  In transition-metal dipnictides $TmPn_2$ ($Tm$ = Ta, Nb; $Pn$ = P, As, Sb),
the origin of extremely large magnetoresistance (XMR) is yet to be studied by
the direct visualization of the experimental band structures. Here, using
angle-resolved photoemission spectroscopy, we map out the three-dimensional
electronic structure of NbAs$_2$. The open-orbit topology contributes to a
non-negligible part of the Fermi surfaces (FSs), like that of the isostructural
compound MoAs$_2$, where the open FS is proposed to likely explain the origin
of XMR. We further demonstrate the observation of open characters in the
overall FSs of W$_2$As$_3$, which is also a XMR semimetal with the same space
group of $C$12/$m$1 as $TmPn_2$ family and MoAs$_2$. Our results suggest that
the open-orbit FS topology may be a shared feature between XMR materials with
the space group of $C$12/$m$1, and thus could possibly play a role in
determining the corresponding XMR effect together with the electron-hole
compensation.
