POLLINATION AND FERTILIZATION 151 



An idea of the number of insect visits made to some flowers 

 may be gathered from the fact that in a single locality one 

 hundred kinds of insects have been seen to frequent dandelion 

 flowers. The statistics in regard to visitors to the flowers of 

 yarrow, Canada thistle, and the willows are fully as remarkable. 



142. Odors of flowers as attractions to insects. It is evident 

 from familiar facts that many insects have an acute sense of 

 smell. The way in which flies are attracted by decaying meat 

 or fish, and bees and wasps by a cider press at work or by 

 fruit-preserving operations, is a matter of common observation. 

 A single cluster of carrion-scented flowers has been known to 

 attract carrion flies and dung beetles from a distance of hun- 

 dreds of yards. Some flowers, such as those of the Virginia 

 creeper (Psedera), the Dutchman's-pipe, the blueberries, and 

 many others, are so inconspicuous that apparently their numer- 

 ous insect visitors must be attracted by an odor which is almost 

 or quite imperceptible to us. 



It seems certain that the odors of flowers have been devel- 

 oped with reference to the sense of smell in animals (usually 

 insects), and that these odors serve as a most efficient means 

 of securing insect visits. 



It is a most interesting fact that many flowers give off their 

 scent mainly at the time of day when the insects which polli- 

 nate them are most active. Thus, some catchflies, the petunias, 

 some kinds of tobacco, and several honeysuckles have little 

 odor by day but are very fragrant at night, when the moths 

 which pollinate them are on the wing. On the other hand, 

 many plants of the Pea family, which are pollinated by day- 

 flying bees and butterflies, give off their scent mostly by day, 

 and especially in strong sunshine. 



143. Colors of flowers as attractions to insects. There has 

 been much discussion among botanists as to how far insects are 

 led to visit flowers by displays of color. It appears to be fairly 

 certain that no insects can make out the forms and sizes of 

 objects at a distance of more than six feet, and that many are 

 unable to see clearly even two feet. In spite of this, however, 



