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ENTOMOLOGY 



these thin spatulate pollinia lie each pair in the same plane, but in a few 

 seconds the two pollinia twist on their stalks and come face to face in 

 such a way that one of them can be easily introduced into the stigmatic 

 chamber of a new flower visited by the insect. Then the struggles of the 

 insect ordinarily break the stem, or retinaculum, of the pollinium and 

 free the insect. Often, however, the insect loses a leg or else is per- 

 manently entrapped, particularly in the case of such large-flowered 

 milkweeds as Asdepias cornuti, which often captures bees, flies and 

 moths of considerable size. Pollination is accomplished by a great 



B 



FIG. 258. Structure of milkweed flower (Asdepias incarnata) with reference to cross 

 pollination. A, a single flower; c, corolla; h, hood; B, external aspect of fissure (/) leading 

 up to disk and also into stigmatic chamber; h, hood; C, pollinia; d, disk. Enlarged. 



variety of insects, chiefly Hymenoptera, Diptera, Lepidoptera and 

 Coleoptera. These insects when collected about milkweed flowers 

 usually display the pollinia dangling from their legs, as in Fig. 259. 



The details of pollination may be gathered by a close observer from 

 observations in the field and may be demonstrated to perfection by using 

 a detached leg of an insect and dragging it upward between two of the 

 hoods of a flower; first to remove the pair of pollinia and then again 

 to introduce one of them into an empty stigmatic chamber. 



Yucca. An extraordinary example of the interdependence of plants 

 and insects was made known by Riley, whose detailed account is here 

 summarized. The yuccas of the southern United States and Mexico 

 are among the few plants that depend for pollination each upon a single 

 species of insect. The pollen of Yucca filamentosa cannot be introduced 

 into the stigmatic tube of the flower without the help of a little white 



