Insects were the first animal herbivores. Plants were more than a diet – they were a lifestyle: something to live on as well as eat. This required being able to hang on; no small feat in the face of stormy wind and rain. Caterpillars evolved sucker-like false legs with rows of small hooks. Crickets and beetles have broad pads on their feet thickly covered in fine hairs. Oily secretions enable the hairs to behave as suckers on smooth leaves.
Desiccation was another challenge. Some insects developed waterproof cuticles to retain moisture. Caterpillars drink the drops of water that plants exude at night. Then there are the sap suckers, such as aphids, which crowd together to reduce evaporation, as well as offering the relative safety of numbers.
The route to insect herbivory was from digesting decaying plant matter to consuming plants while they were still alive. Snacking on spores and pounding down pollen were an interim step.