516 William Morton Barrows 



The experiments recorded in this paper were undertaken to 

 determine, if possible, first, to what substances Drosophila is 

 chemotropic, and secondly, in what way the fly finds its food. 



The work was carried on in the Zoological Laboratory of Har- 

 vard University, under the direction of Prof. W. E. Castle and 

 Prof. G. H. Parker, to whom I am indebted for much valuable 

 advice and careful criticism. 



II EXPERIMENTS 



I Preliminary Experiments 



For the preliminary experiments, which were planned to ascer- 

 tain whether certain substances were stimulating or not for the 

 flies, the following apparatus was devised. A two-dram vial 

 closed at the end by a cork stopper was arranged as a trap. Pierc- 

 ing the stopper and reaching nearly to the bottom of the vial was 

 a glass tube with a caliber of about two millimeters (Fig. i). The 

 bore of the tube was large enough to allow a fly to creep through 

 easily and yet small enough to make it difficult for the fly to turn 

 around after having once started into the tube. The tube pro- 

 jected beyond the cork on the exterior about three millimeters. 

 If a fly once got halfway down the tube leading into the vial, the 

 chance of its backing out or finding its way out later was very 

 small About i cc. of the substance to be tested was placed on a 

 piece of filter paper in the vial. Five vials thus charged with 

 substances to be tested usually formed the set of traps. At least 

 one of these was always used as a control in that it contained filter 

 paper wet with distilled water only or with some other material 

 used as a check. 



The traps, with their open ends directed toward the light, were 

 placed in a vertical glass cylinder 20.5 cm. high and 17.5 cm. in 

 diameter. The bottom edge of the cylinder rested on a sheet of 

 clean filter paper and the top was closed by a glass plate. The 

 atmosphere in the cylinder was kept moist by the evaporation of 

 distilled water exposed in a small vessel Many hungry flies, 

 usually one hundred, were liberated in the glass cylinder and left 

 there for twenty-four hours. By hungry flies is meant those which 



