Many people use “grasshopper” and “locust” interchangeably, or are unsure of the distinction. While closely related, a fundamental difference exists in their behavior and biology. A locust is not a separate species, but rather a specific manifestation of certain grasshopper species under particular conditions. This transformation is key to understanding their relationship.
The World of Grasshoppers
Grasshoppers are a diverse group of insects belonging to the order Orthoptera and the suborder Caelifera. They are characterized by a head, thorax, and abdomen. These insects possess large compound eyes that provide a wide field of vision, along with three simple eyes for detecting light and dark. A pair of short, thread-like antennae aid in touch and smell, while their mouthparts are adapted for chewing plant material.
Grasshoppers are primarily ground-dwelling herbivores, feeding on plants. Their powerful hind legs are a distinctive feature, enabling them to leap considerable distances to escape predators. Most grasshopper species lead a solitary existence, avoiding other individuals and blending into their environment through camouflage. They are found across numerous ecosystems worldwide, from grasslands to semiarid regions.
The Phenomenon of Locusts
Locusts are not a distinct taxonomic group separate from grasshoppers. Instead, “locust” refers to certain short-horned grasshopper species (family Acrididae) that can undergo dramatic changes in behavior and physical form. Their classification as a “locust” hinges on their ability to form large, migratory swarms under suitable environmental conditions.
This capacity for swarming has evolved independently in multiple lineages of grasshoppers, meaning that “locusts” represent a polyphyletic group. Normally, these species exist in a solitary state. However, under specific circumstances, their populations can increase rapidly, leading to mass migrations. This behavioral shift defines them as locusts, transforming them into a gregarious form.
The Transformation: Solitary to Swarming
The shift from solitary grasshopper to swarming locust is a biological mechanism known as phase polyphenism or density-dependent phenotypic plasticity. This transformation is primarily triggered by environmental factors, most notably increased population density and overcrowding. When conditions such as drought followed by significant rainfall lead to abundant vegetation and subsequent population booms, the physical contact among individual grasshoppers increases. This heightened tactile stimulation, particularly to their hind legs, initiates a cascade of physiological and behavioral changes.
One physiological change involves increased serotonin levels within the insect’s nervous system. Research indicates that elevated serotonin is a causal link between crowding and the shift to gregarious behavior. This neurochemical change rapidly alters their fundamental behavior: instead of avoiding each other, individuals in the gregarious phase become attracted to conspecifics, leading to the formation of cohesive groups.
Accompanying these behavioral shifts are distinct morphological changes. Solitary-phase grasshoppers often match their surroundings in color and have shorter wings, while gregarious-phase locusts develop darker coloration, typically black and yellow or orange, and possess longer wings suitable for sustained flight. Their body proportions also change, with gregarious adults having broader shoulders and a more saddle-shaped pronotum. These combined adaptations enable the formation of swarms that can travel long distances, consuming vegetation in their path.