In general, in insects that are considered opistognathous the mouthparts, when not in use, project to the back on the underside of the head and thorax, usually resting between the base of the first pair of legs. However the head remains oriented as I described above for the hypognathous condition, with the antennas inserted on the frontal-most part of the head. When this type of insects use their mouthparts, normally a proboscis or “beak”, they will pierce downwards. This is mostly true for sap feeders.

But in some insects with proboscis the head is oriented as described for the prognathous condition, and the proboscis will project forward, even though it will still fold backwards underneath the body when not in use. This is most common for hunting insects.

Incidentally, proper terminology provides another nice example of what is going on in the head of ants: in ants the frontal region is dorsal.

One little quibble: “what used to be the bottom (the mouth) is now the frontal region.” Since the term frontal region is commonly used to describe the area of the frons, better to use the adjective anterior, or if you want to be really Englishy about it, front-most, or foreward.