Not as familiar with the Insight 1's underbody, but the features behind the the tires on the gen 2 & 3 Prius are classic "boat tail" tapered bodywork meant to minimize the cross sectional area of the trailing wake.
I tried out XXL variants of the same thing when I built & tested my car's boat tail:
You can't see it well in the angle of this photo, but that structure tapers from the full width of the rear tire (as on the gen 2 & 3 Prius) down to about 1 inch wide at its tip.
They may have a dual purpose and serve as a kind of guide/channel for flow exiting the middle of the car; I don't know for sure. Would need to find an aerodynamicist to ask.