COLEOPTERA 543 



The scolytid beetles exhibit two radically different types of 

 habits; and from this point of view they can be grouped into two 

 groups: first, the engraver-beetles or bark-beetles; and second, the 

 ambrosia-beetles or timber-beetles. These two groups, however, do 

 not represent a natural division of the family based on structural 

 characters. The peculiar habits of the ambrosia-beetles are believed 

 to have arisen independently in different parts of the series of scolytid 

 beetles, and in the family Platypodidas as well. 



The Engraver-Beetles or Bark-Beetles 



If the bark be pulled from dead branches or trunks of trees, the 

 inner layer and the sap-wood will be found, in many cases, to be 

 ornamented with burrows of more or less regular form. The smoothly 

 cut figures are the mines of engraver-beetles, which are also known as 

 bark-beetles. Many kinds of these engravings can be found, each 

 characteristic of a particular species of engraver beetles. A common 

 pattern is shown in Figure 668. 



Many figures and detailed descriptions of the burrows of engraver- 

 beetles have been published by writers on forest-insects ; among the 

 more important papers on this subject published in America are those 

 by Hopkins ('09) and Swaine ('18), in which can be found references 

 to many other papers. 



The different species of engraver-beetles vary so greatly in the 

 details of their habits that it is difficult to make generalizations re- 



Fig. 668. 



garding them in the space available here. In a common type, the 

 adult beetle, after penetrating the bark, makes a tunnel in the inner 

 layer of the bark or in the sap-wood or in both ; this is known as the 

 egg-tunnel, and may be either simple or branched. In the sides of 

 the tunnel, most species make niches, the egg-niches, in which the 

 eggs are laid. The larva when hatched feeds on the bark or sap-wood 

 or both and thus makes a lateral tunnel. These lateral tunnels made 

 by the larvs often extend parallel in a more or less regular manner, 

 as shown in Figure 668. 



While most of the engraver-beetles infest forest-trees, the two 

 following species are well-known pests of fruit-trees. 



