204 



ENTOMOLOGY 



tion by insects. As a honey bee or other insect crawls over the flowers 

 (Fig. 255, A) to get the nectar, its legs slip in between the peculiar nec- 

 tariferous hoods situated in front of each anther.. As a leg is drawn up- 

 ward one of its claws, hairs, or spines frequently catches in a V-shaped 

 fissure (f, Fig. 255, B) and is guided along a slit to a notched disk, or cor- 

 puscle (Fig. 255, C, d). This disk clings to the leg of the insect, which 

 carries off by means of the disk a pair of pollen masses, or pollinia (Fig. 

 2 55> O- When first removed from their enclosing pockets, or anthers, 

 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 



B 



FIG. 255. Structure of milkweed flower (Asclepias incarnata) with reference to cross 

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

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



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 Asclepias cornuti, which often captures bees, flies and 

 moths of considerable size. Pollination is accomplished by a great 

 variety of insects, chiefly Hymenoptera, Diptera, Lepidoptera and Cole- 

 op tera. These insects when collected about milkweed flowers usually 

 display the pollinia dangling from their legs, as in Fig. 256. 



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 



