PHYSIOLOGY AND BEHAVIOR OF THE ARTHROPODA 343 



Stimulus is best described as navigation, locomotion at some fixed angle 

 with respect to the stimulus, which acts as a landmark. Many swimming, 

 walking, and especially Hying organisms use landmarks as a means of 

 staying on some particular course. An ant will use a tall tree or a house 

 as a prominent distant object for navigation. If while walking away 

 from the nest the tree was on the left and a little in front, it follows 

 that the proper route home is one that places the tree on the right and 

 a little behind. Navigation is obviously useful to organisms that have 

 homes from which they make journeys, and to organisms that migrate, 

 but it is also useful in many other situations. An insect flying a random 

 path will retrace its path over and over, covering little new territory, 

 while an insect able to fly in straight lines will cover much more ground 

 in its search for food. 



A landmark is useful in proportion to its distance. The relation 

 between a moving organism and a nearby object changes rapidly, render- 

 ing such objects useless as guides. Even with more distant landmarks, 

 the relation changes slowly. This is particularly true in the case of 

 flying insects that may move several miles. The most distant objects are 

 the most useful guides and the most distant of all are the sun and moon, 

 so far away that their relationship to a moving organism remains es- 

 sentially constant except for the rotation of the earth. Perhaps for this 

 reason, the sun or moon is used preferentially as a landmark if it is 

 visible. Night-flying moths use the moon for navigation, particularly to 

 cross open stretches, and bees foraging for nectar and pollen several 

 miles from the hive use the sun. 



During the day, of course, the sun moves across the sky, and the 

 bees must continually make allowance for its movement. Experiments 

 have shown that bees captured in the field and imprisoned for one or 

 two hours do not correct for this movement, but when released fly off 

 using the original bearing with the sun and consequently miss the hive 

 by some distance. They do get close enough to recognize the surround- 

 ings, however. Apparently no adjustments are made in the field, but 

 back at the hive, in more familiar surroundings, the bees do allow for 

 movement of the sun. They take a new bearing each time they leave the 

 hive. 



Civilization has added an ecologic artifact to the world of night, 

 tricking many night flyers into suicidal behavior. The moon is no longer 

 the only light. All too often the light that comes into the view of a 

 flying insect is a street light or, to use a more poetic example, a candle 

 flame. Immediately it uses the light as a landmark, locating it, for ex- 

 ample ahead and somewhat to the left (Fig. 17.17). As the insect con- 

 tinues to fly it must turn repeatedly in order to maintain the bearing, 

 and thus follows a spiral course which will take it inevitably to the 

 light. Many nocturnal insects are unable to cope with such artifacts. 

 Their instinctive responses, which never failed with the moon, are so 

 strong that they are unable to substitute another landmark in place of 

 light. For the moth the candle flame can be the fatal end. In the case 

 of the more prosaic street light the moth will eventually settle beneath 



