114 INSECT ARTIZANS AND THEIR WORK 



necessary that a slight radiation of all should take 

 place, but how this is accomplished by grubs that 

 are entirely shut off from communication is some- 

 thing of a mystery. The explanation is, probably, 

 that each grub can hear the action of its neighbour's 

 jaws transmitted through the cells of the bark and 

 so can judge what is the proper line for its own 

 excavation to take. 



The Bark Beetle is quite small not more than a 

 quarter of an inch long and black or brown in 

 colour. Its smallness would appear to indicate an 

 insignificant enemy to the timber grower, espe- 

 cially as it never penetrates deeply into the wood. 

 Its burrows are half in the bark and half in the 

 surface of the sap-wood. The completion of the 

 work of the grub is made evident by the appearance 

 of a number of what appear to be shot-holes in 

 the bark. These show that the insect having lain 

 for a time as a chrysalis in the broader end of its 

 burrow has changed to a beetle and eaten its way 

 out through the bark. The combined work of the 

 one brood extends over a space of about six inches 

 by four, and the result has been to kill a patch of 

 bark of that size, so that sooner or later it separates 

 from the sapwood and kills that too, by stopping 

 the downward flow of the sap at that place. 



There is some difference of opinion as to whether 

 these beetles attack trees that are really healthy, 

 or whether a certain condition of sickliness is not 

 necessarily precedent to the success of the grubs. 

 If the circulation of the tree were good, the burrows 



