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 l 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, 

 and butterflies often carry considera- 

 ble quantities. As already suggested, 

 insects are led to visit flowers in 

 order to get pollen or nectar. Almost 

 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. 



FIG. 135. Under surface of 

 body of a bumblebee, to the 

 hairs of which many pollen 

 of green milkweed 

 (A cerates) are clinging 



After Robertson 



