Phase polymorphism in locusts

Definition

A density-dependent, reversible polyphenism in which locust populations switch between two or more distinct phenotypic phases—typically solitarious (solitary, cryptic, sedentary) and gregarious (swarming, conspicuous, highly mobile)—in response to population density and associated environmental cues. The transformation involves coordinated changes in coloration, morphology, physiology, behavior, and neurochemistry, and is among the most dramatic examples of phenotypic plasticity in insects.

Etymology

Greek phasis (appearance, phase) + polymorphos (many-formed); applied to locust biology in the early 20th century to distinguish density-dependent forms from genetic polymorphism

Example

Desert locusts (Schistocerca gregaria) in the solitarious phase are green or brown, avoid conspecifics, and have low metabolic rates; when crowded, they transform within hours to hours into the gregarious phase with black and yellow patterning, enhanced serotonin signaling, and coordinated marching or flight behavior that can lead to transcontinental swarms.

Synonyms

• locust phase polyphenism
• density-dependent phase change

Related Terms

• Polyphenism
• phenotypic plasticity
• gregarization
• solitarization
• serotonin
• crowding pheromones
• schistocercal wing shape
• locust swarming
• phase-related traits

Usage Notes

Distinguish from genetic polymorphism, which involves heritable allele differences; phase polymorphism is environmentally cued and reversible within an individual's lifetime. The term applies specifically to locusts (Acrididae: Cyrtacanthacridinae and related subfamilies), though analogous density-dependent changes occur in some other grasshoppers. Intermediate or transient phases (transiens) may occur during transition. Not all locust species show the full syndrome; some exhibit only partial phase characteristics.