VERMES, FLY LARVAE, AND ECHINODERMS 175 



warrant a definite conclusion on this point. Orientation 

 may be the result of a response regulated by the absolute 

 difference of intensity on opposite sides, that is, it may be 

 due to the action of light owing to continued intensity 

 rather than to its action owing to change of intensity. 



2. Blowfly Larvae — Musca sp.{?) 



a. Introduction. — The reactions of the blowfly larvae 

 to light were described by Loeb in 1890. He exposed the 

 larvae in front of a window in diffused and direct sunlight 

 and found them to be negative and to orient very accurately. 

 He says (1905, p. 57), " They crept with mathematical pre- 

 cision in the direction of the rays. When a shadow was 

 thrown on the board by a penholder, it could be noticed 

 that the animals moved away from the light in a direction 

 exactly parallel to the edge of the shadow. . . . They 

 acted as though they were impaled on the ray of light which 

 passed through their median plane. When I turned the 

 board around, the animals immediately turned about also,^ 

 and again placed their median planes in the direction of 

 the rays." Loeb, assuming that the method of orientation 

 in this form is the same as it is in others, concluded that it 

 is controlled by the same factors. In this form as in others 

 light acts constantly as a directive stimulus and " the main 

 feature ... is the fact that symmetrical points of the 

 photosensitive surface of the animal must be struck by the 

 rays of light at the same angle." (1897, p. 440), " Ich 

 glaiihe jetzt, dass hier eine vollkommene Analogie der Licht- 

 und Stromwirkungen zu Tage tritt, derart, dass audi, wie beim 

 Strom, die Lichtintensitdt dauer?id die Spannung der Muskeln 

 heeinflusst, dass aber die Steilheit der Intensitdtsschwankung 



die Fortleitung der Spanmmgsdnderiing bestimmt 



Das Wesen der Orientirung fasste ich dahin auf, dass bei 

 vollendeter Orientirung Symmetriepimkte der Oberfldche des 

 Thieres unter gleichem Winkel von den Lichtstrahlen getroffen 

 werden," According to Loeb then, if the position of the 



