Locusts are short-horned grasshoppers known for their dramatic swarming behavior. These insects transform from solitary individuals into massive, destructive swarms, exhibiting remarkable changes in appearance. Their coloration is complex and dynamic, shifting significantly with their life stage and social environment. Understanding these changes provides insight into their biology and behavior.
The Spectrum of Locust Colors
Locusts display a wide array of colors, closely linked to their life stage and social phase. Solitary locusts often exhibit green or brownish hues, helping them blend into their natural environment, providing camouflage. For instance, the migratory locust (Locusta migratoria) typically appears green when solitary. Solitary hoppers, or nymphs, often maintain this consistent green or brown color throughout their developmental stages.
In contrast, gregarious locusts display striking and distinct color patterns. Immature gregarious desert locusts (Schistocerca gregaria) are typically pink. As gregarious hoppers mature, their color changes from full black in the first stage to a black and yellow pattern in later stages.
Gregarious nymphs, regardless of sex, develop contrasting yellow and black markings. Mature gregarious adults can exhibit vibrant yellows, oranges, and blacks. For example, mature gregarious male desert locusts become bright yellow, while females may be a less bright yellow, primarily on their upper bodies. This dramatic shift in coloration is a clear indicator of their phase transformation.
Environmental and Biological Influences on Color
The diverse coloration observed in locusts is influenced by environmental and biological factors, particularly their phase polymorphism. Locusts exhibit phenotypic plasticity, meaning their physical traits, including color, change based on population density. In the solitary phase, their green or brown colors are an adaptive response for camouflage, helping them hide from predators.
When population density increases, locusts transform to the gregarious phase, which is triggered by physical contact, specifically the repeated touching of their hind legs. This stimulation leads to a surge in serotonin, a neurotransmitter that alters both behavior and appearance. The color change from green to black or brown in gregarious locusts involves a novel “palette effect” mechanism. This change is caused by a red pigment complex formed by beta-carotene-binding protein (βCBP) and beta-carotene, which overlays the existing green coloration. As gregarious locusts mature, their black coloring increases in direct relation to their βCBP levels. Blocking this protein can even reverse the color change, turning black locusts green.
Diet also plays a role in locust coloration. Locusts can become toxic to predators by consuming certain toxic host plants, and this toxicity can be associated with their color change. For instance, the green color in some locust species is derived from a mixture of blue biliverdin pigment and yellow carotenoids. Environmental conditions like humidity and temperature can also influence pigment production and expression, though specific details vary by species. Underlying genetic predispositions also contribute to their capacity for color change.
The Meaning Behind the Hues: Color as a Signal
Locust coloration serves various purposes, acting as a signal to both other locusts and potential predators. For solitary locusts, their green or brown coloration provides effective camouflage, allowing them to blend into their surroundings and avoid detection by predators. This cryptic coloration is a primary defense mechanism in low-density populations.
In contrast, the bright blacks, yellows, and oranges of gregarious locusts often function as a warning signal, a phenomenon known as aposematism. This striking coloration indicates to predators that the locusts may be unpalatable or toxic, deterring attacks. Research has shown that predators, such as lizards, learn to avoid gregarious locusts with this warning coloration, especially when the locusts have consumed toxic plants.
Beyond predator deterrence, color changes in gregarious locusts also serve as social signals within the swarm. The shift to bold colors can help stimulate aggregation and maintain group cohesion. For adult male locusts in dense swarms, the bright yellow coloration acts as an intraspecific warning signal, primarily to other males. This signal helps prevent mistaken sexual harassment among males, reducing male-male competition and facilitating more efficient mating in crowded conditions. The “yellow protein” gene, which controls juvenile warning coloration, is repurposed in adult males for this social signaling function.