376 Eelatiok of Water to the Behatioe of 



much because of darkness as because of their positive phototropism for faint 

 light. "Wilson (1891) shows that Hydra is negative to bright light and positive 

 to dim light. According to McGinnis (1913), Brwnchipus serraius is positively 

 geotropic in light and negatively geotropic in darkness; darkness rather than 

 light may furnish the stimulus to this reversal. In studying the reactions of 

 Drosophila to gravity. Cole (1917) finds that the response to gravity is much 

 less marked in flying than in creeping, where it is very definite. 



Many animals orient in the field in relation to the center of gravity of the 

 earth. Loeb (1905) found that some animals turn their heads upward and 

 others downward. To this latter class belongs the garden spider, which he 

 found may hang in this position in the center of its web for hours. He dis- 

 covered the same behavior in some diptera. Shelford (1917) states that such 

 animals as the grasshopper usually orient with the head up, while aphids and 

 katydids orient with the head down. In the potato beetle the majority of 

 larvae and a large number of adults orient with the dorsal side down. 



There is a vast amount of literature dealing upon reversibility in photo- 

 tropism through chemical agencies. Loeb (1893 and 1904) proves that it was 

 possible to reverse the reactions of a large number of water forms through 

 chemicals such as salts, acids, and the like. According to Moore (1913a, 19136, 

 and 1913), phototropism in Daphnia and Diaptomus may be influenced through 

 the agency of caffein, strychnin, atropin, acids, alcohol, and ether. Moore 

 (1913) says: 



" While negative phototropism ia Diaptomus can be reversed by acids, but 

 positive phototropism brought about by chemical means can not be reversed by 

 strychnin (atropin or caffein)." 



Wolfgang (1913) determines that electrolites influence phototaxis, and 

 Kanda (1914) reversed geotropism ia Arenicola larvae by means of salts. 



EXPERIMENTS UPON THE ROLE OF WATER IN GEOTROPISM. 



On May 15, at 8 p. m., 31 freshly emerged beetles (Tucson A, g. I) were 

 moved to a constant-temperature room and tested 10 times as to their reactions 

 to gravity, and in each test all were negative. These geotropic reactions were 

 tested in the dark, and if the beetles crawled to the top of the tube, when held 

 in a vertical position, they were considered as positive and if they crawled to 

 the bottom as negative. The thermograph tracings showed a constant tempera- 

 ture of 31° C, with a daily variation of 1° C, throughout the test. Again at 

 11 a. m. on May 16, when tested as previously (10 times), they were still 

 negative to gravity, and at this time weighed 3.0009 grams; furthermore, on 

 May 17 at lO*" 30"" a. m., they weighed 1.9394 grams, and again gave the same 

 test to gravity, so that these results proved that the beetles under these con- 

 ditions were uniform for this reaction. For experimental purposes these insects 

 were divided into three groups. The first group of 7 adults was put into a 

 calcium-chloride chamber, which produced so high a rate of evaporation as to 

 desiccate them; the second of 7 individuals was subjected to a low rate of 

 evaporation by placing wet filter-paper under the bell- jar, so that little water 

 was removed from them under these conditions ; in the third chamber 7 adults 



