Psyllid

Psyllids or jumping plant lice are small plant-feeding insects that are very "host specific", i.e. they only feed on one plant species (monophagous) or feed on a few related plants (oligophagous). Together with aphids, coccids and whiteflies they form the group called Sternorrhyncha, which is considered to be a "primitive" group within the "true bugs" (Hemiptera). "Sternorrhyncha" refers to the rearward position of the mouthparts relative to the head. Psyllids have the fewest pest species of these four insect groups, and they are therefore the least well studied.

Psyllid fossils have been found from the early Permian before the flowering plants evolved. The explosive diversification of the flowering plants in the Cretaceous was paralleled by a massive diversification of associated insects, and many of the morphological and metabolic characters that the flowering plants exhibit may have evolved as defenses against herbivorous insects.

Coevolution?

Insect-plant interactions have been important in defining models of coevolution and cospeciation, referring to whether plant speciation drives insect speciation and vice versa, though most herbivorous insects probably evolved long after the plants they feed on.