THE MIGRATION OF THE FLY LARVAE 321 



Influence of Moisture. — The larvae seem to prefer moist air. 

 Enock (1891, p. 335) found that " the progress of the young 

 larvae was very much accelerated when the leaf was moistened, 

 and many died on a hot, dry day." In a condition where the 

 relative humidity of the air is 50%, the larva, if it ever hatches, 

 has great difficulty in making its way down the leaf. In every 

 case under this humidity the larvae failed to move but short 

 distances and invariably died. Too much water, e.g., rain, will 

 also be detrimental for then the larvae are likely to be washed 

 away from the plant. Excessive dew may produce the same 

 result. 



Influence of Light and Darkness. — Not enough work has been 

 done to justify any statement concerning the behavior of larvae 

 under various conditions of light, but judging from the result 

 obtained in an artificial cave w T here the light is very weak, the 

 general behavior of the larvae seemed not at all modified from 

 that in the bright light. 



DISCUSSION AND CONCLUSIONS 



The most interesting thing that was found so far as this 

 study has progressed concerning the behavior of the larva, is 

 the fact of orientation immediately following hatching. Regu- 

 larity of its occurrence is significant. Possible advantages to be 

 derived from this arrangement are not difficult to see. Since 

 the eggs are laid normally with their anterior end pointing away 

 from the base of the leaf, and since the larvae emerge from 

 that end of the egg, the larvae must, under ordinary circum- 

 stances, turn round before they could possibly get down into 

 the leaf-sheath, a process absolutely necessary for the life of 

 the larvae. The orientation is therefore a distinct advantage 

 to the larvae for it helps the latter to find their way quickly and 

 properly. Furthermore, by being set in the right direction, the 

 larvae are so directed as to minimize the period of exposure to 

 the adverse conditions, for it is obvious that the sooner the 

 larvae get down into the leaf -sheath, the safer they will be 

 from the possible dangers, such as mechanical injury, attack 

 from parasites and predaceous enemies, desiccation, etc. It is 

 beyond the scope of this paper to discuss the force that is re- 

 sponsible for this phenomenon of orientation. Whatever the 

 nature of this directing force may be, the fact of orientation 



