530 William Morton Barroivs 



some such method of orientation as is imphed in the theory of 

 tropisms. The more usual conception of the tropism theory, as 

 advocated particularly by Verworn, is to the effect that when an 

 animal is unsymmetrically stimulated it turns until it is symmetric- 

 ally stimulated and either faces toward or away from the source 

 of the stimulus and then moves in the appropriate direction. It is 

 evident from this that symmetrical stimulation is an essential 

 feature of this theory. Loeb has extended this view in the sense 

 that he often implies that the stimulus acts directly on the loco- 

 motor organs, but I do not regard this as an essential part of the 

 tropism theory, and, as I shall show presently, this modification of 

 the theory has no application in this case. The question, then, 

 is, are the accurately directed responses of Drosophila dependent 

 on symmetrical stimulation ? If this is the case, one would natur- 

 ally turn to the antennae of this animal as the symmetrical recep- 

 tive organs of smell for such reactions. 



^ Experunejjts to Determine the Position and Function of the 

 Olfactory Sense Organs 



It is generally believed that in most insects the antennae aie the 

 seat of the olfactory organs. In some species these organs are placed 

 in pits, while in others they are exposed on the surface of the antenna. 

 Mayer ('79) described a pit in the third segment of the antennae 

 of a species of Drosophila which he considered an olfactory organ. 

 I have found that Drosophila ampelophila has a large sac-like pit 

 situated in the end of the terminal (third) segment of the antenna, 

 which contains sense cones. Fig. 5 shows a front view of the head 

 of Drosophila and the position of this pit in the left antenna. The 

 following experiments on flies which had been deprived of the 

 third segments of their antennae show" that without doubt the 

 olfactory organ in Drosophila is located in this segment. 



It was found by repeated trials that the antennae could not be 

 satisfactorily covered with gum to keep out the stimulating odor, 

 nor could they be burned off without considerable injury to the fly. 

 The method finally employed w^as to place a fly, already etherized, on 

 its back on a glass slide and to hold it down with a small camel's- 



