The Glow- Worm and Other Beetles 



reasonably expect in a problem of a physical 

 nature, a problem incompatible with the ab- 

 solute accuracy of abstract truths. 



The Sirex' exit-gallery then is a wide arc 

 of a circle whose lower extremity is con- 

 nected with the corridor of the larva and 

 whose upper extremity is prolonged in a 

 straight line which ends at the surface with 

 a perpendicular or slightly oblique incidence. 

 The wide connecting arc enables the insect to 

 tack about. When, starting from a position 

 parallel with the axis of the tree, the Sirex 

 has passed gradually to a transversal posi- 

 tion, he completes his course in a straight 

 line, which is the shortest road. 



Does the trajectory imply the minimum of 

 work? Yes, under the conditions of the in- 

 sect's existence. If the larva had taken the 

 precaution to place itself in a different di- 

 rection when preparing for the nymphosis, 

 to turn its head towards the nearest point of 

 the bark instead of turning it lengthwise with 

 the trunk, obviously the adult would escape 

 more easily: he would merely have to gnaw 

 straight in front of him in order to pass 

 through the minimum thickness. But rea- 

 sons of convenience whereof the grub is the 

 sole judge, reasons dictated perhaps by 

 weight, cause the vertical to precede the 

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