But in arid times foraging grounds become smaller and the locusts meet each other more often. Rubbing against each other, especially at their hind legs, effects a profound change of phase in the animals which leads to a different behaviour within hours. At this (gregaria) phase the animals meet in swarms, after a generation their body grows bigger, the jaw stronger and they now look a reddish brown.
Right into the 20th c. these were thought to be two different species.
See further M. Rothschild, B. Gardiner, R. Mummery, and G. Valadon, "Carotenoids in the solitaria and gregaria phases of the locust: Schistocerca gregaria fed on artificial and natural diets," Journal of Zoology 181 (1977): 475–494.
