CoNGDON, Reactions to Light. 321 



by jerky and disorganized movements. Pearl and Cole ('01) have described a 

 like photokinetic effect in a variety of animals which they subjected to the light 

 of a projection lantern. A leech, a nemertean worm and a small crustacean are 

 rendered especially active by strong light until they show exhaustion by sluggish- 

 ness and insensibility to tactile stimuli. Holmes ('05a) finds that Ranatra acts 

 like Daphnia in strong light, yet when it has been in the dark for a time it is not 

 only sluggish but negative. The amphipod Orchestia which lives under drift 

 seaweed is negative for a time when exposed to daylight, but turns positive much as 

 does Ranatra. The beach flea Talorchestia, though a nocturnal animal, gives a 

 positive response as strong as any that has been recorded. Ranatra also responds 

 in a positive way with great vigor. The positivity of these dark-loving animals 

 requires explanation. 



Increase of temperature hastens a change of Ranatra in a positive direction and 

 accentuates the positive response when it is already present. Dipping into water 

 gives a negative reaction which is probably a contact effect. Holmes ('01) dis- 

 covered that the immersion of certain terrestrial amphipods will also effect a reversal. 



Labidocera was found by Parker to behave toward light difl^erently from Daph- 

 nia and Ranatra. It reacts positively in diffuse light, but turns strongly negative 

 in direct sunlight. He cites a number of similar cases. The possible adaptive 

 value of the reaction does not need to be pointed out. 



Smith ('05) brings forward a reasonable explanation of the gradual change of 

 sense of response in a number of crustaceans, when subjected to a marked increase 

 or decrease of illumination. It depends upon the fact that in Gammarus annulatus, 

 as in many other crustaceans, the retinal pigment of the individual put from the 

 dark into the light migrates distally at a rapid rate for about fifteen minutes, then 

 moves more slowly for the remainder of an hour. This mechanism protects the 

 more sensitive parts of the eye from over illumination. A large part of a group of 

 animals subjected to strong light change their response within fifteen minutes. 

 At the end of an hour nearly all will be positive. A possible relation between pig- 

 ment migration and photic response is evident. 



diurnal migration. 



Parker and Esterly contribute to the explanation of the movements of plank- 

 ton Crustacea and Harper of insect larvae from their nocturnal position at the sur- 

 face of the water to greater depths during the daytime. Parker ('02) concerned 

 himself with Labidocera aestiva, a typical marine plankton crustacean. He first 

 made sure that geotropism could not account for the migration by any reversal 

 through the agencies of temperature and density. Weak illumination gave a posi- 

 tive response; daylight produced a negative reaction sufficiently strong to overcome 

 the negative geotropism. He thus explains the migration: "Females rise to the 

 surface with the setting of the sun because they are positively phototactic to faint 

 light and negatively geotropic; they descend into deep water at the rising of the sun 

 because they are negatively phototactic to strong light, their negative geotropism 

 being overcome by their negative photopism. The males follow the females in 

 migration because they are probably positively chemotropic toward the females." 



A peculiarity of the method used by Esterly ('07) upon Cyclops consisted in 

 subjecting animals after a long period in the dark to a series of intensities in various 



