Adult female desert locusts require contact chemicals and light for progeny gregarization

Crowding causes many organisms to express phenotypic plasticity in various traits. Phase polyphenism in desert locusts represents one extreme example in which a solitary form (solitarious phase) turns into a gregarious form (gregarious phase) in response to crowding. Conspicuous differences in body size and colour occur even in hatchlings. The phase‐specific differences in hatchling characteristics are caused by the tactile stimuli perceived by the antennae of their mother. However, the nature of the tactile stimuli and the mechanism by which the perceived stimuli are processed as a gregarizing signal remain unknown. To explore this problem, the antennae of solitarious adult females of the desert locust Schistocerca gregaria are touched with the bodies of conspecific locusts at different physiological stages and those of other species. The results suggest that a cuticular chemical factor at a specific developmental stage of conspecific locusts causes the solitarious females to produce large eggs that give rise to black hatchlings characteristic of gregarious forms (progeny gregarization), and that this or a similar compound occurs in other acridids, crickets and cockroaches but not in beetles. The involvement of a chemical substance is also supported by hexane extracts of cuticular surfaces of locusts that induce the same effects. Interestingly, crowding induces such gregarizing effects only when the female receives the appropriate stimulus in the presence of light. Solitarious female S. gregaria with their head capsule coated with phosphorescent paint exhibit progeny gregarization in response to crowding and light pulses in darkness, whereas those treated in the same way without light pulses fail to do so.