The Glow- Worm and Other Beetles 



tide of matter guided in its trajectory solely 

 by the motive forces and the resistance of 

 the medium traversed; it bears within itself 

 conditions which control the others. The 

 adult insect does not even enjoy the larva's 

 privileges; it cannot bend freely in all direc- 

 tions. Under its harness it is almost a stiff 

 cylinder. To simplify the explanation, we 

 may liken the insect to a section of an in- 

 flexible straight line. 



Let us return to the Sirex, reduced by ab- 

 straction to its axis. The metamorphosis is 

 effected not far from the centre of the trunk. 

 The insect lies lengthwise in the tree with 

 its head up, very rarely with its head down. 

 It must reach the outside as quickly as pos- 

 sible. The section of an inflexible straight 

 line that represents it nibbles away a little 

 wood in front of it and obtains a shallow ca- 

 vity wide enough to allow of a very slight 

 turn towards the outside. An infinitesimal 

 advance is made ; a second follows, the re- 

 sult of a similar cavity and a similar turn in 

 the same direction. In short, each change 

 of position is accompanied by the tiny de- 

 viation permitted by the slight excess of 

 width of the hole; and this deviation invari- 

 ably points the same way. Imagine a mag- 

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