INSECTS IN RELATION TO PLANTS 263 
reference to cross pollination by insects. As a honey bee or 
other insect crawls over the flowers (Fig. 254, 4) to get the 
nectar, its legs slip in between the peculiar nectariferous hoods 
situated in front of eachanther. Asa 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 he 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- 
Sects ellie the struscles 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, 
Winicheoiten captures bees, TIES “Al wasp, Spiex ichneumonen rithenon 
emma mous O1comsiderable size, line of milkmeed attached’ ito; its lees 
Slightly enlarged. 
Pollination 1s 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. 
