Wing dimorphism is a common insect solution to a trade-off. Short wings cost less to build and leave more resources for eggs, which pays when a habitat patch is good and stable. Long wings cost more and reduce fecundity but allow escape from a deteriorating or crowded patch.
In Roesel's bush-cricket, the short-winged form is normal and the long-winged form, sometimes called f. diluta, appears at a low frequency that rises with population density and in warm seasons. The developmental switch is condition-dependent rather than a fixed genotype, so the same population produces both forms depending on circumstances. Long-winged individuals can fly and colonise new sites, and the species has expanded its British range substantially over recent decades, with warming and the spread of rough grassland along verges both implicated.
The song is a good identification cue but a poor one for human ears. It is a continuous high-frequency buzz, easily mistaken for a machine, and many people over the age of about forty cannot hear it at all, so the species is systematically under-recorded by hearing alone.