§ 5] THERMOTAXIS 261 



perature ranges from 24° to 28° C. This temperature is thus 

 the optimum temperature for Paramecium, the temperature 

 towards which it tends to move when the extremes are offered 

 to it. Using another nomenclature, we may say, Paramecium 

 is attuned to a temperature of 24° to 28° C.,* and tends to keep 

 in the temperature to which it is attuned. 



Similar results have been obtained by de Wildemann ('94) 

 from Euglense which were kept in damp sand and in the dark, 

 in a horizontal test-tube warmed at one end. Under these 

 conditions they migrated towards the temperature of 30° rather 

 than that of 15 to 22°. 



Finally, we may consider thermotaxis as it is revealed in the 

 higher animals. Loeb ('90, p. 43) enclosed the larvae of the 

 bombycid moth Porthesia in an opaque box, one end of which 

 was next the stove. The animals moved to the warmer end of 

 the box. The migration differed, however, from migrations 

 with reference to light in that the body was not definitely 

 oriented with reference to the source of heat, but the larvffi 

 wandered thither. Similarly some ants (Formica sanguinea) 

 are thermotactic according to Wasmann ('91, p. 22); and the 

 cockroach (Graber, '87, p. 254) moves towards that tempera- 

 ture which is more nearly normal for it. Graber ('83, p. 230) 

 has likewise shown that the salamander Triton is similarly 

 responsive. Thus some Metazoa as well as Protista are clearly 

 thermotactic. 



Looking now for the cause of thermotaxis, we see at the out- 

 set that it is necessary to distinguish between two possibilities : 

 a movement towards a greater or less intensity of heat, and a 

 movement with reference to the direction of the heat rays in 

 radiant heat. Now we have seen in earlier chapters, in con- 

 sidering the action of gravity, the electric current, and light, 

 that these agents determine the direction of locomotion by 

 determining the orientation of the axis of the body; and since 

 radiant heat passes in lines, it might be possible to have a 

 similar effect here. But there is no evidence that radiant 

 heat acts here. In the case of the Myxomycete, it is clear that 



* Adequate control experiments with dead Paramecia and fine suspended 

 particles demonstrated that the movement was not purely passive ; i.e. due to 

 currents in the water. 



