Bark-boring is done by many species of beetles, especially Scolytidae, which excavate characteristically formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (e.g. Tomicus bidentatus) or even burrow right into the latter (e.g. T. lineatum). It commonly happens that the external apertures show up clearly, owing to the brown dust and excrement, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc.; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do.