396 CLARENCE HAMILTON KENNEDY. 



shaded stream when they started to oviposit. So by these 

 reversed reactions guttulata is able to occupy a habitat that the 

 normal members of the genus Ephemera are not able to occupy, 

 one that is ecologically outside of the general habitat of the genus. 



Certain experimental studies in the behavior of mayfly nymphs 

 point to possible explanations of some of this series of reactions. 

 Wodsedalek 2 has demonstrated that the nymph of Heptagenia 

 is negatively phototropic. Also that COo in the water will make 

 a nymph which has been negatively phototropic, reverse its 

 reaction and become positively phototropic. It is then possible 

 that the change in reaction at the time of emergence, when the 

 nymph ceases to burrow and swims up to the light, is due to an 

 accumulation of CO 2 in the nyrnph after its tracheal system has 

 detached itself from the gills in the last nymphal ecdysis. The 

 shedding c.f the skin actually starts by a general loosening of the 

 chitin several hours before the final emergence occurs. During 

 this time, if the gills are early detached, much CO 2 could accumu- 

 late in the tissues, enough eventually to reverse the phototropism 

 and so cause the nymph to rise to the surface. 



On emergence the nymph fills its tracheae with air and simul- 

 taneously becomes negatively phototropic, so that it flies toward 

 dark land. It appears to retain this reaction until the next 

 evening at dusk or for about twenty hours when it mates and 

 the female at once becomes positively phototropic and flies 

 towards the light surface of the open lake. During the intense 

 nuptial dance her tissues have been accumulating COg and in 

 some way the sexual orgasm overbalances the condition, giving 

 the acid condition full sway. 



The experiments of Alice and Stein 3 show that the reactions 

 are not as simply explained as they have been sketched in the 

 preceding paragraphs. The actual intensity of the light probably 

 also figures in some of the reactions. Krecker's work 4 in the 

 subimagoes of Hexagenia, a close relative, shows that these, in 

 spite of flying to the dark land, are positive to certain bright 



2 VVodsedalek, " Phototactic Reactions and their Reversal in the Mayfly N'ymphs 

 of Heptagenia inter punctata, (Say)." BIOL. HULL., Vol. 21, pp. 265-271, 1911. 



'Alice and Stein, "Light Reactions and Metabolism in Mayfly Nymphs," 

 Jour. Exp. Zoo/., Vol. 26, pp. 423-458, 1918. 



4 Krecker, "Phenomena of Orientation Exhibited by EphemeridaV' BIOL. 

 BULL., Vol. 29. pp. 381-388. 1915. 



