The Glow-Worm and Other Beetles 



responds on the one side with the minimum 

 of resistance to be overcome and on the other 

 with the maximum. Nothing prevents the 

 Sirex from tracing his path in any one of the 

 multitude of planes on which the path would 

 possess an intermediate value between the 

 shortest and the longest. The insect refuses 

 them all and constantly adopts the one which 

 passes through the axis, choosing, of course, 

 the side that entails the shortest path. In 

 brief, the Sirex' gallery is contained in a plane 

 pointing towards the axis of the tree and the 

 starting-point; and of the two portions of this 

 plane the channel passes through the less ex- 

 tensive. Under the conditions, therefore, 

 imposed upon him by his stiffness the hermit 

 of the poplar-tree releases himself with the 

 minimum of mechanical labour. 



The miner guides himself by the compass 

 in the unknown depths underground, the 

 sailor does the same in the unknown ocean 

 solitudes. How does the wood-eating insect 

 guide itself in the thickness of a tree-trunk? 

 Has it a compass? One would almost say 

 that it had, so successfully does it keep to the 

 quickest road. Its goal is the light. To 

 reach this goal, it suddenly chooses the 

 economical plane trajectory, after spending 

 its larval leisure in roaming tortuous pass- 

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