Grasshoppers are common insects belonging to the order Orthoptera. While most live out their lives as solitary individuals, certain species can undergo a dramatic physical and behavioral transformation. This change converts them into a highly mobile, collective form that engages in long-distance migration. Understanding this unique biological phenomenon requires clarifying the difference between the typical grasshopper and the few species capable of mass migration.
Locusts vs. Grasshoppers: Clarifying the Migratory Species
The insects known as “locusts” are not a separate species from grasshoppers, but rather specific types of short-horned grasshoppers that exhibit a unique biological trait. They are distinguished by their capacity for density-dependent phenotypic plasticity, a reversible change that alters their form and behavior entirely. Most of the time, these insects exist in the solitary phase, where they are shy, cryptically colored, and avoid one another, behaving exactly like a typical grasshopper.
The key difference emerges when population density increases, which triggers a switch to the gregarious phase. This transformation involves profound changes in the insect’s physiology, morphology, and behavior. Gregarious individuals develop different body shapes, often with longer wings suited for sustained flight, and change their color to more conspicuous patterns. Crucially, their behavior shifts from avoidance to attraction, making them highly social and forming the basis for coordinated migration.
The gregarious phase, or locust, is defined solely by its potential to form dense, destructive, and migratory swarms. Only about a dozen of the 12,000 known grasshopper species possess this capacity for transformation, making the migratory behavior a rare adaptation. The ability to switch between these two phases allows the species to survive in environments that fluctuate between supporting low and high populations.
Environmental Triggers That Initiate Mass Movement
The transformation from the solitary phase to the gregarious phase is primarily driven by environmental conditions and population density. This process begins in arid or semi-arid regions following unseasonable or heavy rainfall. The sudden abundance of vegetation supports a successful breeding season, leading to a massive surge in the population of new nymphs.
As the temporary vegetation starts to dry out, the large number of young, flightless insects are forced to congregate in the remaining patches of green food. This forced proximity leads to an increase in physical contact, which is the direct trigger for the phase change. Repeated tactile stimulation signals to the individual that it is in a crowded environment.
This physical stimulus initiates a rapid neurochemical change within the insect’s nervous system, specifically an increase in the neurotransmitter serotonin. For the Desert Locust, this surge is directly linked to the induction of gregarious behavior, occurring in as little as a few hours. The crowding triggers the biological change that makes mass movement inevitable.
The Organization and Scale of Swarm Migration
Once the gregarious phase is initiated, the insects organize their movement in two distinct forms, depending on their life stage. The flightless juveniles, known as nymphs, form dense, marching groups called “hopper bands,” which move across the ground consuming nearly all vegetation in their path. When these nymphs mature into winged adults, they merge to form the immense, flying swarms that are the ultimate expression of locust migration.
These adult swarms are highly coordinated and can reach staggering sizes, sometimes covering hundreds of square kilometers. A typical swarm contains between 80 million and 150 million individuals per square kilometer. They fly downwind, using prevailing air currents for lift and direction, and can cover distances of 100 to 200 kilometers in a day.
The sheer volume of insects and their collective appetite drive the migratory necessity, as they consume their own body weight in food each day. The migratory flight is a survival strategy, ensuring the population moves en masse to find new food sources, often crossing international boundaries. A swarm covering just one square kilometer can eat the same amount of food in a day as approximately 35,000 people.