 

 flies, visit many of the same flowers that are frequented by bees and 

 such flowers cannot, of course, be designated strictly as fly flowers. 

 Syrphid flies are especially important as pollinators of some fruit 

 trees, though these trees may also be pollinated by bees. There are 

 some flowers, however, that are pollinated almost exclusively by 

 flies (Fig. 34). Many of these are ill-smelling and attract mainly 

 carrion flies. Familiar examples are the carrion flower {Smilax 

 herbacea), the purple trillium {Trillium erectum), the skunk cab- 

 bage {Syrnplocarpus foetidus) (Fig. 35) and the water arum {Calla 

 palustris) . 



