A locust is biologically a specific kind of grasshopper, which often confuses people. They are the same insect but display dramatically different life cycles and behaviors. The fundamental difference lies in the capacity of certain grasshopper species to undergo a transformation. This allows them to switch from a solitary existence to a highly social one, separating the common field grasshopper from its notorious, swarming relative.
Classification: Clarifying the Relationship
Both grasshoppers and locusts belong to the insect order Orthoptera, characterized by powerful hind legs adapted for jumping. They are classified within the suborder Caelifera, which comprises the short-horned grasshoppers distinguished by their antennae being shorter than their body. This taxonomic placement confirms their close biological kinship.
The term “locust” is not a formal scientific classification. It is a designation applied to approximately 20 species of grasshoppers within the family Acrididae. These species possess the genetic blueprint allowing them to transition between two distinct morphological and behavioral states. Therefore, all locusts are grasshoppers, but only a small fraction of grasshopper species can become locusts.
Physical Characteristics
Even when a locust is in its solitary phase, it can exhibit subtle anatomical differences compared to a typical non-swarming grasshopper species. Solitary-phase locusts often develop longer wings relative to their body size. This physical adaptation prepares them for long-distance migration should a population boom occur.
The most pronounced physical differences, however, are triggered by population density. Grasshoppers typically maintain cryptic coloration—greens, browns, and mottled patterns—that allows them to blend into their environment consistently throughout their lives.
In contrast, locusts exhibit density-dependent color changes, a phenomenon known as polyphenism. When isolated, a locust nymph is usually green, mimicking a typical grasshopper. When population density increases and the behavioral phase shift occurs, the new generation of nymphs develops striking black and yellow or black and orange patterns. This gregarious coloration promotes group cohesion during swarming.
The Defining Difference: Phase Polymorphism and Swarming
The true biological divergence between a grasshopper and a locust is defined by the latter’s ability to undergo phase polymorphism. This mechanism allows a single species to exist in two completely different forms: the solitary phase and the gregarious phase. The trigger for this switch is typically environmental, specifically a rapid increase in population density coupled with abundant food resources following rainfall in arid regions.
The transition from solitary to gregarious behavior is initiated by physical contact among individuals, particularly stimulation of the hind legs. This sensory input rapidly affects the insect’s neurochemistry, leading to a cascade of internal changes. Studies have shown that the neurotransmitter serotonin plays a direct role in initiating the behavioral shift, changing the insect’s response to other locusts from repulsion to attraction within hours.
A locust in the solitary phase is shy, sedentary, and actively avoids contact with other individuals, preferring to forage alone. Its movements are slow and uncoordinated with its peers, resembling the behavior of a standard grasshopper. The switch to the gregarious phase results in the opposite temperament: the insects become highly active, mutually attracted, and begin to move and feed in cohesive, coordinated groups.
This behavioral shift is accompanied by further morphological changes in subsequent generations, leading to the fully developed gregarious form. These changes include a shorter lifespan, increased metabolic rate, and the development of the distinctive dark coloration mentioned previously. The insects become physically optimized not for camouflage and survival in isolation, but for rapid movement and mass migration.
When population density reaches a threshold, the coordinated movement of the gregarious locusts results in the formation of massive bands of nymphs, which eventually mature into flying swarms. These swarms are cohesive entities driven by collective decision-making and attraction to pheromones. A single Desert Locust swarm can cover hundreds of square kilometers and contain up to 80 million individuals per square kilometer.
The sheer size and movement of these aerial armies are what give the locust its destructive reputation, a behavior never exhibited by typical grasshoppers. The swarms migrate long distances, sometimes crossing continents, consuming their own body weight in vegetation daily. This collective, migratory feeding behavior is the definitive functional distinction that sets the locust apart from its solitary grasshopper relatives.