Scientists believe that insect hematophagy evolved from piercing-sucking mouthparts used to extract plant fluids but have difficulty studying this evolution due to the lack of insect fossil records.
The features of an insect's mouthparts can help identify which order it belongs to, while also providing clues about how it evolved and what it feeds on.
You can find this mouthpart on ants from the Hymenoptera order, grasshoppers and crickets of the Orthoptera order, dragonflies of the Odonata order, and beetles of the Coleoptera order.
And while it's hard to catch an insect in the act of pollinating, the fossil record gives us several lines of indirect evidence of pollination, like specialized plant structures, specialized insect mouthparts, and pollen grains.