INSECTS IN RELATION TO PLANTS 



263 



FIG. 255. 



reference to cross pollination by insects. As a honey 1> 

 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- 

 linia of milkweed attached to its legs. 

 Slightly enlarged. 



