Locusts are, in fact, a specific type of grasshopper, but this relationship is more complex than a simple difference in species. A locust is defined not by its unique taxonomy but by its capacity for a dramatic, reversible biological shift. This shift transforms an otherwise harmless, solitary insect into a highly mobile, coordinated agent of mass destruction. Understanding the distinction requires examining the classification of these insects and the unique mechanism that triggers this profound biological change.
Grasshoppers, Locusts, and the Taxonomic Family Tree
Both grasshoppers and locusts belong to the Order Orthoptera, which includes crickets and katydids, and are specifically classified within the Family Acrididae. The term “locust” is not a separate scientific classification but a functional label applied to a limited number of grasshopper species that possess the ability to swarm. Every locust is a grasshopper, but only about 18 genera worldwide can become locusts.
These insects share the same basic physical characteristics, such as powerful hind legs adapted for jumping and relatively short antennae. Locusts are defined by density-dependent phenotypic plasticity—the ability to display alternative physical and behavioral forms based on population density. This capacity for transformation separates a common, solitary grasshopper from one capable of forming devastating migratory swarms.
Phase Change: The Key Difference
The biological phenomenon that defines a locust is phase polymorphism, which involves two interconvertible life phases: the solitary phase and the gregarious phase. When population densities are low, the insect exists in the solitary phase, acting much like a normal grasshopper. These individuals are typically camouflaged with green or brown coloration, are relatively inactive, and actively avoid contact with other members of their species.
A dramatic transformation occurs when the insect switches to the gregarious phase, driven by increasing population density. The insects undergo significant physical and behavioral changes, including a change in coloration, often developing bright, contrasting yellow and black markings. They also develop different body proportions, such as stronger wings and larger muscles, which equip them for prolonged, long-distance flight. Their behavior flips entirely, moving from aversion to attraction, where they actively seek out other locusts to form cohesive, migrating groups.
What Triggers Mass Swarming
The switch from the solitary to the gregarious phase is initiated by environmental conditions that lead to consistent physical contact among individuals. Typically, a period of drought concentrates grasshoppers into dwindling patches of vegetation, forcing them into close proximity. The primary trigger is the repeated tactile stimulation of the insect’s hind legs as they rub against one another.
This external sensory input rapidly causes an internal, neurochemical response in the insect’s central nervous system. Within just a few hours of consistent crowding, the levels of the neurotransmitter serotonin increase significantly. This increase in serotonin is the mechanism that flips the behavioral switch. This chemical change causes the newly gregarious insects to become mobile, attracted to their neighbors, and ready to form marching bands of nymphs or flying swarms of adults.
The Devastating Impact of Locust Plagues
Once the gregarious phase is established, the resulting swarms pose one of the oldest and most destructive threats to agriculture. A single square kilometer of a dense desert locust swarm can contain up to 80 million adult insects. This aggregation is capable of consuming the same amount of food in one day as approximately 35,000 people.
Locust plagues can devastate crops and pastures across over 60 countries. The sheer scale of destruction can lead to severe food shortages, market price fluctuations, and long-term negative impacts on the affected communities. The Food and Agriculture Organization of the United Nations (FAO) runs the Desert Locust Information Service (DLIS) to monitor populations and provide early warnings to mitigate the devastating consequences of these swarms.