322 JAMES W. McCOLLOCH AND H. YUASA 



certainly is an adaptation, a process distinctly advantageous in 

 the life economy of the insect. As to the nature of the stimulus 

 or stimuli in response to which the larvae manifest the migratory 

 behavior, experimental data are lacking, but from the nature of 

 the case, this phenomenon of migration might be interpreted as 

 the result of positive thigmotropism and also possibly of posi- 

 tive geotropism. It is interesting to note that Enock (1891) 

 found that the larvae moved towards the tip of the leaf when 

 the eggs were deposited in an inverted position but he failed 

 to notice the orientation of the larvae on hatching. Gossard 

 and Houser (1906, pp. 4-5) seemed to have had difficulty in 

 making the larvae ascend a slope of about 45 degrees. In the 

 present work, however, it was found that the larvae are not 

 only able to ascend an inclined leaf (to the height of 139 mm., 

 in one case) standing almost perpendicularly, but they do so 

 regularly if the eggs are laid in an inverted position. 



The reason quoted by Packard (1883, p. 212) why the red 

 wheat is less injured by the fly than the white wheat needs 

 revision, because the sloping leaf has nothing directly to do 

 with the larvae working down " the wrong way." Whether the 

 larvae are assisted by dew in their migration down the leaf 

 blade, as suggested by Headlee and Parker (1913, pp. 95-96), 

 needs, in the writer's opinion, closer scrutiny for the data on 

 hand seem to indicate that the larvae find great difficulty in 

 overcoming the surface tension of drops of water and, further- 

 more, dewdrops do not always roll to the base of the leaf-blade 

 and stop there until the larvae are safely discharged. 



SUMMARY 



1. The direction of the migration of the larva in its initial 

 stage is predetermined by the orientation of the eggs. The 

 larvae on hatching always turn from the anterior toward the 

 posterior end of the eggs. 



2. The degree of inclination of the leaf has nothing to do 

 with the direction of the larval migration. 



3. The larvae are capable of locomotion on either an ascend- 

 ing or descending incline of anywhere between zero and 90 

 degrees. 



4. When the eggs are laid with their anterior ends toward 

 the base of the leaf, the larvae, on hatching, crawl up the leaf 



