THE MIGRATION OF THE FLY LARVAE 319 



leaf where the grooves converge into a point at the extremity 

 of the blade, it is then thrown upon its own resources in finding 

 its way. It naturally performs random movements and in so 

 doing it is likely to place itself now in inverted position in one 

 of the grooves. This opens a way for the larva to escape the 

 distracting maze of the tip of a leaf, and, after adjusting itself 

 to the groove, it starts back down the long way it has so labor- 

 iously climbed up. 



Variations in Behavior. — Although the larva is not known to 

 refuse to turn round away from the direction of the anterior 

 end of the egg, it may show individual differences or devia- 

 tions from the ordinary course of behavior during migration on 

 the leaf. Occasionally, a larva is found to cross the leaf-blade 

 from one surface to another. This may happen at any point 

 on the leaf but it usually takes place at or near the tip where 

 the larva is forced to find a new way by random movements. 

 When a larva meets an obstacle, e.g., a dirt particle, it usually 

 seeks to avoid it by moving to an adjacent groove. Sometimes 

 it may overcome the difficulty by actually crawling over the 

 obstacle, or it may be forced to carry the impediment on its 

 back, if the object is light enough to be lifted or pushed along. 

 Small drops of water may wash the larva down away from 

 the plant. A very small amount of water is found to be sufficient 

 to trap the larva which loses it hold, and in case of a droplet, 

 the maggot is not able to overcome the surface tension and free 

 itself from watery imprisonment. It is not known whether the 

 larva is capable of feeding on the leaf while migrating, although 

 it seems to be the general feeling among the entomologists that it 

 does not feed during this time. Enock (1891, 9, 334), however, 

 states, that " the larva increases in width even before it dis- 

 appears out of sight, leading one to suppose that it imbibes 

 moisture as it journeys down the furrows of the leaf." Several 

 cases of reversal of the direction of migration without apparent 

 causes were noticed. In one case the larva, hatching from an 

 egg laid on the lower side of the leaf, passed down to the stalk 

 near the point where the latter passed into the culture solution. 

 The larva turned around and started upward on the stalk which 

 was standing vertically. After moving about 20 mm. the larva 

 again reversed its direction of progress and started downward. 

 In other cases, larvae were found to climb up the central stalk 



