The galleries of the bark-beetles are as a rule such characteristic features of the different species that sometimes these are more easily recognised by the shape of their galleries than by their own structure, as, for example, in the case of the two pine-beetles, Blastophagus piniperda and B. minor, in which the distinguishing features of the insects cannot be discerned without the help of a good pocket-lens. But, on the other hand, the galleries of many species are subject to a certain amount of variation and show typical modifications which are responses to various external factors, such as the size of the trunk or of the branches, the thickness of the bark, and the orientation of the trunk, viz., whether the tree is standing or felled.