Grasshoppers and locusts often appear indistinguishable to the casual observer. While both share a similar appearance and belong to the same biological family, the distinction between them is not based on a difference in species. The primary difference is a dramatic behavioral and physical transformation that only certain species of grasshoppers are capable of undergoing. This change is entirely dependent on environmental conditions, making the term “locust” a description of a specific life phase rather than a separate taxonomic category.
Defining the Taxonomic Connection
Locusts are specific species of short-horned grasshoppers that possess the genetic potential for a remarkable transformation. All locusts are a type of grasshopper, but only a small fraction of the world’s grasshopper species can become locusts. These insects belong to the family Acrididae, which includes thousands of grasshopper species. Only about a dozen species have the capacity to switch between two very different life forms. This capability is known as density-dependent phase polyphenism, meaning an individual organism can express two or more distinct phenotypes based on population density. The physical form of a locust is the specialized, high-density form of what is otherwise an ordinary, solitary grasshopper.
The Role of Density and Phase Change
The trigger for a grasshopper to transform into a locust is a sudden increase in population density, or crowding. When environmental conditions, such as rainfall followed by rapid vegetation growth, lead to a high concentration of individuals, the stage is set for this phase shift. This crowding causes frequent physical contact between individuals, particularly the repeated touching of their hind legs and antennae. This physical stimulation acts as the direct environmental cue for the transformation.
Neurochemical and Behavioral Shift
This external sensory input rapidly triggers a neurochemical change within the insect’s central nervous system. A temporary but intense surge of the neurotransmitter serotonin occurs in nerve bundles in the insect’s midsection. This increase in serotonin levels initiates the behavioral shift from a solitary existence to a social one. This behavioral transformation, which can begin in a matter of hours, is the defining characteristic of the “gregarious phase.” The solitary phase grasshopper, which actively avoids others, becomes a gregarious phase locust that seeks out and aggregates with its conspecifics.
Distinguishing Features of Swarming Locusts
Once the gregarious phase is triggered, the insect undergoes a full morphological transformation that results in a distinct physical appearance. Solitary grasshoppers are typically green or brown and blend with their surroundings, but gregarious locusts develop striking coloration, often featuring black and yellow or orange markings. Their body shape also changes, with their wings and muscles developing to support long-distance, sustained flight.
Swarm Formation and Agricultural Impact
The most noticeable feature of the gregarious phase is the formation of coordinated, massive swarms. Solitary grasshoppers fly individually and randomly, but locusts fly in dense, highly directional formations that can contain billions of individuals. These swarms are capable of sustained, long-range migration, with some species able to cover distances of 81 miles or more in a single day. This ability to form vast, mobile swarms is what makes the distinction important for human agriculture. The collective feeding and movement of these dense populations can completely strip vegetation from an area, causing devastating crop damage across entire regions.