6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



two symmetrically located sense organs has been prevented from func- 

 tioning, the animal no longer orients but turns toward one side when 

 stimulated. Certain flies with one eye blackened turned toward the 

 functioning eye. Parker (69) obtained similar results with a butterfly 

 {Vanessa antiopa). Loeb (44, pp. 52-61) cites the results of several 

 writers, who conducted similar tests on insects, as giving " direct proof 

 of the muscle tension theory of heliotropism in motile animals." 

 Dolley (10) experimented with Vanessa antiopa and concluded that 

 his results contradicted Loeb's " continuous action theory." According 

 to this view the tension of the muscles of the appendages on both 

 sides of the body is controlled through direct reflex arcs by the photo- 

 chemical changes produced by light in the two retinas. Dolley says 

 that these butterflies with one eye blackened can orient and can turn 

 under certain conditions toward either side, all of which indicates 

 that orientation in them is not wholly dependent upon the relative 

 intensity of light on the two eyes. The same author (11) determined 

 that J\inessa antiopa moves faster in weak light than in strong light. 

 This behavior is not in accord with the above theory. Dolley also 

 determined that these butterflies move faster in intermittent light than 

 in continuous light, which indicates that orientation in them is " due 

 to the time rate of change of intensity." 



(f) What ivave lengths stimulate insects most? — The determining 

 of this is perhaps the most difficult of all the problems encountered 

 in a study of tropisms, and much confusion has arisen while trying 

 to solve it. Many erroneous conclusions have been derived ; first, 

 because the investigators, as a rule, have had a poor knowledge of the 

 physics of color ; and second, because in most cases they have not been 

 properly equipped with apparatus to study the effects of various wave 

 lengths on insects. 



Loeb (42, p. 18) remarks that all authors who have studied the 

 behavior of plants behind screens have usually concluded that only 

 the more refrangible rays are heliotropically active. Using two colored 

 screens (red and blue), he concluded that the more refrangible rays 

 of the visible spectrum are more effective than the less refrangible 

 rays in causing orientation in animals (p. 82). He tested the cater- 

 pillars of Euproctis chrysorrhoea (pp. 29-31) with these screens, 

 which had been examined spectroscopically, and determined that they 

 reacted most decidedly to the shorter wave lengths. 



Mast (56, pp. 302-365) in 191 1 reviewed the entire subject of wave 

 lengths or colors and discussed insects in particular (pp. 343-355)- 

 He reviewed Loeb's work on animals and fails to understand how the 



/-'tik 



