Triangulated boron networks can be described in terms of the deviation of their local vertex environments from the degree 5 vertices found in ideal icosahedra. Vertices of degrees other than 5 or equivalent are considered to be defective vertices. This method, which was previously applied to deltahedral borane anions BnHn2- and nido-BnHn+4 boranes, has now been applied to arachno boranes of the types BnHn+6 and BnHn+5- (4 less than or equal to n less than or equal to 10). The known structures of the neutral arachno boranes B4H10, B-8-H-14, and n-B9H15 consist of triangulated boron networks with no defective vertices in accord with their higher stabilities relative to other neutral arachno boranes. In other structures of known arachno boranes, there are relatively small numbers of defective vertices, and these are isolated as far as possible from each other.