INSECTS IN RELATION TO PLANTS 



263 



FIG. 255. 



reference to cross pollination by insects. As a honey bee or 

 other insect crawls over the flowers (Fig. 254, A) to get the 

 nectar, its legs slip in between the peculiar nectariferous hoods 

 situated in front of each anther. As a leg is drawn upward one 

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

 fissure (f, Fig. 254, B) and is guided along a slit to a notched 

 disk, or corpuscle (Fig. 254, C, d). This disk clings to the 

 leg of the insect, which carries off by means of the disk a pair 

 of pollen masses of pollinia (Fig. 254, C). When first re- 

 moved from their enclosing pockets, or anthers, these thin 

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

 minutes 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 in- 

 sect. 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 permanently 

 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, Lepi- 

 doptera and Coleoptera. These insects when collected about 

 milkweed flowers usually display the pollinia dangling from 

 their legs, as in Fig. 255. 



The details of pollination may be gathered by a close ob- 

 server 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 re- 

 move the pair of pollinia and then again to introduce one of 

 them into an empty stigmatic chamber. 



A wasp, Sphex ichneumonea, with pol- 



Jj f milkweed attached to its le s s - 



Slightly enlarged. 



