  Combining 3D smoothed-particle hydrodynamics and Monte Carlo radiative
transfer calculations, we examine the morphology of spiral density waves
induced by gravitational instability (GI) in protoplanetary disks, as they
would appear in direct images at near-infrared (NIR) wavelengths. We find that
systems with disk-to-star-mass ratios q=M_disk/M_star that are ~0.25 or more
may produce prominent spiral arms in NIR imaging, remarkably resembling
features observed in the MWC 758 and SAO 206462 systems. The contrast of
GI-induced arms at NIR wavelengths can reach a factor of ~3, and their pitch
angles are about 10-15 degree. The dominant azimuthal wavenumber of GI-induced
spiral arms roughly obeys m~1/q in the range 2<~1/q<~8. In particular, a
massive disk with q~0.5 can exhibit grand-design m=2 spirals. GI-induced arms
are in approximate corotation with the local disk, and may therefore trap dust
particles by pressure drag. Although GI can produce NIR spiral arms with
morphologies, contrasts, and pitch angles similar to those reported in recent
observations, it also makes other demands that may or may not be satisfied in
any given system. A GI origin requires that the spirals be relatively compact,
on scales <~100 AU; that the disk be massive, q>~0.25; and that the accretion
rate Mdot be high, on the order of 1e-6 solar mass per year.
