An objects momentum is its mass multiplied by its velocity. This means that Mass is Inertia and Momentum is
inertia in motion.
Actually, momentum is mass × velocity. That means that, because the bug's mass is neglible compared to the truck's, the bug's momentum is negligible, so there is no observable change in velocity (or momentum) for the truck. However, there are two things to remember:
The bug is not stopped. It has had an inelastic collision with the truck, which continues to move. So the bug's momentum has merged with the truck's, just like the rail cars in the example we worked in class today.
The total momentum of the bug plus the truck is a constant. That means that the truck's momentum is reduced by the amount of momentum that the bug had (since the collision was head-on). In practice, you could not measure the reduction without really, really sensitve instruments.