The Problem of the Sirex 



ning along the grain, and sometimes by way 

 of the side, by means of a curved road cut- 

 ting across the grain. 



- Is the compass a chemical influence, or 

 electrical, or calorific, or what not? No, 

 for in an upright trunk the emergence is ef- 

 fected as often by the north face, which is 

 always in the shade, as by the south face, 

 which receives the sun all day long. The 

 exit-door opens in the side which is nearest, 

 without any other condition. Can it be the 

 temperature? Not that either, for the shady 

 side, though cooler, is utilized as often as 

 the side facing the sun. 



Can it be sound? Not so. The sound 

 of what, in the silence of solitude ? And are 

 the noises of the outside world propagated 

 through half an inch of wood in such a way 

 as to make differences perceptible? Can it 

 be weight? No again, for the trunk of the 

 poplar shows us more than one Sirex travel- 

 ling upside down, with his head towards the 

 ground, without any change in the direction 

 of the curved passages. 



What then is the guide ? I have no idea. 

 It is not the first time that this obscure quest- 

 ion has been put to me. When studying the 

 emergence of the Three-pronged Osmia from 

 the bramble-stems shifted from their natural 

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