150 INTRODUCTION TO BOTANY 
The strong odor of many kinds of milkweed flowers and 
the abundance of nectar which they afford bring them many 
insect visitors. On the flowers of one common milkweed ! in a 
single locality one hundred fifteen kinds of insect visitors have 
been found, including bees, wasps, flies, butterflies, and beetles. 
141. Insects as carriers of pollen. Most flowers which re- 
quire or are benefited by cross-pollination, and which are not 
wind-pollinated, depend upon insects as pollen carriers. It is 
not an over-statement to say that, 
in general, flowers seem to have 
acquired their odors and their colors 
(other than green) as means of at- 
tracting insects which may serve to 
cross-pollinate them. Insects vary 
greatly in their efficiency as polli- 
nators; the small ones with smooth 
surfaces on the head, legs, and abdo- 
men (such as ants and many beetles) 
carry little pollen, while bees, moths, 
ie 198 Gdes autase we “One butterflies often carry considera- 
body of a bumblebee, to the ble quantities. As already suggested, 
hairs of which many pollen insects are led to visit flowers in 
masses of green milkweed order to get pollen ornectar. Almost 
(Acerates) are clinging ‘ ; 
geareruate any insect can obtain pollen from 
flowers of the ordinary type, but the 
nectar seekers are frequently provided with a very long sucking 
tube, or proboscis. The honeybee (fig. 130) and the sphinx, 
or hawk, moth (fig. 131) are good examples of nectar-sucking 
insects ; and the sphinx, with its slender sucking tube, often 
many inches in length, is especially well equipped for getting 
nectar from narrow corolla tubes. Cucumbers grown under 
glass afford a good practical illustration of the importance of 
insect visits; it is found necessary to keep hives of bees in 
the cucumber houses in order to insure pollination, fertili- 
zation, and consequent crops of cucumbers. 
1 Asclepias verticillata, 
