218 GEOLOGICAL SURVEY OF NEW JERSEY. 



when to all appearance they are sound as a dollar ; and. this may 

 be so, too, where woodland has been too thoroughly opened up or 

 where natural water-courses have been diverted and the usual 

 supply of moisture has been either decreased or added to abnor- 

 mally. When once these beetles have secured a good foothold 

 the tree is doomed, and good forestry practice is to take it out 

 and use it up immediately. 



Almost every species of bark-beetle makes a gallery peculiar 

 to it, so that to one familiar with their habits a look at their 

 work tells the species even if no adult insect is available for 

 examination. Yet there are certain features that most of them 

 have in common ; usually the female parent bores into a proper 

 tree, through the bark to the sap wood, and in the layer between 

 bark and trunk, though mostly in the bark, a vertical channel 

 or burrow is made. A series of small chambers are nicked out at 

 each side of this channel and in each of them an egg is laid. 

 The larva, when hatched, at once starts a channel or burrow of 

 its own, narrow and thread-like at first, but becoming larger as 

 the grubs increase in size and each channel diverging from all 

 the others so that they rarely cross. The pupa is formed at the 

 end of this larval gallery in a little cell eaten out for that pur- 

 pose, and the adults come out through little round holes bored 

 by them when fully matured. 



In some cases the central gallery is even throughout ; some- 

 times there is an enlargement or an oblique spur at one or both 

 ends, or a chamber somewhere in its course in which the parent 

 can turn around if it so desires. 



Of this type is the Hickory bark-beetle that at Glen Ridge, 

 New Jersey, killed off so many trees a few years ago. (See 

 Figure 4.) 



The Scolytids boring in the solid wood are usually called 

 shot-hole borers, because of the size of the hole and because the 

 edges are generally blackened as by fire. The work of these 

 beetles is not so obvious, nor is their ill effect so immediately 

 visible as in the case of the previous type, yet they are always 

 ready to do their share when the proper conditions arrive. 



The typical shot-hole borers differ altogether in many respects 

 from the bark-boring type. Their galleries go variable dis- 

 tances in the solid wood, then laterals no larger or longer than the 



